Einstein College of Engineering

(An Institution Affiliated To Anna University, Tirunelveli)

(III sem Mechanical Engg.) (ODD SEMESTER 2010-2011)

Einstein College of Engineering

OBJECTIVE    1. To understand the basic concepts of different types of electrical machines and their performance. To study the different methods of starting D.C motors and induction motors. To study the conventional and solid-state drives. 8

Basic Elements – Types of Electric Drives – factors influencing the choice of electrical drives – heating and cooling curves – Loading conditions and classes of duty – Selection of power rating for drive motors with regard to thermal overloading and Load variation factors 2. DRIVE MOTOR CHARACTERISTICS 9

Mechanical characteristics – Speed-Torque characteristics of various types of load and drive motors – Braking of Electrical motors – DC motors: Shunt, series and compound - single phase and three phase induction motors. 3. STARTING METHODS 8

Types of D.C Motor starters – Typical control circuits for shunt and series motors – Three phase squirrel cage and slip ring induction motors. 4. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES 10

Speed control of DC series and shunt motors – Armature and field control, Ward-Leonard control system Using controlled rectifiers and DC choppers –applications. 5. CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES 10

Speed control of three phase induction motor – Voltage control, voltage / frequency control, slip power recovery scheme – Using inverters and AC voltage regulators – applications. TOTAL : 45 TEXT BOOKS 1. 2. VEDAM SUBRAHMANIAM, “Electric Drives (concepts and applications)”, Tata Hill, 2001 NAGRATH.I.J. & KOTHARI.D.P, “Electrical Machines”, Tata McGraw-Hill, 1998 REFERENCES 1. PILLAI.S.K “A first course on Electric drives”, Wiley Eastern Limited, 1998 2. M.D.SINGH, K.B.KHANCHANDANI, “Power Electronics”, Tata McGraw-Hill, 1998 H.Partab, “Art and Science and Utilisation of electrical energy”, Dhanpat Rai and Sons, 1994 McGraw-

Einstein College of Engineering

Drives are employed for systems that require motion control – e.g. transportation system, fans, robots, pumps, machine tools, etc. Prime movers are required in drive systems to provide the sources: di esel engines, petrol engines, hydraulic motors, electric motors etc. movement or motion and energy that is used to provide the motion can come from various Drives that use electric motors as the prime movers are known as electrical drives There are several advantages of electrical drives: a. Flexi ble control characteristic – This is particularly true when power electronic converters are employed where the dynamic and steady state characteristics of the motor can be controlled by controlling the applied voltage or current. b. Available in wide range of speed, torque and power c. High efficiency, lower noi se, low maintenance requirements and cleaner operation d. Electric energy is easy to be transported. A typical conventional electric drive system for variable speed application employing multimachine system is shown in Figure 1. The system is obviously bul ky, expensive, inflexible and require regular maintenance. In the past, induction and synchronous machines were used for constant speed applications – this was mainly because of the unavailabili ty of variable frequency supply.

W ith the advancement of power electronics, microprocessors and digital electronics, typical electric drive systems nowadays are becoming more compact, efficient, cheaper and versatile – this is shown in Figure 2. The voltage and current applied to the motor can be changed at will by employing power electronic converters. AC motor is no longer limited to application where only AC source is available, however, it can also be used when the power source available is DC or vice versa

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Electric drives is multi-disciplinary field. Various research areas can be sub-divided from electric drives as shown in Figure 3.

COM PONENTS OF ELECTRICAL DRIVES The main components of a modern electrical drive are the motors, power processor, control unit and electrical source. These are briefly discussed bel ow a) Motors Motors obtain power from electrical sources. They convert energy from electrical to mechanical - therefore can be regarded as energy converters. In braking mode, the flow of power is reversed. Depending upon the type of power converters

Einstein College of Engineering used, it is also possible for the power to be fed back to the sources rather than dissipated as heat There are several types of motors used in electric drives – choice of type used depends on applications, cost, environmental factors and also the type of sources available.. Broadly, they can be classified as either DC or AC motors they can be classified as either DC or AC motors: DC motors (wound or permanent magnet) AC motors Induction motors – squirrel cage, wound rotor Synchronous motors – wound field, permanent magnet Brushless DC motor – require power electronic converters Stepper motors – require power electronic converters Synchronous reluctance motors or switched reluctance motor – require power electronic converters b) Power processor or power modulator Since the electrical sources are normally uncontrollable, it is therefore necessary to be able to control the flow of power to the motor – this i s achieved using power processor or power modulator. W ith controllable sources, the motor can be reversed, brake or can be operated with variable speed. Conventional methods used, for example, variable impedance or relays, to shape the voltage or current that is supplied to the motor – these methods however are inflexible and inefficient. Modern electric drives normally used power electronic converters to shape the desired voltage or current supplied to the motor. In other words, the characteristic of the motors can be changed at will. Power electronic converters have several advantages over classical methods of power conversion, such as 1)More efficient – since ideally no losses occur in power electronic converters 2)Flexi ble – voltage and current can be shaped by simply controlling switching functions of the power converter. 3) Compact – smaller, compact and higher ratings solid–state power electronic devices are continuously being devel oped – the prices are getting cheaper Converters are used to convert and possibly regulate (i.e. using closed-loop control) the available sources to sui t the load i.e. motors. These converters are efficient because the switches operate in either cut-off or saturation modes Several conversion are possi ble

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b)Control Unit The complexity of the control unit depends on the desired drive performance and the type of motors used. A controller can be as simple as few op-amps and/or a few digital ICs, or it can be as complex as the combinations of several ASICs and digital signal processors (DSPs). The types of the main controllers can be • analog - which is noisy, inflexible. However analog circuit ideally has infinite bandwidth. • digital – immune to noise, configurable. The bandwidth is obviously smaller than the analog controller‟s – depends on sampling frequency

i. but note that 230V units will be damaged if operated at 400V. typically by 2 -3 times.determine the ratings of converters and the quadrant of operation required. • Check the Current rating of the inverter and the motor. d) Source Electrical sources or power supplies provide the energy to the electrical motors. maintenance c) c) Space and weight restrictions d) Environment and location 3.1 Overall Considerations. and hence the current. • Check the speed range you require. The inverter will limit current to 150 or 200 % of full current very quickly .e. when a drive system is selected from first principles.g. MIDIMASTERs will operate with three phase only. 3. W ith DSP/microp. . consider using a braking resistor (braking unit on MIDIMASTERs) to absorb the energy. This is because the power factor. lower bandwidth compared to above. careful consideration may avoid problems in installation and operation. Operation at low frequency and high torque can cause the motor to overheat due to lack of cooling • Synchronous motors require de-rating. 230V three phase input MICROMASTERs will operate with single or three phase inputs.a standard. b) Capital and running cost – Drive systems will vary in terms of start-up cost and running cost. e. • Do you need to stop quickly? If so. Single phase input units can be more cost effective in some cases. as a motor is already installed and the speed range requirement is not excessive. DSPs perform faster operation than mi croprocessors (multiplication in single cycle). For high efficiency operation. fixed speed motor will tolerate these overloads. their magnitudes or frequencies are fixed or depend on the sources of energy such as solar or wind.. complex estimations and observers can be easily implemented. Operation above normal supply frequency (50 or 60Hz) is usually only possible at reduced power.Einstein College of Engineering • DSP/microprocessor – flexible. Power rating is only a rough guide • Check that you have selected the correct operating voltage. Selecting a Drive Often drive selection is straight forward. However. • Check overload performance. the power obtained from the electrical sources need to be regul ated using power electronic converters Power sources can be of AC or D C in nature and normally are uncontrollable. can be very high at low frequency. AC source can be either three-phase or single-phase. 3-phase sources are normally for high power applications There can be several factors that affect the selection of different configuration of electrical drive system such as a) Torque and speed profile . and may also save significant cost.

The motor slows down a little as the load increases and the slip increases. The motor speed is determined mainly by the applied frequency. Based on the assumptions that the temperature limits of all parts does not exceed the temperature limits under certain operating conditions. The various parts of the machine use different type of insulation materials which have different temperature limits. or screened or armoured cables longer than 25m? If so. . Obviously. the motors can therefore adequately modeled as homogeneous bodies. Allowable power losses are higher for materials which can withstand higher temperature which translates to higher costs. 3. Particularly important is the insulation used for the windings which give rise to the different classes of machines. brushes. see later. and etc. commutator. slip rings. Core losses: Mainly due to eddy current and hysteresis losses Friction and w indage losses: Mainly due to ball bearings. this assumpti on cannot determine the specific internal thermal conditi ons for the motors. If the load is too great the motor will exceed the maximum torque and stall or „pull out‟. and Moment of Inertia. ventilation losses The constructions of the machines are very complex. brushes.2 Motor limitations For more information concerning calculation of Power requirements. Most motors and inverters will operate at 150% load Thermal considerations The losses in the machines contribute to the temperature increase in the machine. Three main cause of power losses are: Conductor losses : Exist in the windings. cables. Torque. or fit a choke to compensate for the cable capacitance. normally built from various types of materials (heterogeneous) with complex geometrical shapes. it may be necessary to de-rate. To exactly predict the heat flow and hence the temperature distribution is extremely difficult.Einstein College of Engineering • Do you need to operate with cables longer than 50m.

\ During cooling. The output power due to radiation is assumed negligible because of the low operating temperature and back radiation. the input power equals the output power.Einstein College of Engineering Let us assume that a homogeneous body shown in Figure 12 represents a motor which has a thermal capacity C. The equation describing the power balance is given by The heat dissipated by convection is given by where is the coefficient of heat transfer If we let equation (12) can be written as where is the thermal time constant. The input power. is represented by p1 whereas the output power. With power input p1 from 0 to ph at t=0. which is the losses incurred in the motor. the solution for is and a step change in the At steady state. is represented by p2 . the temperature of the body decays to the ambient temperature. Under a steady state condition. . when heat is removed at t=0. i.e. which is the power released as heat by convection. this is when the steady state temperature is reached.

a temporary overload is therefore possible without exceeding the temperature limits. the motor torque has to be larger than the load torque. According to (1).Continuous duty . the faster is the speed gets to its reference . the speed and torque responses for a closed-loop speed control DC drive with two different torque limit setting (10 Nm and 15 Nm) is shown in Figure 7. the difference between T1 and Te determines how fast the angular acceleration is. Three typical modes of operation are: . For example.Einstein College of Engineering If the thermal time constant is large.Short time intermittent duty .Periodic intermittent duty\ Ratings of converters and motors In order to accelerate to a given reference value. The higher the torque during the speed transient.

. Above the base speed. This momentary high torque is possible due to the large thermal capacity of the motor with suitable insulators used for the winding. The speed limit basically depends on the mechanical limitation of the motor. the toque is limited by the maximum allowable power. the current cannot be higher than its rated value.Einstein College of Engineering In most cases. The operating area of a 4-quadrant motor drive is shown in Figure 8. must be able to sustain this condition. The converter is normally protected from the over-current condition by the current limiter mechanism within the converter system. The maximum allowable torque during transient of a drive system is determined by the current rating of the converter used whereas the continuous torque limit depends on the current rating of the motor. the torque during this transient conditi on can be up to 3 times the rated torque of the motor and for servo motor. However since the thermal capacity of the converter is small. Consequently. which means that sustained overloads on the motor has to be protected by an additional thermal protection mechani sm. it can be as high as 8 to 10 times the rated value. the current rating of the converter is normally set to equal the maximum allowable motor current and this can be as high as the 3 times the motor rated current. The converter. which depends on whether the transient or continuous torque limit is considered. which conducts the motor current.

the speed will return to the stable equilibrium speed. if the steady-state speed is of stable equilibrium.Einstein College of Engineering Fig. if the speed i s not of the stable equilibrium. It can be shown that the condition for stable equilibrium is: . Limits for torque. The stable equilibrium speed is investigated using steady-state torque. A disturbance in any part of the drive will result in a speed to depart from the steady state speed. speed and power for drive system Steady-state stability The motor will operate at the steady-state speed (point where T1 = Te) provided that the speed is of stable equilibrium. the disturbance will results in the speed to drift away from the equilibrium speed. However.speed characteristics of the load and motor. On the other hand.

varies the armature drop varies proportionally and one can plot the variation of the induced emf E. Knowing the value of E and flux one can determine the value of the speed. With highly saturated machines . Thus the speed of the machine remains more or less constant with load. The flux inside the machine however slightly falls due to the effect of saturation and due to armature reaction. As the armature current Ia. Also knowing the armature current and the flux. The mmf of the field is assumed to be constant. (a). (b)(i). a graph indicating speed as a function of torque or the torque-speed characteristics is plotted Fig. the value of the torque is found out. This procedure is repeated for different values of the assumed armature currents and the values are plotted as in Fig. From these graphs. The variation of these parameters is shown in Fig.Einstein College of Engineering UNIT-II DRIVE MOTOR CHARACTERISTICS Torque speed characteristics of a shunt motor: A constant applied voltage V is assumed across the armature. As seen from the figure the fall in the flux due to load increases the speed due to the fact that the induced emf depends on the product of speed and flux.

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(a) shows the variation of E1. flux.(b) The initial portion of this torquespeed curve is seen to be a rectangular hyperbola and the final portion is nearly a straight line. This may be simply achieved by providing a series stability winding which aids the shunt field mmf. (b)(ii) and hence must be avoided. At large value of the armature currents the useful flux would be less than the no-load magnetization curve for the machine. The speed under light load conditions is many times more than the rated speed of the motor. This type of external characteristics introduces instability during operation Fig. Fig. Load characteristics of a series motor Following the procedure described earlier under shunt motor. as the centrifugal forces acting on the armature and . the torque speed Characteristics of a series motor can also be determined. torque and speed following the above procedure from which the torque-speed characteristics of the series motor for a given applied voltage V can be plotted as shown in Fig. The armature current also happens to be the excitation current of the series field and hence the flux variation resembles the magnetization curve of the machine.Einstein College of Engineering the on-load speed may even slightly increase at over load conditions. This effect gets more pronounced if the machine is designed to have its normal field ampere turns much less than the armature ampere turns. Such high speeds are unsafe. Similarly for small values of the load currents the torque varies as a square of the armature currents as the flux is proportional to armature current in this region. As the magnetic circuit becomes more and more saturated the torque becomes proportional to Ia as flux variation becomes small.

in the modern machines. In order to safeguard the motor and personnel. a 'weak' shunt field is provided . Hence series motors are not recommended for use where there is a possibility of the load becoming zero.Einstein College of Engineering commutator can destroy them giving rise to a catastrophic break down.

say by the failure of the prime mover. value of flux even when the armature current is nearly zero. It is needless to say. The major benefit of the compounding is that the field is strengthened on load. this field should be connected so as to aid the series field. This mode may sometimes result due to the motoring operation of a level-compounded generator. This feature makes a cumulatively compounded machine well suited for . Also. The mmf of the shunt field and series field may oppose each other or they may aid each other. Thus the torque per ampere of the armature current is made high. This way the no-load speed is limited to a safe maximum speed. The first configuration is called differential compounding and is rarely used. differential compounding may result in large negative mmf under overload/starting condition and the machine may start in the reverse direction. Cumulatively compounded motors are very widely used for industrial drives. They lead to unstable operation of the machine unless the armature mmf is small and there is no magnetic saturation. though small. In motors intended for constant speed operation the level of compounding is very low as not to cause any problem.Einstein College of Engineering on series motors to ensure a definite. Load characteristics of a compound motor Two situations arise in the case of compound motors. High degree of compounding will make the machine approach a series machine like characteristics but with a safe no-load speed.

Extremely large brake power is needed under those conditions. Where the safety of the personnel or the equipment is at stake the machine may be required to stop instantly. This reduces the wear and tear of the mechanical brakes and reduces the frequency of the replacement of these parts. it can be seen that all modern machines are compound machines. The difference between them is only in the level of compounding. Due to the rotation of the armature during motoring mode and due to the inertia. Electric braking can help in these situations also. Braking the d. Reverse voltage braking or plugging These are now explained briefly with reference to shunt. In many cases mechanical braking is adopted. Regenerative 3. To improve the throughput in many production processes by reducing the stopping time. a y wheel can be used with such motors with advantage. 4. In many cases electric braking makes more brake power available to the braking process where mechanical brakes are applied. The electric braking may be done for various reasons such as those mentioned below: 1. 5. Motors When a motor is switched off it `coasts' to rest under the action of frictional forces. In and Fig. This is called regeneration. In the case of emergencies to step the machine instantly. To regenerate the electrical power and improve the energy efficiency. 3.c. 2. By recovering the mechanical energy stored in the rotating parts and pumping it into the supply lines the overall energy efficiency is improved. Due to the large speed variation between light load and peak load conditions.Einstein College of Engineering intermittent peak loads. series and compound motors. In processes where frequent starting and stopping is involved the process time requirement can be reduced if braking time is reduced. Due to the reasons provided under shunt and series motors for the provision of an additional series/shunt winding. Dynamic 2. The reduction of the 1. 49 this is done by changing the switch from position 1 to 2. This voltage drives a . To augment the brake power of the mechanical brakes. To save the life of the mechanical brakes. An emf is induced due to the presence of the field and the rotation. The supply to the field should not be removed. the armature continues to rotate. Dynamic braking Shunt machine In dynamic braking the motor is disconnected from the supply and connected to a dynamic braking resistance RDB. Braking is employed when rapid stopping is required.

shows the dynamic braking of a shunt excited motor and the corresponding torque-speed curve. Basically the electric braking involved is fairly simple. The positive torque corresponds to the motoring operation.Einstein College of Engineering current through the braking resistance. Fig. (b) for a particular value of RDB. Braking can be broadly classified into: . The machine acts like a brake. The electric motor can be made to work as a generator by suitable terminal conditions and absorb mechanical energy. torque developed also gets reversed. This converted mechanical power is dissipated/used on the electrical network suitably. Here the machine behaves as a self-excited generator. The direction of this current is opposite to the one which was owing before change in the connection. Process time improves the throughput. Therefore. The torque speed characteristics separate by excited shunt of the machine under dynamic braking mode is as shown in Fig. Below a certain speed the self-excitation collapses and the braking action becomes Zero.

Einstein College of Engineering Figure : Dynamic Braking of a shunt motor .

Rather. A cumulatively connected motor becomes differentially compounded generator and the braking torque generated comes down. Series connection of all the series fields with parallel connection of all the armatures connected across a single dynamic braking resistor is used in that case.Einstein College of Engineering Figure : Dynamic braking of shunt excited shunt machine Series machine In the case of a series machine the excitation current becomes zero as soon as the armature is disconnected from the mains and hence the induced emf also vanishes. When several machines are available at any spot. Compound generators In the case of compound machine. . as in railway locomotives. dynamic braking is feasible. A separately excited shunt field and the armature connected across the braking resistance are used. the motor is made to work like a separately excited machine. the situation is like in a shunt machine. In order to achieve dynamic braking the series field must be isolated and connected to a low voltage high current source to provide the field. It is therefore necessary to reverse the series field if large braking torques are desired.

Increase in speed does not result in braking and the increase in excitation is feasible only over a small range. Hence plugging is also termed as reverse voltage braking. The armature current can be made to reverse for a constant voltage operation by increase in speed/excitation only. Plugging The third method for braking is by plugging. Hence the best method for obtaining the regenerative braking is to operate. The machine instantly comes to rest. This has many advantages over its rotating machine counter part.c. power source. has higher efficiency. Series motors cannot be regeneratively braked as the characteristics do not extend to the second quadrant. Initially the machine is connected to the supply with the switch S in position number 1. shows the method of connection for the plugging of a shunt motor. Thus this type of braking improves the energy efficiency of the machine. requires lesser space. Plugging is a convenient mode for quick reversal of direction of rotation in reversible rives.c. As the voltage is continuously pulled below the value of the induced emf the speed steadily comes down. The variable d. then we have to open the switch S at zero speed. This produces a large negative torque or braking torque. during . shown schematically in Fig. Braking torque can be obtained right up to zero speed. voltage.c.Einstein College of Engineering Regenerative braking In regenerative braking as the name suggests the energy recovered from the rotating masses is fed back into the d. supply voltage can be obtained by Ward-Leonard arrangement. In modern times static Ward-Leonard scheme is used for getting the variable d. the machine on a variable voltage supply. This type of braking therefore has two modes viz. Bidirectional power flow capacity is a must if regenerative braking is required. and silent in operation. . which may be of the order of 10 to 15%. The field current is held constant by means of separate excitation. Fig. Static set is compact. unidirectional power flow and low over load capacity. however it suffers from drawbacks like large ripple at low voltage levels. The induced armature voltage E and supply voltage V aid each other and a large reverse current flows through the armature. If the motor is not switched off at this instant the direction of rotation reverses and the motor starts rotating the reverse direction. then a reverse voltage is applied across the armature. If we need the plugging only for bringing the speed to zero. If now the switch is moved to position 2. 1) plug to reverse and 2) plug to stop. Just as in starting. If nothing is done it is plug to reverse mode.

Einstein College of Engineering Figure Regenerative braking of a shunt machine .

Einstein College of Engineering Figure : Plugging or reverse voltage braking of a shunt motor .

We will Consider the approximate equivalent circuit of the machine. The torque available can be obtained by dividing this number by the shaft speed. In such cases the braking proceeds just as in a shunt motor. Series motors In the case of series motors plugging cannot be employed as the field current too gets reversed when reverse voltage is applied across the machine. the current drawn by the circuit is given by . If plugging is done to operate the motor in the negative direction of rotation as well. We will begin by looking at the speed-torque characteristic of the machine.Einstein College of Engineering Plugging also it is necessary to limit the current and thus the torque. in operating a d. supply. A cumulatively compounded motor becomes differentially compounded on plugging. Deducing the machine performance. This decreases the braking torque. We have reasoned earlier that the power consumed by the 'rotor-portion' of the equivalent circuit is the power transferred across the air-gap. to reduce the stress on the mechanical system and the commutator. this is the power available at the shaft. or a. Out of that quantity the amount dissipated in R0 r is the rotor copper loss and the quantity consumed by R0r(1 + s)=s is the mechanical power developed. Neglecting mechanical losses. Unlike dynamic braking and regenerative braking where the motor is made to work as a generator during braking period. plugging makes the motor work on reverse motoring mode. (Single phase Induction motor) From the equivalent circuit. This keeps the direction of the torque produced unchanged. many aspects of the steady state behavior of the machine can be deduced.c. then the series field has to be reversed and connected for getting the proper mmf. Recalling that the equivalent circuit is the per-phase representation of the machine.c.c. The mmf due to the series field can 'over power' the shunt field forcing the flux to low values or even reverse the net field. This fact is used with advantage. However some precautions have to be observed due to the presence of series field winding. Compound motors Plugging of compound motors proceeds on similar lines as the shunt motors. This is done by adding additional resistance in series with the armature during plugging. The complete torque-speed characteristic of Induction motor In order to estimate the speed torque characteristic let us suppose that a sinusoidal voltage is impressed on the machine. series motor on d. Series motors thus qualify to be called as `Universal motors'. and increases the duration of the large braking current. To avoid this it may be advisable to deactivate the series field at the time of braking by short-circuiting the same.

we need to look at the torque for a given speed after all transients have died down. scilab available free under GNU at the time of this writing) may find out the difference caused by using the `exact' equivalent circuit by using the script found here. with a rated speed of 1440 rpm. this curve is obtained by varying slip with the applied voltage being held constant. This plot corresponds to a 3 kW. In general. One cannot. 50 machine. A typical torque-speed characteristic is shown in _g. We can see that the slope and shape of the characteristics are dependent intimately on the machine parameters. Readers with access to MATLAB or suitable equivalents (octave. this is the torque produced per phase.Thus we have. The torque is obtained by dividing this by the shaft speed . it implies that if this characteristic is to be measured experimentally. The plots correspond to a 3 kW. this expression should be multiplied by p. A comparison between the two is found in the plot of fig. Hence the overall torque is given by The torque may be plotted as a function of `s' and is called the torque-slip (or torquespeed. since slip indicates speed) characteristic | a very important characteristic of the induction machine.60 Hz machine. 4 pole. It can be seen that the approximate equivalent circuit is a good approximation in the operating speed range of the machine. Further. Further. for example. Since this current is owing through .Einstein College of Engineering Where Vs is the phase voltage phasor and Is is the current phasor. Comparing the two figures. the number of pole-pairs. 4 pole. The magnetizing current is neglected. 22. Coupled with the fact that this is an equivalent circuit valid under steady state. the air-gap power is given by The mechanical power output was shown to be (1_s) Pg (power dissipated in R0r=s). Equation 16 is valid for a two-pole (one pole pair) machine. Where! S is the synchronous speed in radians per second and s is the slip. We must note that the approximate equivalent circuit was used in deriving this relation. try . The rated operating speed is 1780 rpm.

A typical curve is shown in fig for a four-pole machine. the procedure is not as straightforward. the rotor maybe driven to higher speeds by a prime mover or may also be rotated in the reverse direction. Nm to obtain this curve by directly starting the motor with full voltage applied to the terminals and measuring the torque and speed dynamically as it runs up to steady speed. Note that negative speeds correspond to slip values greater than 1. With respect to the direction of rotation of the air-gap flux. The plot also shows the operating modes of the induction machine in various regions.Einstein College of Engineering Torque. With non-sinusoidal voltage waveforms. . the synchronous speed being 1500 rpm. and speeds greater than 1500 rpm correspond to negative slip. Another point to note is that the equivalent circuit and the values of torque predicted is valid when the applied voltage waveform is sinusoidal. The slip axis is also shown for convenience. The torque-speed relation for the machine under the entire speed range is called the complete speed-torque characteristic.

38. the maximum torque remaining constant all the while. The torque at synchronous speed is therefore zero. which for the machine of fig is 0. . But this is a subject to be discussed later. The value of the stalling torque may be obtained by differentiating the expression for torque with respect to zero and setting it to zero to find the value of ^s. At values of slip lower than ^s. The expression shows that ^ Te is the independent of R0 r. and is called as stalling torque. Using this method. If it is possible to change R0 r. At values of slip higher than s = ^s. while ^s is directly proportional to R0 r. the curve falls slowly to a minimum value at s = 1. The torque at s = 1 (speed = 0) is called the starting torque. we see that the curve has a peak point.Einstein College of Engineering Restricting ourselves to positive values of slip. then we can get a whole series of torque-speed characteristics. This is the maximum torque that the machine can produce. If the load torque is more than this value. the curve falls steeply down to zero at s = 0. It occurs at a slip ^s. Substituting ^s into the expression for torque gives us the value of the stalling torque ^ T the negative sign being valid for negative slip. This fact can be made use of conveniently to alter ^s. the machine stops rotating or stalls.

This would shift the point of operation to a point 10 at which the slip would be less and the developed torque higher. disturbances at point 2 will not cause a runaway effect. In other words. Any overshoot in speed as it approaches the point 10 will cause it to further accelerate since the developed torque is increasing..Einstein College of Engineering We may note that if R is chosen equal to becomes unity. the characteristics are never fixed. For an induction machine. The difference in torque-developed 4Te. (generator mode) we note that the maximum torque is higher than in the positive slip region (motoring mode). they change slightly with time. where the change is exaggerated for clarity. Therefore we conclude that point 2 is a stable operating point.e. The load is a constant torque load i. Let us consider the point 2. the slip is now higher (speed is lower) and the positive difference in torque will accelerate the machine. Therefore we may conclude that 1 is not a stable operating point. the torque required for operation is fixed irrespective of speed. we note that there are two such points. Similar arguments may be given for the case where the load characteristic shifts down. This being the case let us considers that the system is operating at point 1. and the load torque demand increases slightly. This is shown in fig. in practice. wherever possible can serve as a means to control the starting torque. To answer this we must note that. From the above plot. The system consisting of the motor and load will operate at a point where the two characteristics meet. While considering the negative slip range. It would be appropriate to consider a small band around the curve drawn where the actual points of the characteristic will lie. . We therefore need to find out which of these is the actual operating point. Thus changing of R r. This behavior will tend to bring the operating point towards 2 once again. which p means that the maximum torque occurs at starting. having the load characteristic also superimposed on it. Operating Point Consider a speed torque characteristic shown in fig. Similar arguments may be used to show that if for some reason the developed torque becomes smaller the speed would drop and the effect is cumulative. If this point shifts to 20. being positive will accelerate the machine.

When the rotating magnetic field is set up due to stator currents. Nm From the foregoing discussions.Einstein College of Engineering torque. Therefore the machine will always operate between s = 0 and s = ^s. A little reaction on the definition of the slip indicates that we are at the point s = 1. while the region from s = ^s to s = 0 is a stable region. we can say that the entire region of the speed-torque characteristic from s = 0 to s = ^s is an unstable region. Modes of Operation The reader is referred to fig which shows the complete speed-torque characteristic of the induction machine along with the various regions of operation. Let us consider a situation where the machine has just been excited with three phase supply and the rotor has not yet started moving. it is the induced emf that causes current in the rotor. It has already been pointed out that it is the presence of the non-zero slip that causes a torque to be developed. and the interaction between the two causes torque. Thus the region of the .

Suppose when the rotor is rotating. This would cause the rotating stator field to reverse its direction | the rotating stator mmf and the rotor are now moving in opposite directions. Further. or by wind blowing. In order to do this it has to contend with an opposing torque generated by the machine preventing the speed build up. Consider a situation where the induction machine is operating from mains and is driving an active load (a load capable of producing rotation by itself). There is yet another situation. we change the phase sequence of excitation to the machine. the wind is rotating the rotor to a speed higher than what the electrical supply alone would cause. Note further that the direction of rotation of the rotor is the same as that of the air gap flux. it would now tend to slow down. If we adopt the convention that positive direction is the direction of the air gap flux. Further suppose that both acting independently cause rotation in the same direction. Further.Einstein College of Engineering curve between Figure : Stability of operating point s = 0 and s = 1 is the region where the machine produces torque to rotate a passive load and hence is called the motoring region. a strong wind may cause the rotor to rotate faster than the mmf produced by the stator excitation. where the fan like blades of the windmill are connected to the shaft of the induction machine. Therefore this region (s > 1) is called the braking region. A typical example is that of a windmill. The torque generated is therefore negative. Rotation of the blades may be caused by the motoring action of the machine. A little reaction shows that slip is then negative. the rotor speed would then be a negative quantity. (What would happen if the supply is not cut-off when the speed reaches zero?) . Indeed this is at present the most commonly used approach in wind-energy . The slip would be a number greater than unity. The region of slip s > 1 is the generating mode of operation. and reach zero speed. the rotor as we know should be "dragged along" by the stator field. Since the rotor is rotating in the opposite direction to that of the field. Now when both grid and windact. It is this action of the wind against the torque of the machine that enables wind-energy generation.

E stored in the rotor is either dissipated in an external resistance or fed back to the supply or both. This is a good method no doubt. by forcing the motor to operate as a generator during braking. but will take longer time particularly for large motors having high rotational inertia. S is a single pole double throw switch and can be connected either to position 1 or to position 2. Therefore.Einstein College of Engineering generation. one at the wheel-brake shoe interface (where most of the energy is dissipated) and the other at the road-tier interface.c shunt motor operating from a d. Rheostatic braking Consider a d. as usual for motor mode in .E now gets two ways of getting dissipated. a torque opposite to the direction of rotation will be imposed on the shaft. We know that any moving or rotating object acquires kinetic energy. The idea can be understood remembering that in motor mode electromagnetic torque acts along the direction of rotation while in generator the electromagnetic torque acts in the opposite direction of rotation. drawing an armature current Ia and the back emf is Note the polarity of Eb which. it will eventually come to a stop eventually after moving quite some distance. Braking of d. thereby helping the machine to come to stop quickly.c shunt motor: basic idea It is often necessary in many applications to stop a running motor rather quickly. During braking action. The initial kinetic energy stored. Let with S in position 1. but regular maintenance of brake shoes due to wear and tear is necessary. the initial K. Thus stored K. However. how fast we can bring the object to rest will depend essentially upon how quickly we can extract its kinetic energy and make arrangement to dissipate that energy somewhere else. The situation can be improved. Because here the stored energy has to dissipate mainly through bearing friction and wind friction. brake is applied with the help of rubber brake shoes on the rim of the wheels.c supply with the switch S connected to position 1 as shown in figure. One end of an external resistance Rb is connected to position 2 of the switch S as shown. to make the stopping faster. It may be noted from the torque expression of equation that torque is negative for negative values of slip. motor runs at n rpm. If you stop pedaling your bicycle. Thus by forcing the machine to operate as generator during the braking period. If a motor is simply disconnected from supply it will eventually come to stop no doubt. in this case dissipates as heat in the friction of the road.

the machine is operating as generator dissipating power to Rb and now the electromagnetic torque Te must act in the opposite direction to that of n since Ia has changed direction but has not As time passes after switching. armature current will be Ia = Eb/(ra + Rb) and with reversed direction compared to direction prevailing during motor mode at t = 0-.25 and 39. Also note Te and n have same clockwise direction. Plugging or dynamic braking This method of braking can be understood by referring to figures 39. the armature gets disconnected from the supply and terminated by Rb with field coil remains energized from the supply. Thus at t = 0+. Obviously for t > 0. Here S is a double pole double throw switch. Now if S is suddenly thrown to position 2 at t = 0.E and as a consequence both Eb and Ia decrease. In other words value of braking torque will be highest at t = 0+. . S is connected to positions 1 and 1'.Einstein College of Engineering opposition with the supply voltage. n decreases reducing K. Since speed of the rotor can not change instantaneously.26. For usual motoring mode. and it decreases progressively and becoming zero when the machine finally come to a stop. the back emf value Eb is still maintained with same polarity prevailing at t = 0-.

However. if S continuous to be in position 1' and 2' even after zero speed has been attained? The answer is rather simple. Ia can not become zero at any time due to presence of supply V. a series combination of an external resistance Rb and supply voltage with polarity as indicated is connected. the machine will start picking up speed in . Here also Ia reverses direction-producing Te in opposite direction to n. substantial magnitude of braking torque prevails. So unlike rheostatic braking. Ia decreases as Eb decreases with time as speed decreases. thereby disconnecting the armature from the left hand supply. the switch is thrown to position 2 and 2' at t = 0. Here at t = 0+. To initiate braking. Hence stopping of the motor is expected to be much faster then rheostatic breaking. However. during motor mode this part of the circuit remains inactive.Einstein College of Engineering Across terminals 2 and 2'. But what happens. the armature current will be Ia = (Eb + V)/(ra + Rb) as Eb and the right hand supply voltage have additive polarities by virtue of the connection.

when the track has a downward gradient (shown in figure 39. Obviously under this condition the direction of Ia will reverse imposing torque which is opposite to the direction of rotation. Imagine the d.28). Since The question is how speed on its own become large enough to make Eb < V causing regenerative braking.Einstein College of Engineering the reverse direction operating as a motor.27 where armature motoring current Ia is drawn from the supply and as usual Eb < V. Eb > V. So care should be taken to disconnect the right hand supply. The normal motor operation is shown in figure 39. The situation is explained in figures 39.27 and 39. Regenerative braking here will not stop the motor but will help to arrest rise of dangerously high speed. the moment armature speed becomes zero.. component of gravitational force along the track also appears which will try to accelerate the motor and may increase its speed to n2 such that Eb In such a scenario. Machine is running as a motor at a speed of n1 rpm. direction of Ia reverses. Such a situation may occur in practice when the mechanical load itself becomes active.c motor is coupled to the wheel of locomotive which is moving along a plain track without any gradient as shown in figure. feeding power back to supply.28. However. .e. Regenerative braking A machine operating as motor may go into regenerative braking mode if its speed becomes sufficiently high so as to make back emf greater than the supply voltage i.

the torque developed by the motor at zero speed must exceed that demanded by the load. machine is such as to cause 1 to 5 percent drop at full load current. The starting is now discussed with respect to specific machines. The induced emf at starting point is zero as the i = 0 The armature current with rated applied voltage is given by V=Ra where Ra is armature circuit resistance. The same can be told of the torque if full flux is already established. DC shunt motor . This continues till the load torque and the motor torque are equal to each other. Then TM _ TL will be positive so also is di=dt. Normally the armature resistance of a d. Machine tends to run continuously at this speed. Hence the starting current tends to rise to several times the full load current. The machine instantly picks up the speed. so also the torque.Einstein College of Engineering UNIT-III STARTING METHODS STARTING OF D. As the speed increases the induced emf appears across the terminals opposing the applied voltage. and the machine accelerates.C. MACHINES: For the machine to start. as the acceleration is zero at this point of operation. The current drawn from the mains thus decreases.c.

shunt motor are energized together.Einstein College of Engineering If armature and field of d. The current and the torque get reduced.C motor is attempted to start with full voltage. This has to be done progressively. Thus the level of high starting current may be maintained for quite some time so as to cause serious damage to the brush/ commutator and to the armature winding. Also the source should be capable of supplying this burst of large current. rotor speed n = 0. Better starting torques are obtained for compound motors as the torque per ampere is more. The torque speed curve under these conditions is shown in Fig.c.c motor is given by At the instant of starting. (a). large current is drawn at start but the torque builds up gradually as the field flux increases gradually. would experience a dip in the terminal voltage. hence starting armature current is Since. characterized by large rotor inertia (J). which is used. will pick up speed rather slowly. at start. Problems of starting with full voltage We know armature current in a d. A large machine. This arrangement is called Ward-Leonard arrangement. To improve the torque per ampere of line current drawn it is advisable to energize the field first. the starting resistance wastes large amount of power. Characteristics for series motors are given in fig. for starting the motor. Grading of starting resistance for a shunt motor If the starting resistor is reduced in uniform steps then the current peaks reached as we cut down the resistances progressively increase. The other loads already connected to the same source. armature resistance is quite small. A schematic diagram of WardLeonard arrangement is shown in Fig. the voltage V can be reduced or armature circuit resistance Ra can be increased. The starting current is given by V=Ra and hence to reduce the starting current to a safe value. By controlling the field of the Ward-Leonard generator one can get a variable voltage at its terminals. It can be readily seen from this graph that the unloaded machine reaches its final speed but a loaded machine may crawl at a speed much below the normal speed. This dip in supply voltage is caused due to sudden rise in voltage drop in the source's internal resistance. To ascertain that at no step does Starting of D. every time a D.C shunt motor 1. Also. making sure that the current does not jump up to large values. Hence the starting resistance must be reduced to zero at the end of the starting process. The duration for which this drop in voltage will persist once again . starting current may be quite high (many times larger than the rated current). The second method of starting with increased armature circuit resistance can be obtained by adding additional resistances in series with the armature. Variable voltage V can be obtained from a motor generator set. Starting of series motor and compound motors are similar to the shunt motor.

for small D. the basic working principle is same as that of plain resistance starter The starter is shown enclosed within the dotted rectangular box having .C motors extra precaution may not be necessary during starting as large starting current will very quickly die down because of fast rise in the back emf. a starter is to be used during starting. Now if the supply goes off (due to some problem in the supply side or due to load shedding). let us imagine. 3. Although. to have sufficient starting torque. the motor to be running with Rext = 0 (Figure (b)). A simple starter To limit the starting current.Einstein College of Engineering depends on inertia (size) of the motor. for large motor. It not only overcomes the difficulty of a plain resistance starter. But each time one has to restart the motor.C shunt motor. the external armature resistance must be set to maximum value by moving the jockey manually. All on a sudden. This is then nothing but full voltage starting. 3-point starter A “3-point starter” is extensively used to start a D. one should be constantly alert to set the resistance to maximum value whenever the motor comes to a stop. This is one major limitation of a simple rheostatic starter. supply is restored. In other words. However. As the motor picks up speed. the value of Rext is gradually decreased to zero so that during running no external resistance remains in the armature circuit. a suitable external resistance Rext is connected in series (Figure (a)) with the armature so that At the time of starting. 2. to zero value. field current is maximized by keeping the external field resistance Rf. the circuit looks a bit clumsy at a first glance. Imagine. but also provides additional protective features such as over load protection and no volt protection. The diagram of a 3-point starter connected to a shunt motor is shown in figure . Hence. motor will come to a stop.

2 etc. (the function of which we shall discuss a little later) and enters through the handle shown by dashed lines. The spring tension keeps the handle in the OFF position when no one attempts to move it. The junctions of the resistances are brought out as terminals (called studs) and marked as 1. The other end of this strip is brought as terminal F after going through an electromagnet coil (called NVRC). Just beneath the resistances. Terminal F is connected to one field terminal F1 of the motor and terminal L is connected to one supply terminal as shown. the rectangular strip will make contact with the continuous arc copper strip.12. a continuous copper strip also in the form of an arc is present. F2 terminal of field coil is connected to A2 through an external variable field resistance and the common point connected to supply (-ve). L and F for external connections. . progressively. the circular strip of the handle will make contacts with resistance terminals 1. Terminal A is connected to one armature terminal Al of the motor. On the other hand. The wire from L passes through a small electro magnet called OLRC.2.. Now let us trace the circuit from terminal L (supply + ve). Near the end of the handle two copper strips are firmly connected with the wire. Terminal F is finally connected to motor field terminal Fl. There is a handle which can be moved in the clockwise direction against the spring tension.. . The external armatures resistances consist of several resistances connected in series and are shown in the form of an arc. The furthest strip is shown circular shaped and the other strip is shown to be rectangular. When the handle is moved to the right.Einstein College of Engineering three terminals marked as A.

The other electromagnet. OLRC overload release coil along with a soft iron piece kept under it. all resistances will be left out when stud number 12 (ON) is reached. Carefully note that B and C are nothing but the two ends of the NVRC. In this position armature and all the resistances in series gets connected to the supply. Even if the operator removes his hand from the handle. In this position the external resistance in the armature circuit is less as the first resistance is left out. The current flowing through OLRC is the line current IL drawn by the motor. Therefore.. Restoring force of the spring will bring the handle back in the OFF position. Field coil gets full supply as the rectangular strip makes contact with arc copper strip. the electromagnet (NVRC) will attract the soft iron piece attached to the handle. The moment it gets deenergised. Neither armature nor the field of the motor gets supply. To the iron a copper strip (Ä shaped in figure) is attached. Hence NVRC will not carry any current now and gets deenergised. Three-point starter has one disadvantage. In this position. The starter also provides over load protection for the motor. Initially the handle is in the OFF position. However. if rated I I force of attraction will be sufficient to pull up iron piece. field coil current will decrease to zero. During over loading condition. Ia hence IL increases. continues to get full voltage by virtue of the continuous arc strip. Suppose we want that the motor should not be over loaded beyond rated current. spring action will bring the handle in the OFF position thereby disconnecting the motor from the supply. Now the handle is moved to stud number 1. by reducing field current). As the machine picks up speed handle is moved further r to stud number 2. it will still remain in the ON position as spring restoring force will be balanced by the force of attraction between NVRC and the soft iron piece of the handle. As the motor is loaded.Working principle Let us explain the operation of the starter. this copper strip will also move up and put a short circuit between two terminals B and C. Continuing in this way. when over load occurs a short circuit path is created across the NVRC. Hence NVRC will be deenergised and will not be able to exert any force on the soft iron piece of the handle.Einstein College of Engineering 4. This upward movement of the iron piece of OLRC is utilized to de-energize NVRC. DIFFERENT TYPES OF STARTERS FOR 3 PHASE INDUCTION MOTOR (IM) INSTRUCTIONAL OBJECTIVES . Now gap between the electromagnet and the soft iron piece is so adjusted that for the iron piece will not be pulled up. Thus we find that a false disconnection of the motor takes place even when there is neither over load nor any sudden disruption of supply.e. In case supply voltage goes off. IL is a measure of loading of the motor. If we want to run the machine at higher speed (above rated speed) by field weakening (i. the strength of NVRC magnet may become so weak that it will fail to hold the handle in the ON position and the spring action will bring it back in the OFF position. Field however. is used to achieve this. In other words. The no volt release coil (NVRC) carries same current as that of the field coil.

starting with the need for using starters in IM to reduce the starting current. starting current. which means that the rated voltage is supplied to the stator. with the variations in input (stator) voltage and rotor resistance. in IM has been derived first. the expression of maximum torque developed and also the slip. the starting current in the stator windings . and also the starting torque developed using the above starters. first two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage IM and then. along with the explanation of their features. are presented along with the starting current drawn from the input (supply) voltage. i. where it occurs. need for starters. Neglecting stator impedance. In this lesson. are presented. starting torque. Star-delta starter. have been derived. the expression of gross torque developed. auto-transformer starter. the starter using additional resistance in rotor circuit. lesson of this module. starters for squirrel cage and wound rotor induction motor. fourth.e. for Wound rotor (Slip-ring) IM. For the cage rotor. Lastly. rotor resistance starter. as a function of slip (speed). the rotor bars are short circuited via two end rings. The sketches of the different torque-slip (speed) characteristics. Keywords: Direct-on-Line (DOL) starter. for Wound rotor (Slip-ring) Induction motor Introduction In the previous. with the rotor terminals short-circuited in a wound rotor (slip-ring) motor. Direct-on-Line (DOL) Starters Induction motors can be started Direct-on-Line (DOL).Einstein College of Engineering • Need of using starters for Induction motor • Two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage Induction motor • Starter using additional resistance in rotor circuit.

as compared to no load current. as the starting current increases. In the formula for starting current. depending on the rating of IM. the starting torque also increases. no load current is neglected. with the motor reaching rated or near rated speed quickly. about 4-6 times the current at full load. may be higher.Einstein College of Engineering The input voltage per phase to the stator is equal to the induced emf per phase in the stator winding. The starting torque is which shows that. as the stator impedance is neglected (also shown in the last lesson (#32)). This results in higher accelerating torque (minus the load torque and the torque component of the losses). It may be noted that the starting current is quite high. Need for Starters in IM .

). which can be started DOL. i. the supply authorities set a limit on the rating or size of IM. for which a starter is to be used to reduce the current drawn at starting. which is higher than the current for which this line is designed. to as the voltage drop in the stator impedance is small compared to the input voltage. and those. This will cause a drop (dip) in the voltage. is not permitted to be started DOL. Only for the current lower the current for which the line is designed.e. in a (squirrel) cage induction motor. Any motor exceeding the specified rating.e. i. . So. where the supply voltage is constant. or if the stator impedance is neglected. So. both for the consumers between the substation and this consumer. who are in the line after this consumer. because the current drawn is more than the current for which the line is designed. drawing a high current from the line. Starters for Cage IM The starting current in IM is proportional to the input voltage per phase the motor (stator). This has been shown earlier. As described later. where.Einstein College of Engineering The main problem in starting induction motors having large or medium size lies mainly in the requirement of high starting current. of which one consumer has an induction motor with a DOL starter. . The line feeds a no. of consumers. this also results in decrease of starting torque. when started direct-on-line (DOL). higher than the limit as per ISS. the starter is used only to decrease the input voltage to the motor so as to decrease the starting current. This drop in the voltage is more than the drop permitted. all along the line. Assume that the distribution line is starting from a substation (Fig. the drop in voltage is lower the limit.

Einstein College of Engineering .

the starting current using star-delta starter is reduced by 33. The advantage is that. Alternatively. the voltage per phase supplied to each winding is reduced by )1/3(. 33. 33.3%. 33. if the motor is started direct-on-line (DOL). can be found by using star- . The line current or the input current to the motor is which is the current. as the ratio of the two currents is ( 3 / 1 ). which is a disadvantage of the use of this starter. If the above winding is reconnected as star (Fig.577). where is the impedance of the motor per phase at standstill or start (stator impedance and rotor impedance referred to the stator. at standstill). thus making it simple.3%. except that shown in Fig. the stator winding of which is nominally delta-connected (Fig. which can be easily reconfigured as shown in Fig. As for starting torque.Einstein College of Engineering This type is used for the induction motor. The voltage per phase in each stator winding is now (. is also reduced by ( 2 ) 3 / 1 ( 3 / 1 = ). same as that (ratio) of the voltages applied to each winding as shown earlier. 3 / s V ). As the voltage per phase in delta connection is V s. Now.2b). no extra component. if the motor is to be started using this starter. must be lower than the starting torque. So. This is a simple starter. being proportional to the square of the current in each of the stator windings in two different connections as shown earlier. the phase or line current drawn from supply at start (standstill) is which is of the starting current. The load torque and the loss torque. the phase current in each stator winding is .2c. the starting current in the second case with the stator winding reconnected as star. As shown later.2c. need be used. if DOL starter is used. 33. the starting torque is reduced by 33. this is an autotransformer starter with the voltage ratio as 57.7%. So.2a). if the stator winding is connected as star.

with the impedance per phase after converting to delta.Einstein College of Engineering delta conversion as given in lesson #18. and the starting current now being reduced to (1/3 ) of the starting current obtained using DOL starter. found as ( s Z · 3 ). Auto-transformer Starter . with the stator winding connected in delta.

If %) 70 ( 7 . The output voltage/input voltage ratio is x . is also reduced by . which is nearly 2 times the values obtained using star-delta starter. 0 ( 2 2 = = x times the values of starting current and torque with DOL starting. %) 7 . both starting current and torque are %) 80 ( 8 . both starting current and .e applying rated input voltage. So. neglecting losses and assuming nearly same power factor on both sides. 0 ) 8 . such that the motor can start against higher load torque.0 and 1. the value of which lies between 0. being proportional to the square of the input current to IM in two cases. i. which is the output voltage of the auto-transformer. The input current of auto-transformer.3). with the result that lower rated motor can now be started. which is the starting current drawn from the supply. So. is used to start the induction motor. the starting torque. If the ratio is . while the advantage is that the starting torque is now doubled. when this starter is used with input voltage as . with and without autotransformer (i.e. the disadvantage is that starting current is increased. 57 ( 577 . 0 = x . the starting torque is reduced by the same ratio as that of the starting current.0 Let be the starting current. the input voltage being V s. as the ratio of the two currents is same as that (ratio) of the voltages applied to the motor as shown earlier. which is the output current of auto-transformer. direct). The input voltage of IM is . when the motor is started using DOL starter. The input current of IM. 0 = x . 0 = x %) 64 ( 64 . is. obtained by equating input and output volt-amperes. The star-delta starter can be considered equivalent to an autotransformer starter with the ratio. as the current drawn from the supply is to be kept within limits. 33. whose output is fed to the stator and input is from the supply (Fig. As discussed earlier.Einstein College of Engineering An auto-transformer.

0). Though the starting current decreases.e. being rated for intermittent duty. 32. i. for increasing values of the resistance in the rotor circuit. Finally. thus leaving the slip-rings short-circuited. The starting current in the rotor winding is where = Additional resistance per phase in the rotor circuit.2b.5 times the values obtained using star delta starter. also the additional cost of the external resistance with intermittent rating is to be incurred. So. This is only used in case higher starting torque is needed to . 33. having same power rating. along with increase of starting torque. the resistance is to be decreased in steps. as the total resistance in the rotor circuit is increased. thus resulting in increase of starting torque as shown in Fig. The input (stator) current is proportional to the rotor current as shown earlier. If the additional resistance is used only for starting. Rotor Resistance Starters for Slip-ring (wound rotor) IM In a slip-ring (wound rotor) induction motor. which is nearly 1. The starting current (input) reduces. in both cases. which results in decrease of starting current.Einstein College of Engineering torque are times the values of starting current and torque with DOL starting. Also it may be noted that the cost of a slip-ring induction is higher than that of IM with cage rotor. But the starting torque increases. so as to increase the starting torque. as resistance is inserted in the rotor circuit. as the motor speed increases. resistance can be inserted in the rotor circuit via slip rings (Fig. both being advantageous. additional cost is to be incurred to obtain the above advantages. Here.4). the external resistance is to be completely cut out. to be made equal to zero (0. and also obtained by using the expression given earlier. the total resistance increases.

because the This is. the various types of single-phase induction motors.e. has been described first.2b that the starting torque increases till it reaches maximum value. for cage type IM. the rotor resistance starter for slip-ring (wound rotor) IM has been discussed. In this lesson . i. like the addition of another winding in the stator.Einstein College of Engineering start IM with high load torque. even if the total resistance in the rotor circuit is increased. The starting if the external resistance the external resistance in the if the external resistance in the rotor circuit is increased the starting torque decreases.0) and torque is equal to the maximum value. the rotor resistance starter for slip-ring (wound rotor) IM is briefly described. used for cage rotor IM. and autotransformer. has been described first. along with the need for other types of starters.star-delta. Introduction In the previous. the range of total resistance being The range of external resistance is between zero (0. the direct-on-line (DOL) starter used in three-phase IM. along with the need for starters. 2 r x -). i.star-delta and autotransformer. where starting current decreases along with increase in starting torque. fifth. It may be observed from Fig. because the starting current decreases at a faster rate. 32. for motors with nominally deltaconnected stator winding. the direct-on-line (DOL) starter used for IM. are then presented. with the introduction of additional features. In all such . will be presented. Lastly. Then. where both decrease in starting current and torque occur. are presented. In the next (sixth and last) lesson of this module. two types of starters .e. Lastly. STARTING METHODS FOR SINGLE-PHASE INDUCTION MOTOR Instructional Objectives • Why there is no starting torque in a single-phase induction motor with one (main) winding in the stator? • Various starting methods used in the single-phase induction motors. Two types of starters . along with the starting methods. inserted is equal to further. This is. lesson of this module. and/or capacitor in series with it. rotor circuit is increased.the fifth one of this module.

which is lower than its synchronous speed. firstly it is shown that there is no starting torque in a single-phase induction motor with only one (main) winding in the stator. say. unlike a three-phase one. motor with capacitor start/run. starting torque. the addition of another (auxiliary) winding in the stator. split-phase. the flux in the air gap is alternating only. Single-phase Induction Motor The winding used normally in the stator (Fig. However. The rotor is of squirrel cage type.or three-) winding in the stator of IM. and/or capacitor in series with it. unlike that for a threephase IM.Einstein College of Engineering cases. Then. the various starting methods used for such motors. capacitor type. a single-phase IM is not self-starting. Double field revolving theory . This type of alternating field cannot produce a torque if the rotor is stationery so. As the stator winding is fed from a single-phase supply. In the last (sixth) lesson of this module. not a synchronously rotating one produced by a polyphase (may be two. if the rotor is initially given some torque in either direction then immediately a torque is produced in the motor. starting methods.) of the single-phase induction motor (IM) is a distributed one. main and auxiliary windings. like. as the rating of this type of motor is low. The motor then accelerates to its final speed. as shown later. additional cost is to be incurred. Keywords: Single-phase induction motor. This is now explained using double field revolving theory. which is a cheap one.

as the time axis (angle) is changing from Fig. 34. in opposite directions.Einstein College of Engineering When the stator winding (distributed one as stated earlier) carries a sinusoidal current (being fed from a single-phase supply). a sinusoidal space distributed mmf. whose peak or maximum value pulsates (alternates) with time. . is produced in the air gap. This is shown in Fig.shows the alternating or pulsating flux (resultant) varying with time or angle. This sinusoidally varying flux is the sum of two rotating fluxes or fields.1a (i-iv)) show the resultant sum of the two rotating fluxes or fields. The first set of figures (Fig. the magnitude of which is equal to half the value of the alternating flux and both the fluxes rotating synchronously at the speed.

0 (zero). taken as negative. with bars short circuited via end rings. The other part of flux or field rotates at the same speed in the opposite (clockwise) direction. which is termed as positive (+ve). say in the anticlockwise (forward) direction. 34. So. The motor speed is decided by the load torque supplied. say. there is no starting torque in a singlephase IM. and the electromagnetic torque is produced in the same direction as given above. The resultant torque is thus positive as the motor rotates in the forward direction. in the anticlockwise direction. if the motor (rotor) is started or rotated somehow. with the forward torque being more than the backward torque. The motor accelerates in the forward direction. The two torques are equal and opposite. with the resultant torque now being positive. is described as (space . same as that of the direction of rotation of this field. Similarly.Einstein College of Engineering The flux or field rotating at synchronous speed. the same direction.3). the slip due to backward rotating field is also sb = 0 . The current flows in the rotor conductors. the torque produced by this field is negative (-ve). the mmf. The rotor is a squirrel cage one. 1 = sf . as that of the motor (rotor) taken as positive induces emf (voltage) in the rotor conductors. and the resultant (total) torque is the difference of the two torques produced (Fig. So. the forward torque is more than the backward torque.1. But. including the losses (specially mechanical loss). If the rotor is stationary the slip due to forward (anticlockwise) rotating field is 0 .e. which is distributed sinusoidally in space. as it is in the clockwise direction. Mathematically. and the resultant torque is 0. Two torques are in the opposite direction. with its peak value pulsating with time. i.

The resultant torque is. The . is distributed both in space and time. with both as resultant torque at start is 0. Similarly. the the slip is. the speed of which is fields are in opposite direction. as in a three-phase IM. So.Einstein College of Engineering angle) measured from the winding axis. but has resultant torque. It was earlier shown that. When the motor rotates in the forward (anticlockwise) direction with angular speed the slip due to the forward rotating field is. So. The torques produced by the two STARTING METHODS The single-phase IM has no starting torque. in addition to the main winding. with two voltages equal in magnitude. but placed at a space angle of 90o(electrical).e. in a balanced two-phase IM having two windings. starting torque is produced. and are fed from a balanced two-phase supply. in a single-phase IM. having both starting and also running torque as shown earlier. each having equal number of turns and placed at a space angle of 90o(electrical). i. The torque-speed characteristic is same as that of a three-phase one. It is also known that. at an angle of 90o.0 (zero). backward rotating field is. the mmf which shows that a pulsating field can be considered as the sum of two synchronously rotating fields and the The forward rotating field is. Now. Both the fields have the same amplitude equal to where is the maximum value of the pulsating mmf along the axis of the winding. This can be expressed as. when it rotates at any other speed. except synchronous speed. Therefore. the rotating magnetic fields are produced. due to the backward rotating field. if an auxiliary winding is introduced in the stator. when the rotor is stationary.

This results in a small amount of starting torque. The motor has a starting torque of 100-200% of full load torque. The phase angle between the voltage (V ) by an angle. whereas the current (Im ) in the main winding lags the which is nearly 90o. giving rise to starting torque. The switch. to produce maximum starting torque. rotating magnetic field is produced in such motor. another (auxiliary) winding with a high resistance in series is to be added along with the main winding in the stator. The current (Ia ) in the auxiliary winding lags the voltage (V ) by an angle. . The phasor diagram of the currents in two windings and the input voltage is shown in Fig. Resistance Split-phase Motor The schematic (circuit) diagram of this motor is given in Fig. 34. Thus. two currents is which should be at least 30°. as shown in a balanced two-phase stator. This winding has higher resistance to reactance ratio as compared to that in the main winding. The various starting methods used in a single-phase IM are described here. which is small. S (centrifugal switch) is in series with the auxiliary winding. with the starting current as 5-7 times the . when the motor attains a speed close to full load speed.Einstein College of Engineering currents in the two (main and auxiliary) stator windings also must be at an angle of 90o. It automatically cuts out the auxiliary or starting winding. and is placed at a space angle of from the main winding as given earlier. As detailed earlier.4b.

It does not need any other thing. But the main problem is low starting torque in the motor. The torque-speed characteristics of the motor with/without auxiliary winding are shown in Fig. This can be can be achieved by having a capacitor in series with the auxiliary winding. etc. As the current in the main winding is lagging by the current in the auxiliary winding has to lead the input voltage by with is taken as negative (-ve). The change over occurs. This motor is used in applications. which is there in nearly all such motors as discussed here. The direction of rotation is reversed by reversing the terminals of any one of two windings. requiring only second (auxiliary) winding placed at a space angle of 90 o from the main winding. except for centrifugal switch. Capacitor-start Motor . 34. washing machine. The motor described earlier.Einstein College of Engineering full load current. with the increase in starting torque. but not both. office equipment. is a simple one. when the starting torque will be proportional to the product of the magnitudes of two currents. The two types of such motors are described here. centrifugal pump.5b). which results in additional cost. the phase difference required is 90°(Fig. saw. before connecting the motor to the supply terminals. To get high starting torque. as this torque is a function of. such as fan. or related to the phase difference (angle) between the currents in the two windings. as the auxiliary winding is used as a starting winding. blower. while is positive (+ve). when the auxiliary winding is switched off as given earlier. small lathe.

and the torque-speed characteristics of the motor with/without auxiliary winding. conveyor. refrigeration and air-conditioning equipment. The phasor diagram of two currents as described earlier.Einstein College of Engineering The schematic (circuit) diagram of this motor is given in Fig. It may be observed that a capacitor along with a centrifugal switch is connected in series with the auxiliary winding. respectively. the cost of which decreases. are shown in Fig. as it is used only at the time of starting. The capacitor may be rated only for intermittent duty. which is being used here as a starting winding. Capacitor-start and Capacitor-run Motor . such as compressor. machine tool drive. The function of the centrifugal switch has been described earlier. etc. This motor is used in applications.

Einstein College of Engineering .

The first capacitor is rated for intermittent duty. The second one is to be rated for continuous duty. as it is used for running. are also satisfied. are used. This motor is used in applications. are shown in Fig.6c respectively. 34. In the second case. being used only for starting. 34. The phasor diagram of two currents in both cases. A centrifugal switch is also needed here. only the forward rotating field is present. So.6a). and the torque-speed characteristics with two windings having different values of capacitors. and Cr for running. while it is for 90° second case (running).Einstein College of Engineering In this motor (Fig. 34. the two windings having same number of turns and other conditions as given earlier. refrigerator. and the no backward rotating field exists. etc. The efficiency of the motor under this condition is higher. using two capacitors. as described earlier. . Hence. The phase difference between the two currents is in the first case (starting). two capacitors -Cs for starting.6b and Fig. such as compressor. the performance of the motor improves both at the time of starting and then running. the motor is a balanced two phase one.

such as ceiling fans. This is a singlephase induction motor.Einstein College of Engineering Beside the above two types of motors. a Permanent Capacitor Motor (Fig. A small portion of each pole is covered with a short-circuited. The power factor of this motor. blower. Let the main winding flux be . Shaded-pole Motor A typical shaded-pole motor with a cage rotor is shown in Fig. etc. is high.) with the same capacitor being utilised for both starting and running. The sinusoidally varying flux created by ac (single-phase) excitation of the main winding induces emf in the shading coil. air circulator. when it is operating (running). The operation is also quiet and smooth. As a result. with main winding in the stator. This motor is used in applications. induced currents flow in the shading coil producing their own flux in the shaded portion of the pole. single-turn copper coil called the shading coil. is also used.

Obviously the phasor with sum by the angle while the flux is in phase The resultant flux in the shaded pole is given by the phasor as shown in Fig. It is seen from the phasor diagram (Fig.Einstein College of Engineering where As per the above equations. It is used for domestic fans. The motor thus has a definite direction of rotation. which cannot be reversed. the shading coil current (I sc ) and flux phasors lag behind the induced emf (Esc ) by angle phasor leads the induced emf (Esc ) by 90o. The shaded-pole principle is used in starting electric clocks and other single-phase synchronous timing motors. The reversal of the direction of rotation. The fact that the shaded-pole motor is single-winding (no auxiliary winding) selfstarting one. small business machines. etc. can be achieved by providing two shading coils. slide projectors.8b) that the net flux in the shaded portion of the pole lags the flux in the unshaded portion of the pole resulting in a net torque. which causes the rotor to rotate from the unshaded to the shaded portion of the pole. The motor has low efficiency and is usually available in a range of 1/300 to 1/20 kW. one on each end of every pole. It may be noted that the motor is self-starting unlike a single-phase single-winding motor. record players and tape recorders. and lags the flux The two sinusoidally varying fluxes of the remaining pole are displaced in space as well as have a time phase difference thereby producing forward and backward rotating fields. which produce a net torque. makes it less costly and results in rugged construction. and by open-circuiting one set of shading coils and by short-circuiting the other set. . 34. humidifiers. where desired.

no starting torque is produced in the single-phase induction motor with only one (main) stator winding. the motor accelerates in that direction. will be discussed.main and auxiliary. Using double revolving field theory. are presented.the sixth and last one of this module. the various types of single-phase induction motors. and also the torque is produced in that direction. it is shown that. Then. The process of production of starting torque in shade-pole motor is also described in brief. the torque-speed characteristics of this type of motor are described. along with the starting methods used in each one are presented. . if the motor is initially given some torque in either direction. along with their application area. with the winding being fed from single phase supply. both as generator and motor.Einstein College of Engineering In this lesson . firstly. are needed to produce the starting torque. In the next module consisting of seven lessons. The merits and demerits of each type. as the flux produced is a pulsating one. Two stator windings . and it is also shown that. the construction and also operation of dc machines.

Einstein College of Engineering .

This causes operation at higher speeds than the nominal speed. The voltage control can be from a variable voltage source like WardLeonard arrangement or by the use of series armature resistance.C. Above the nominal speed field weakening is adopted. In the case of series motors however field weakening is done by the use of „diverters‟. Thus operation below nominal speed is done by voltage control. Va is the voltage applied across the armature and ö is the flux per pole and is proportional to the field current If. namely by: . Thus for shunt motor we have essentially two methods for controlling speed.Einstein College of Engineering UNIT-IV CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D. For weakening the field. by varying Va and If we can vary n. Strengthening the field has little scope for speed control as the machines are already in a state of saturation and large field mmf is needed for small increase in the flux. Unlike the starting conditions the series resistance has to be in the circuit throughout in the case of speed control.C. If of course can be varied by controlling external field resistance Rf connected with the field circuit. armature voltage control and flux control methods are available. That means considerable energy is lost in these resistors. MOTORS: In the case of speed control. The variable voltage source on the other hand gives the motor the voltage just needed by it and the losses in the control gear is a minimum. Field control or flux control is also used for speed control purposes. series resistances are used for shunt as well as compound motors. Therefore. Even though flux weakening gives higher speeds of operation it reduces the torque produced by the machine for a given armature current and hence the power delivered does not increase at any armature current. armature current Ia is decided by the mechanical load present on the shaft. as also the power rating. Normally field weakening is used. The machine is said to be in constant power mode under field weakening mode of control. Further these resistors must be adequately cooled for continuous operation. but this is never done as the stress on the commutator insulation increases. For fixed supply voltage and the motor connected as shunt we can vary Va by controlling an external resistance connected in series with the armature. Diverters are resistances that are connected in parallel to the series winding to reduce the field current without affecting the armature current. This method is commonly used when the speed ratio required is large. DRIVES SPEED CONTROL OF D. Speed control of shunt motor We know that the speed of shunt motor is given by: where. As explained earlier. Above the nominal speed of operation. constant ux mode with increased applied voltage can be used.

speed n versus armature current Ia characteristic will be a straight line with a small negative slope as shown in figure . Speed control by varying armature resistance The inherent armature resistance ra being small. Since for constant operation. as the motor is loaded. 2. a d. varying armature resistance. Ia = 0) speed is highest and Note that for shunt motor voltage applied to the field and armature circuit are same and equal to the supply voltage V. armature current characteristic as shown in figure. In the discussion to follow we shall not disturb the field current from its rated value. It is for this reason. This drop in speed from no load to full load condition expressed as a percentage of no load speed is called the inherent speed regulation of the motor.c shunt motor is said to be practically a constant speed motor (with no external armature resistance connected) since speed drops by a small amount from no load to full load condition. torque characteristic is also similar to speed vs. However. varying field resistance.Einstein College of Engineering 1. Te becomes simply proportional to Ia. speed vs. Therefore.. Iara drop increases making speed a little less than the no load speed n0. At no load (i. One can get a family of speed .e. For a well-designed shunt motor this drop in speed is small and about 3 to 5% with respect to no load speed. 1. The slope of the n vs Ia or n vs Te characteristic can be modified by deliberately connecting external resistance rext in the armature circuit.

Te = a k I ö too will remain constant. since steady state operation demands Te = TL. Let us assume that the load torque TL is constant and field current is also kept constant. Variation of the speed is smooth and speed will decrease smoothly if rext is increased. then at rated load torque. new steady state operating speed will be n1 corresponding to the operating point D. Obviously. . Suppose rext = 0. If additional resistance rext1 is introduced in the armature circuit. which means Ia will not change. when rext2 is introduced in the armature circuit. armature curves as shown in figures for various values of rext. operating point will be at C and motor speed will be n.Einstein College of Engineering vs. this method provides smooth wide range speed control (from base speed down to zero speed). Therefore. has a serious draw back since energy loss takes place in the external resistance rext reducing the efficiency of the motor. From these characteristics it can be explained how speed control is achieved. Although. In this way one can get a speed of n2 corresponding to the operating point E. This same load torque is supplied at various speed. this method is suitable for controlling speed below the base speed and for supplying constant rated load torque which ensures rated armature current always.

let us assume that the load torque is constant at rated value. hence speed will vary. this method will not be suitable for constant load torque. The field coil produces rated flux when no external resistance is connected and rated voltage is applied across field coil. However. Let us rewrite . So from the initial steady condition. Speed control by varying field current In this method field circuit resistance is varied to control the speed of a d. will be suitable for a load whose torque demand decreases with the rise in speed keeping the output power constant as shown in figure.Einstein College of Engineering 2. If we vary If. hence new armature current will be greater than the rated armature current and the motor will be overloaded. Speed versus armature current characteristic is shown in figure for two flux values and Since no load speed for flux value is than the no load speed no corresponding to . flux will change. . To change If an external resistance is connected in series with the field windings. It should be understood that we can only decrease flux from its rated value by adding external resistance. Therefore at higher speed main flux may become so weakened.c shunt motor.the basic equation to understand the method. Obviously this method is based on flux weakening of the main field. . To make this point clear. This method therefore. that armature reaction effect will be more pronounced causing problem in commutation. Thus the speed of the motor will rise as we decrease the field current and speed control above the base speed will be achieved. we have torque remains constant and flux is reduced to state is obtained from Therefore new armature current is If load new armature current in the steady But the fraction.

c voltage source.Einstein College of Engineering 3. Since the no load speed the speed versus Ia characteristic will shift parallely as shown in figure for different values of Va. Here the armature resistance and field current are not varied. armature is supplied from a separate variable d. Speed control by armature voltage variation In this method of speed control. while the field is separately excited with fixed rated voltage as shown in figure. .

obtaining a variable d. In a way armature voltage control method is similar to that of armature resistance control method except that the former one is much superior as no extra power loss takes place in the armature circuit. 4. Obviously to run this generator. One has to simply connect an appropriate rheostat in the field circuit for this purpose.Einstein College of Engineering As flux remains constant. a prime mover is required. Arrangement for field control is rather simple. A 3-phase induction motor is used as the prime mover which is supplied from a 3-phase supply.c supply was not easy and a separately excited d. However.c generator was used to supply the motor armature. the generated emf. both field and armature control are integrated as shown in figure. By controlling the field current of the generator. as one should not apply across the armature a voltage. . this method is suitable for constant torque loads. which is higher than the rated voltage. Ward Leonard method: combination of Va and If control In this scheme. in the pre power electronic era. Armature voltage control method is adopted for controlling speed from base speed down to very small speed.

c generator and a 3-phase induction motor of simialr rating as that of the d.Einstein College of Engineering hence Va can be varied. The potential divider connection uses two rheostats in parallel to facilitate reversal of generator field current. variable d.c motor is varied maintaining constant Va.c motor whose speed is intended to be controlled. can now be gradually increased to the rated value by slowly increasing the generator field current.c supply by using controlled rectifiers thus avoiding the use of additional induction motor and generator set to implement Ward leonard method. In this scheme. no starter is required for the d. The applied voltage to the motor Va. To control the speed of the d. Field supply of the motor is switched on with motor field rheostat set to zero. wide range smooth speed control is achieved. the cost involved is rather high as we require one additional d. First the induction motor is started with generator field current zero (by adjusting the jockey positions of the rheostats). excitation of the d.c generator is varied. Series motor .c motor below base speed by armature voltage.c motor as the applied voltage to the armature is gradually increased. In present day.c supply can easily be obtained from a. Reversal of direction of rotation of the motor can be obtained by adjusting jockeys of the generator field rheostats. while to control the speed above base speed field current of the d. Although.

starting torque of a series motor is quite high compared to a similarly rated d. here field current is not independent of armature current. the relation between speed and armature current can be obtained as follows: . 1. Now torque produced in a d. Ia speed vs.Characteristics of series motor Torque vs.c motor is: Since torque is proportional to the square of the armature current. armature current From the KVL equation of the motor.c voltage as depicted in figure 39.e.Einstein College of Engineering In this motor the field winding is connected in series with the armature and the combination is supplied with d. the T vs.13..c shunt motor. In fact. armature current characteristic Since in the linear zone and characteristic is as shown in figure the saturation zone. field and armature currents are equal i. Unlike a shunt motor.

a series motor should never be operated under no load condition. is a measure of degree of loading. Therefore. Speed versus armature current characteristic is shown in figure nvsia:side: . Unlike a shunt motor. a series motor has no finite no load speed.Einstein College of Engineering The relationship is inverse in nature making speed dangerously high as Remember that the value of Ia.

Since the machine resistance a s r +re is quite small. series motor speed approaches a dangerously high value. Speed control below base speed For constant load torque. the relationship between speed and torque is represent appropriate constants to take into account the proportionality that exist between current. steady armature current remains constant. speed too will be reduced. For this reason. the back emf Eb is approximately equal to the armature terminal voltage Va. the applied voltage across each motor is V. If Va is reduced. Therefore. For same armature current in both the cases (which means flux per pole is same). the applied voltage across each motor is V/2 while in parallel connection shown in figure. Series-parallel connection of motors If for a drive two or more (even number) of identical motors are used (as in traction). Speed control of series motor 1. torque and flux in the linear zone.Einstein College of Engineering Since in the linear zone. If belt snaps. This relation is also inverse in nature indicating once again that at light load or no load condition. a series motor is never connected to mechanical load through belt drive. hence flux also remains constant. . The back emf in the former case will be approximately half than that in the latter case. speed is proportional to Va. This Va can be controlled either by connecting external resistance in series or by changing the supply voltage. speed will be half in series connection compared to parallel connection. In series connection of the motors shown in figure . The characteristic is shown in figure. the motors may be suitably connected to have different applied voltages across the motors for controlling speed. the motor becomes unloaded and as a consequence speed goes up unrestricted causing mechanical damages to the motor.

So field current is now not equal to the armature current. independent control of field current is not so obvious as armature and field coils are in series.19. this can be achieved by the following methods: 1. raises the speed of the motor. . In this method shown in figure 39.However. Speed control above base speed Flux or field current control is adopted to control speed above the base speed. in fact it is less than the armature current. a portion of the armature current is diverted through the diverter resistance. In a series motor.Einstein College of Engineering 2. Using a diverter resistance connected across the field coil. Flux weakening thus caused.

field mmf se f N I changes. 3.c machine are brought out after connecting the field coils (wound over each pole) in series.20. However provision is kept to change the number of turns of the field coil.Einstein College of Engineering 2. When number of turns changes. parallel. If the terminals of the individual coils are . Changing number of turns of field coil provided with tapings. Connecting field coils wound over each pole in series or in. Consider a 4-pole series motor where there will be 4 individual coils placed over the poles. Generally the field terminals of a d. armature and field currents are same. changing the flux hence speed of the motor. In this case shown figure 39.

The four coils could be connected in series as in figure 39. then there exist several options for connecting them. n figure For series connection of the coils (figure 39. In a similar way.Einstein College of Engineering brought out. reader can work out the variation of speed possible between (i) all coils connected in series and (ii) all coils connected in parallel. flux will be doubled in the second case and naturally speed will be approximately doubled as back emf in both the cases is close to supply voltage V. for same armature current Ia. .22) flux produced is proportional to Therefore. the 4 coils could be connected in parallel or parallel combination of 2 in series and other 2 in series as shown in figure 39.22.21. Thus control of speed in the ratio of 1:2 is possible for series parallel connection.21) flux produced is proportional to Ia and for series-parallel connection (figure 39.

have typically constant speed requirements and hence the induction machine is ideally suited for these. . 1. we can see that the torque depends on the square of the applied voltage. typically for fan or pump applications. the curve is rather steep and goes from zero torque at synchronous speed to the stall torque at a value of slip s = ^s. the induction machine. The machine speed is quite with respect to load changes. while the value of stall torque comes down with decrease in applied voltage. Therefore it is good candidate for variable speed applications if it can be achieved. Speed control by changing applied voltage From the torque equation of the induction machine . we also note that the starting torque is also lower at lower voltages. However. especially the squirrel cage type.3 or less. The entire speed variation is only in the range ns to (1 _ ^s)ns. even if a given voltage level is sufficient for achieving the running torque. These curves show that the slip at maximum torque ^s remains same. the machine may not start. DRIVES SPEED CONTROL OF INDUCTION MACHINES: We have seen the speed torque characteristic of the machine. The foregoing discussion shows that the induction machine. Normally ^s may be such that stall torque is about three times that of the rated operating torque of the machine. the speed change is quite small.C. In the stable region of operation in the motoring mode. and hence may be about 0. This means that in the entire loading range of the machine.Einstein College of Engineering UNIT-V CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A. is quite rugged and has a simple construction. The speed range for stable operation remains the same. This method of trying to control the speed is best suited for loads that require very little starting torque. when operating from mains is essentially a constant speed machine. Many industrial drives. The variation of speed torque curves with respect to the applied voltage is shown in Fig. but their torque requirement may increase with speed. ns being dependent on supply frequency and number of poles. Thus. Further.

the variation of torque with speed is such that T /! 2. Whether the load is of constant torque type or fan-type. the machine can start even at reduced voltages. the voltage across the magnetizing branch also comes down. it is . Here one can see that it may be possible to run the motor to lower speeds within the range ns to (1 _ ^s) ns. Note that while the maximum torque and synchronous speed remain constant. Another use of voltage control is in the so-called `soft-start' of the machine. This in turn means that the magnetizing current and hence ux level are reduced. while retaining a constant torque. Figure shows a family of torque-speed characteristic obtained by changing the rotor resistance. Under such conditions. inefficient proposition). however. Rotor resistance control The reader may recall the expression for the torque of the induction machine. Further. reduction in magnetizing current improves the power factor of operation. Voltage control may be achieved by adding series resistors (a lossy. Therefore. If. then operating under rated flux levels is not required. Some amount of energy saving may also be achieved. Further. Clearly. the maximum torque point shift to higher slip values. In a fan (blower) type of load. and so does the starting torque. we may expect that if the rotor resistance is changed. 2. or a series inductor / autotransformer (a bulky solution) or a more modern solution using semiconductor devices. Reduction in the ux level in the machine impairs torque production (recall explanations on torque production). The slip at maximum torque dependent on the rotor resistance.Einstein College of Engineering Figure : Speed-torque curves: voltage variation Figure also shows a load torque characteristic | one that is typical of a fan type of load. the slip at which maximum torque occurs increases with increase in rotor resistance. A typical solid-state circuit used for this purpose is the AC voltage controller or AC chopper. that the maximum value is independent of the rotor resistance. it is dependent on the rotor resistance. One may note that if the applied voltage is reduced. This is discussed in the section on starting methods. This will not be possible with constant torque type of loads. since the load torque at zero speed is zero. the machine is running under lightly loaded conditions.

This. the method is not very efficient since the additional resistance and operation at high slips entails dissipation resistors connected to the slip-ring brushes should have good power dissipation capability. in order to vary the rotor resistance. Let the frequency of supply given to the first machine be f1. it is necessary to connect external variable resistors (winding resistance itself cannot be changed). A `solid-state' alternative to a rheostat is a chopper controlled resistance where the duty ratio control of the chopper presents a variable resistance load to the rotor of the induction machine. Secondly. if n is the speed of the rotor . Since the machines are coupled at the shaft. Hence. For all its advantages. rotor resistance control could also be used as a means of generating high starting torque.Einstein College of Engineering evident that the speed control range is more with this method. A still better option would be to mechanically couple the shafts of the two machines together. Figure: Speed-torque curves : rotor resistance variation 3. Therefore f2 = s1f1. Let f2. the speed of the rotor is common for both. there are no rotor terminals. and its slip of operation be s1. Apart from using the heat generated in meaning full ways. therefore necessitates a slip-ring machine. The stator of the second machine would carry slip frequency currents of the first machine. The frequency of currents owing in the rotor of the first machine and hence in the stator of the second machine is s1f1. p2 and s2 be the corresponding quantities for the second machine. Cascade control The power drawn from the rotor terminals could be spent more usefully. Firstly. the scheme has two serious drawbacks. since only in that case rotor terminals are available outside. which would generate some useful mechanical power. Water based rheostats may be used for this. Further. its number poles be p1. This sort of a connection is called cascade connection and it gives some measure of speed control as shown below. the slip ring output could be connected to another induction machine. For cage rotor machines.

the resultant stator mmf of the second machine may set up an air-gap flux which rotates in the same direction as that of the rotor. The physical interpretation is that we now have an active source connected on the rotor side. This is equivalent to saying that across the rotor terminals a voltage source of zero magnitude is connected. Note that while giving the rotor output of the first machine to the stator of the second. This results in values for speed as The latter expression is for the case where the second machine is connected in opposite phase sequence to the first.Einstein College of Engineering Figure: Generalized rotor control in radians. Such circuits are called static kramer drives. Let the power consumed by that source be Pr. where the rotor circuit has been terminated with a voltage source Er. In general the circuitry connected to the rotor may not be a simple resistor or a machine but a power electronic circuit. the value of s can be changed by the value of Pr. As Pr becomes positive. speed reduces. it behaves like a normal induction machine. If the rotor terminals are shorted. Different situations could then be considered if this voltage source Er had a non-zero magnitude. Speed control through rotor terminals can be considered in a much more general way. or opposes it. Then considering the rotor side circuit power dissipation per phase Clearly now. 4. Consider the induction machine equivalent circuit. The cascade-connected system can therefore run at two possible speeds. Pole changing schemes . For Pr = 0. When Pr = _I02 2 R2 the entire copper loss is supplied by the external source. This circuit may drive a machine or recover power back to the mains. As Pr becomes negative. for all other circuit conditions remaining constant. which is able to supply part of the rotor copper losses. which can process this power requirement. s increases or in the other words. This corresponds to operation at synchronous speed. The RHS and hence the slip is zero. the machine is like a normal machine with a short circuited rotor. the right hand side of the equation and hence the slip decreases.

then at any winding connection. thus establishing a two-pole structure. A1 with the other terminal at C then form the terminals of the overall combination. say into terminal A1. then the pole configurations produced by these connections is a two-pole arrangement in fig and a four-pole arrangement. The two coils on A & C may be connected in series in two different ways | A2 may be connected to C1 or C2. the operating speed is changed. the pole emanating flux lines is considered as north pole and the pole into which they enter is termed as south. Coils are wound on A & C in the directions shown. for a given direction of current flow at terminal A1. B. Thus two connections result as shown in fig. In case (a). Since the synchronous speed of the induction machine is given by ns = fs=p (in rev. The flux lines will be then have to complete the circuit by owing into the pole structures on the sides. the flux lines are out of the pole A (seen from the rotor) for and into pole C. Further note that irrespective of the connection. The following figure serves to illustrate the basic principle. the same maximum torque is achievable. Thus by changing the terminal connections we get either a two pole air-gap field or a four. Such winding arrangements are therefore referred to as constant-torque connections.pole field. C. Figure: Pole arrangement In case (b) however./s) where p is the number of pole pairs. then such connections are called constant-horsepower type. Cases (a) and (b) therefore form a pair of constant torque connections. the flux lines are out of the poles in A & C. D in fig. the flux directions within the poles are shown in the figures. In an induction machine this would correspond to a synchronous speed reduction in half from case (a) to case (b). Therefore the airgap flux in both cases is the same. Now. this would correspond to changing the synchronous speed. when seen from the rotor. This method of speed control is a stepped variation and generally restricted to two steps. the applied voltage is balanced by the series addition of induced emfs in two coils. With the slip now corresponding to the new synchronous speed. . If. Consider a magnetic pole structure consisting of four pole faces A. If the changes in stator winding connections are made so that the air gap flux remains constant.Einstein College of Engineering Sometimes induction machines have a special stator winding capable of being externally connected to form two different number of pole numbers. If however such connection changes result in air gap flux changes that are inversely proportional to the synchronous speeds.

The terminals T1 and T2 are where the input excitation is given. the current through one coil (out of two. It is important to note that in generating a different pole numbers. T3 open. If however the excitation is given to Ta. In the case of a threephase machine. T2. T3 are supplied with three phase excitation and Ta. Stator frequency control The expression for the synchronous speed indicates that by changing the stator frequency also it can be changed. Depending on the type of . This can be achieved by using power electronic circuits called inverters. coil C in this case) is reversed. The other coils also face similar conditions. 5. However. and hence this would result in a four-pole structure. thereby bringing down the speed. T2. there is only one coil-induced emf to balance the applied voltage. in fig. Therefore flux in case (c) would therefore be halved compared to that of case (b) (or case (a). Cases (a) and (c) therefore form a pair of constant horsepower connections. The applied voltage must be balanced by induced emf in one coil only (C1 & C2 are parallel). Thus the effective number of poles would increase. for that matter). which convert dc to ac of desired frequency. Note that current direction in the coils now resembles that of case (b). Tb& Tc with T1. Let the machine have coils connected as shown [C1 _ C6] The current directions shown in C1 & C2 correspond to the case where T1. the following example serves to explain this. Tb & Tc are shorted to each other (STAR point). on the other hand a connection as shown in the fig. then current through one of the coils (C1 & C2) would reverse.Einstein College of Engineering Consider.

Einstein College of Engineering control scheme of the inverter, the ac generated may be variable-frequency-fixedamplitude or variable-frequency- variable-amplitude type. Power electronic control achieves smooth variation of voltage and frequency of the ac output. This when fed to the machine is capable of running at a controlled speed. However, consider the equation for the induced emf in the induction machine.

Figure : Pole change example: three phase

where N is the number of the turns per phase, _m is the peak flux in the air gap and f is the frequency. Note that in order to reduce the speed, frequency has to be reduced. If the frequency is reduced while the voltage is kept constant, thereby requiring the amplitude of induced emf to remain the same, flux has to increase. This is not advisable since the machine likely to enter deep saturation. If this is to be avoided, then flux level must be maintained constant which implies that voltage must be reduced along with frequency. The ratio is held constant in order to maintain the flux level for maximum torque capability. Actually, it is the voltage across the magnetizing branch of the exact equivalent circuit that must be maintained constant, for it is that which determines the induced emf. Under conditions where the stator voltage drop is negligible compared the applied voltage, is valid. In this mode of operation, the voltage across the magnetizing inductance in the 'exact' equivalent circuit reduces in amplitude with reduction in frequency and so does the inductive reactance. This implies that the current through the inductance and the flux in the machine remains constant. The speed torque characteristics at any frequency may be estimated as before. There is one curve for every excitation frequency considered corresponding to every value of synchronous speed. The curves are shown below. It may be seen that the maximum torque remains constant.

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Figure : Torque-speed curves with E=f held constant This may be seen mathematically as follows. If E is the voltage across the magnetizing branch and f is the frequency of excitation, then E = kf, where k is the constant of proportionality. If! = 2_f, the developed torque is given by

If this equation is differentiated with respect to s and equated to zero to find the slip at maximum torque ^s, we get ^s = _R0 r=(! L0 lr). The maximum torque is obtained by substituting this value into eqn.

Equation shows that this maximum value is independent of the frequency. Further ^s! is independent of frequency. This means that the maximum torque always occurs at a speed lower than synchronous speed by a fixed difference, independent of frequency. The overall effect is an apparent shift of the torque-speed characteristic as shown in fig. Though this is the aim, E is an internal voltage, which is not accessible. It is only the terminal voltage V that we have access to and can control. For a fixed V, E changes with operating slip (rotor branch impedance changes) and further due to the stator impedance drop. Thus if we approximate E=f as V=f, the resulting torque-speed characteristic shown in fig. is far from desirable.

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Figure : Torque-speed curves with V=f constant At low frequencies and hence low voltages the curves show a considerable reduction in peak torque. At low frequencies (and hence at low voltages) the drop across the stator impedance prevents sufficient voltage availability. Therefore, in order to maintain sufficient torque at low frequencies, a voltage more than proportional needs to be given at low speeds. Another component of compensation that needs to be given is due to operating slip. With these two components, therefore, the ratio of applied voltage to frequency is not a constant but is a curve such as that shown in fig. With this kind of control, it is possible to get a good starting torque and steady state

Figure : Voltage boost required for V=f control performance. However, under dynamic conditions, this control is insufficient. Advanced control techniques such as field- oriented control (vector control) or direct torque control (DTC) are necessary.

electric locomotives trains. I. i. iv. 3. iii. Give some examples of Electric Drives. Driving fans. Define Drive and Electric Drive. Lifting goods by hoists and cranes. Turbine or electric motors. 2. I . ii. Ability to provide a wide range of torques over wide range of speeds. compressors and pumps. mines and warehouses Running excavators & escalators.C Engines. What are the types of electric drives? Group electric drives (Shaft drive). Electric motors are available in a variety of design in order to make them compatible to any type of load. i. cars trolley buses. Imparting motion to conveyors in factories. lifts & drum winders etc. Steam engine. iii. ii. 5. Availability of electric drives over a wide range of power a few watts to mega watts. List out some advantages of electric drives. List out some examples of prime movers. 4. ventilators. transmission equipment and mechanical working load is called a drive Electric drive: An Electric Drive can be defined as an electromechanical device for converting electrical energy to mechanical energy to impart motion to different machines and mechanisms for various kinds of process control.Einstein College of Engineering Einstein College of Engineering Department of Electrical & Electronics Engineering Electrical Drives and Control – ME 36 (For Mechanical Engineering) Third Semester 2 marks (Questions & Answers) UNIT – I INTRODUCTION 1. Drive: A combination of prime mover.

What are the advantages and disadvantages of Group drive (Shaft drive)? Advantages:  A single large motor can be used instead of a number of small motors.  The rating of the single motor may be appropriately reduced taking into account the diversity factor of loads. 2. Addition of an extra machine to the main shaft is difficult. Multi motor electric drives. which drives one or more line shafts supported on bearings. The efficiency of the drive is low. because of the losses occurring in several transmitting mechanisms. Disadvantages:      There is no flexibility. Single spindle drilling machine. What is a Group Electric Drive (Shaft Drive)?   This drive consists of single motor. Lathe machines etc. In this drive. This motor also imparts motion to various other parts of the machine. 7. The complete drive system requires shutdown if the motor. In this drive. each of which serves to activate on of the working parts of the driven mechanisms. Manual. 6. The line shaft may be fitted with either pulleys & belts or gears. . What is an individual electric drive? Give some examples. 1. there are several drives. each individual machine is driven by a separate motor. by means of which a group of machines or mechanisms may be operated.Einstein College of Engineering ndividual Drives. 3. Semiautomatic. Classify electric drives based on the means of control. Automatic. The system is not very safe to operate The noise level at the work spot is very high. What is a multi motor electric drive? Give some examples. requires servicing or repair.

14. rolling mills. Types:   One for the static or steady state process Other for the dynamic process. Torque & power) as a function of time are required to draw…. operates the drive in accordance with a preset sequence or order. traction drive.) and an automatic device that in response to command.Einstein College of Engineering Metal cutting machine tools. Semiautomatic control: This control consists of a manual device for giving a certain command (Starting.. when the dynamic components of torque are induced by the inertia of the motor & load. 4. paper making machines. Traveling cranes etc.. What is a load diagram? What are its types? What are required to draw a load diagram? A load diagram is the diagram which shows graphically the variation of torque acting on the electric drive. acceleration. Write about manual control. semiautomatic control & Automatic control? Manual control: The electric drives with manual control can be as simple as a room fan. incorporating on switch and a resistance for setting the required speed. braking. (Instantaneous speed. What are the types Drive systems? . Automatic control: The electric drives with automatic control have a control gear. change of speed etc. without manual devices 12. What are the Typical elements of an Electric Drive? Power Supply Geared Coupling Mechanical Laod Speed & Torque Control Motor 13.. reversing. The motor of the electric drive has to overcome the load torque expressed as a function of time.

Einstein College of Engineering Electric Drives Electromechanical Drives Mechanical Drives Hydraulic drives. Efficiency 3.Service cost 20. What are the assumptions made while performing heating & cooling calculation of an electric motor? i. Heat dissipation taking place is proportional to the difference of temperature of the body and surrounding medium. What are the factors that influence the choice of electrical drives? 1. ii. Service capacity & rating 18. Accessibility 8. Maintenance 14. Power rating: Correct selection of power rating of electric motor is of economic interest as it is associated with capital cost and running cost of drives. The rate of dissipation of heat is constant at all temperatures. Special competence 5. iii. Nature of electric supply 18. Availability 17. 15. Types of drive 19. No heat is radiated. Power range 12. Nature of load 9. Total purchase cost 15. 16. Indicate the importance of power rating & heating of electric drives. Shaft power & speed 11. Influence on the supply network 4. The machine is considered to be a homogeneous body having a uniform temperature gradient. All the points at which heat is dissipated are also at same temperature. All the points at which heat generated have the same temperature. Influence on power supply 16. Speed range 2. Heating: For proper selection of power rating the most important consideration is the heating effect . Electrical Characteristics 10. Starting torque 13. 17. The losses occurring in a machine is given by W = W c + x 2 Wv Where Wc = Constant losses Wv = Variable losses at full load X = load on the motor expressed as a function of rated load. Environment 7. Give an expression for the losses occurring in a machine. Cost of energy losses 6.

fans. In this connection various forms of loading or duty cycles have to be considered. Continuous duty with starting & braking 8. List out some applications for which continuous duty is required. conveyors & compressors 23. 19. Continuous duty 2. 20. Intermittent periodic duty 4. How will you classify electric drives based on the method of speed control? 1. 24. This is because the motor does not require frequent starting it is started only once in its duty cycle and the losses during starting do not have much influence on heating. motor winding & bearing friction. 6. 5. Intermittent periodic duty with starting & braking 6. What is meant by “short time rating of motor”? Any electric motor that is rated for a power rating P for continuous operation can be loaded for a short time duty (Psh) that is much higher than P. Why the losses at starting is not a factor of consideration in a continuous duty motor? While selecting a motor for this type of duty it is not necessary to give importance to the heating caused by losses at starting even though they are more than the losses at rated load.Einstein College of Engineering of load. Centrifugal pumps. Continuous duty with intermittent periodic loading 7. . Continuous duty with periodic load changes 21. Intermittent periodic duty with starting 5. Reversible &non reversible in controlled constant speed Reversible and non reversible step speed control Reversible and non reversible smooth speed control Constant predetermined position control Variable position control Composite control. 4. 3. if the temperature rise is the consideration. Short time duty operation of motor Main classes of duties 3. What are the classes of duties? 1. 22. 2. How heating occurs in motor drives? The heating of motor due to losses occurring inside the motor while converting the electrical power into mechanical power and these losses occur in steel core.

Imparting motion to conveyors in factories. Draw the heating & Cooling curve of an electric motor. if the motor is not to be reversed. Method of 4.dt 30. ventilators. Method of 3.. the motor should have a prominent drooping characteristic so that on load there is a considerable speed drop. electric locomotives. 2. Running excavators & escalators. Method of 2.dt = p. 29. cars. What are the various function performed by an electric drive? 1. How a motor variable load ? 1. 26. What is meant by “load equalization”? In the method of “load Equalization” intentionally the motor inertia is increased by adding a flywheel on the motor shaft. Driving fans. Method of 27. Write down the heat balance equation.Einstein College of Engineering 25. rating is determined in a continuous duty and Average losses equivalent power equivalent current equivalent Torque Define heating time constant & Cooling time constant? 28. For effectiveness of the flywheel. trains. trolley buses and lifts etc. Lifting goods by hoists & cranes 3. . Heat balance equation is given by Ghd0 + S0 . compressors & pumps etc. mines & warehouses and 4.

rapid transit systems. Mechanical Brakes require frequent maintenance Not smooth Can be applied to hold the system at any position torque. fed from 2 or 3 phase supply. This can be achieved by means of additional resistance in the armature circuit or by using thyristor power converter. blowers and fans. What is the effect of variation of armature voltage on N-T curve and how it can be achieved? The N-T curve moves towards the right when the voltage is increased. trolley cars. centrifugal pumps. What are the different types of electric braking? Dynamic or Rheostatic braking. Counter current or plugging and Regenerative braking 5. 6. What is meant by plugging? The plugging operation can be achieved by changing the polarity of the motor there by reversing the direction of rotation of the motor.Einstein College of Engineering UNIT – II ELECTRICAL MOTOR CHARACTERISTICS 1. This can be achieved in ac motors by changing the phase sequence and in dc motors by changing the polarity. Shunt : driving constant speed. air compressors. conveyors Compound : elevators. Compare electrical and mechanical braking. reciprocating pumps Series : electric locomotives. 4. Electrical very little maintenance smooth cannot produce holding . 2. heavy planners. lathes. Its stator winding produces a flux which only alternates along one space axis. cranes and hoists. machine tools. rolling mills. It is not a synchronously revolving field. as in the case of a 2 or 3phase stator winding. 3. Why a single phase induction motor does not self start? When a single phase supply is fed to the single phase induction motor. Give some applications of DC motor.

Where Ns = synchronous speed. Ns Nr S = synchronous speed in rpm. Shunt : driving constant speed. part of which is returned to the supply and rest as heat in the winding and bearing. Define slip. = rotor speed in rpm = Slip 9. Mechanical energy is converter into electrical energy. . 5. rapid transit systems. the motor operates as a generator. Its stator winding produces a flux which only alternates along one space axis. 6. heavy planners. S = Ns – Nr Ns Where. while it is still connected to the supply here. blowers and fans. centrifugal pumps. of stator poles. 8. trolley cars. cranes and hoists. It is given by Ns = 120f / p rpm. What is meant by regenerative braking? In the regenerative braking operation. the slip becomes negative. It is not a synchronously revolving field. Regenerative braking occurs and the K.E. reciprocating pumps Series : electric locomotives. fed from 2 or 3 phase supply. When does an induction motor behave to run off as a generator? When the rotor of an induction motor runs faster than the stator field. of the rotating parts is return back to the supply as electrical energy and thus the machine generates power. f = supply frequency in Hz 10.Einstein College of Engineering 7. conveyors Compound : elevators. p = no. as in the case of a 2 or 3phase stator winding. Give some applications of DC motor. Why a single phase induction motor does not self start? When a single phase supply is fed to the single phase induction motor. machine tools. rolling mills. Define synchronous speed. the motor speed is grater that the synchronous speed. air compressors. lathes.

Differentiate cumulative and differential compound motors.L Starter (Direct Online Starter) Star-Delta Starter Auto Transformer Starter Reactance or Resistance starter Stator Rotor Starter (Rotor Resistance Starter) 3.L. differential series flux opposes UNIT – III STARTING METHODS 1. Mention the Starters used to start an Induction motor. D. Compare electrical and mechanical braking. Cumulative The orientation of the series flux aids the shunt flux shunt flux 9. Mechanical Brakes require frequent maintenance Not smooth Can be applied to hold the system at any position torque. Mention the Starters used to start a DC motor. Electrical very little maintenance smooth cannot produce holding 8. What are the protective devices in a DC/AC motor Starter. Over load Release (O.Einstein College of Engineering 7.R) or No volt coil Hold on Coil Thermal Relays Fuses(Starting /Running) Over load relay . Two point Starter Three point Starter Four point Starter 2. What is meant by mechanical characteristics? A curve drawn between the parameters speed and torque.O.

Is it possible to include/ Exclude external resistance in the rotor of a Squirrel cage induction motor?. the rotors bars are permanently short circuited by means of circuiting rings (end rings) at both the ends. 9. which are short circuited. Give the prime purpose of a starter for motors. Since rotor of an induction motor behaves as a short circuited secondary of a transformer whose primary is stator winding. 5. . i. 8.Einstein College of Engineering 4. when induction motor is switched on to the supply.e by reducing the applied voltage to the stator windings. By increasing the resistance in the rotor circuit By reducing the magnitude of rotating magnetic field i. So Starters are necessary 6. it takes about 5 to 8 times full load current at starting. 10. This can be implemented only on a slip ring induction motor. heavy rotor current will require corresponding heavy stator balancing currents. Why squirrel cage induction motors are not used for loads requiring high starting torque? Squirrel cage motors are started only by reduced voltage starting methods which leads to the development of low starting torque at starting. Justify No it is not possible to include/ Exclude external resistance in the rotor of a Squirrel cage induction motor because. Thus motor draws heavy current at starting 7. What are the methods to reduce the magnitude of rotor current (rotor induced current) at starting?. This magnetic field is cut by the rotor conductors. magnetic field rotating in space at synchronous speed is produced.e. This starting current may be of such a magnitude as to cause objectionable voltage drop in the lines. Why motor take heavy current at starting? When 3 phase supply is given to the stator of an induction motor. What is the objective of rotor resistance starter (stator rotor starter)? To include resistance in the rotor circuit there by reducing the induced rotor current at starting. This is the reason Why squirrel cage induction motors are not used for loads requiring high starting torque. This gives to induced current in them. How reduced voltage starting of Induction motor is achived?. no slip rings to do so.

Give the relation between line voltage and phase voltage in a (i) Delta connected network (ii) Star connected network Delta connected network: Vphase = Vline Star connected network: Vphase = Vline / √3 12.8)2 Tdol Full load torque(T) Ts = 6T Ts = 2/3T Ts = 1.L Starter Star Delta starter Auto transformer .L starter.O.O. Give some advantages and disadvantages of D.L Starter (Direct Online Starter) Star-Delta Starter Auto Transformer Starter Reactance or Resistance starter 11. Explain double stage reduction of line current in an Auto transformer starter. Advantages: Highest starting torque Low cost Greatest simplicity Disadvantages: The inrush current of large motors may cause excessive voltage drop in the weak power system The torque may be limited to protect certain types of loads.8)2 Idol Full load current(I ) Is = 6I Is = 2I Is = 3.7% 80% Starting current (Is)compared with D.L Torque(Tdol) Ts = Tdol Ts = (1/√3)2 Tdol Ts = =(0.Einstein College of Engineering D.84 I Starting torque (Ts)compared with D.28 T Description of Starter D.O. Compare the Induction motor starters % of line voltag e applie d 100% 57. First stage reduction is due to reduced applied voltage Second stage reduction is due to reduced number of turns 14. 13.L current(Idol) Is = Idol Is = (1/√3)2 Idol Is =(0.O.O.

4)2 Idol Is = (0.by varying the terminal voltage -for above rated speed -for below rated speed .425)2 Tdol Ts = 0. Give the expression for speed for a DC motor. Draw the Speed-Torque characteristics of an Induction motor with various values of Rotor Resistance. Armature control.Einstein College of Engineering starter 60% 40% Reactanceresistance starter 64% Is =(0.35T 15.96 I Is = 2.6)2 Tdol Ts = =(0.16 I Is = 0. What are the ways of speed control in dc motors? Field control by varying the flux per pole.32 T Ts = 0.5 I Ts = =(0. Speed N = k (V-IaRa)  where V = Terminal Voltage in volts Ia = Armature current in Amps Ra = Armature resistance in ohms = flux per pole.64)2Idol Is = 2.6)2 Idol Is =(0. 2.4)2 Tdol Ts =(0. Rotor Resistance Increasing Tmax Torque Speed UNIT – IV CONVENTIONAL SPEED CONTROL 1.72 T Ts = 0.

This control is not suitable to application needing speed reversal. It gives speed below normal.  Loss of power is directly proportional to the reduction in speed. more efficient and convenient. Speed lower than the rated speed cannot be obtained.  A large amount of power is wasted in the controller resistance. 8.Einstein College of Engineering 3. not above. 7. b. It can cope with constant kW drives only. c. Compensating winding can be used to increase the speed range in field control method 5. . Electric excavators In elevators Main drives in steel mills and blooming and paper mills. Hence    efficiency is decreased. What are the 3 ways of field control in DC series motor?      Field diverter control Armature diverter control Motor diverter control Field coil taps control Series-parallel control 6. What are the advantages of field control method?  More economical. What are the main applications of Ward-Leonard system?     It is used for colliery winders. What are the merits and demerits of rheostatic control method?  Impossible to keep the speed constant on rapidly changing loads. Maximum power developed is diminished in the same ratio as speed. Give the Limitation of field control a. It needs expensive arrangements for dissipation of heat produced in the controller resistance. 4.

Einstein College of Engineering  It can give speeds above normal speed. While controlling the speed of a dc shunt motor what should be done to achieve a constant torque drive? Applied voltage should be maintained constant so as to maintain field strength . 12. 11. Mention the speed control method employed in electric traction. flux will decrease. 9.  The torque is four times that produced by the motor when in parallel. 10. Compare the values of speed and torque in case of motors when in parallel and in series. What is the effect of inserting resistance in the field circuit of a dc shunt motor on its speed and torque? For a constant supply voltage. Series-parallel speed control.  The speed is one fourth the speed of the motor when in parallel. speed will increase and torque will increase.

but the current remains unidirectional because of unidirectional thyristors. What is forced commutation? The commutation process which takes place by the action of an external force is called forced commutation. This angle is known as firing angle. What is natural or line commutation? The commutation which occurs without any action of external force is called natural or line commutation.Einstein College of Engineering UNIT – V SOLID STATE SPEED CONTROL 1. What is firing angle? The control of dc voltage is achieved by firing the thyristor at an adjustable angle with respect to the applied voltage. What is a full converter? A full converter is a tow quadrant converter in which the voltage polarity of the output can reverse. 2. with motors ratings as large as several MW‟s. 4. . 6. Give some applications of phase control converters. What is a chopper? A chopper is essentially an electronic switch that turns on the fixed-voltage dc source for a short time intervals and applies the source potential to motor terminals in series of pulses. Phase control converters are used in the speed control of fractional kW dc motors as well as in large motors employed in variable speed reversing drives for rolling mills. What is a controlled rectifier? A controlled rectifier is a device which is used for converting controlled dc power from a control voltage ac supply. 5. 3. What is the main purpose of free wheeling diode? Free wheeling diode is connected across the motor terminal to allow for the dissipation of energy stored in motor inductance and to provide for continuity of motor current when the thyristors are blocked. 8. 7.

What is load commutation? The load current flowing through the thyristor either becomes zero (as in natural or line commutation employed in converters) or is transferred to another device from the conducting thyristor. 10. This is known as current commutation.    Classify commutation. 13. Filter design for variable frequency operation is difficult. What is current commutation? A current pulse is forced in the reverse direction through the conducting thyristor. 15. Slip power recovery control. the thyristor is turned OFF. What are the different means of controlling induction motor?     Stator voltage control. What are the two ways of controlling the RMS value of stator voltage?   Phase control Integral cycle control 16. As the net current becomes zero.Einstein College of Engineering 9. 12. What are the two main difficulties of variable frequency system? Control of Va requires variation of chopper frequency over a wide range. What is voltage commutation? A charged capacitor momentarily reverse-bias the conducting thyristor to turn it off. Mention the two slip-power recovery schemes. a large value of toff makes the motor current discontinuous. Frequency control Pole changing control. 14. Voltage commutation Current commutation. This is known as load commutation. This is known as voltage commutation. . At low voltage. 11.

21. it is used to drive an auxiliary motor which is mechanically coupled to the induction motor shaft. costly and less efficient. Give the basic difference between the two slip-power recovery schemes.   Static scherbius scheme   The scheme has applications in large power fan and pump drives which requires speed control in anrrow range only. The dc source could be converted to ac form by an inverter. 18. then power rating of diode. the setup is bulky. Write short notes on inverter rectifier. What are feeders? . What are the 4 main parts of distribution system?    Feeders. 22. transmission and distribution system of electrical power is called electrical power supply system. 19.  20. Give the special features of static scherbius scheme.. If max.Einstein College of Engineering Static Kramer drive scheme. What is electrical power supply system? The generation. Because of two stage of conversion. slip is denoted by S max. Distributors and Service mains. inverter and transformer can be just Smax times motor power rating resulting in a low cost drive. This drive provides a constant torque control. transformed to a suitable voltage and then rectified to dc form. 17. What are the advantages of static Kramer system. The slip is returned to the supply network in scherbius scheme and in Kramer scheme. it causes less reactive power and smaller harmonic contents of current than a static scherbius. over static scherbius system?    Since a static Kramer system possesses no line commutated inverter.

25.Einstein College of Engineering Feeders are conductors which connect the stations (in some cases generating stations) to the areas to be fed by those stations. 23. What are the different methods of providing neutral earthing?     Solid earthing Resistance earthing Reactance earthing Arc suppression coil or Peterson coil earthing. What do you mean by the term earthing? The term “earthing” means connecting the non-current carrying parts of electrical equipment to the neutral point of the supply system to the general mass of earth in such a manner that at all time an immediate discharge of electrical energy takes place without danger.   24. . capacitance. phase displacement and surge problem. cross section of line conductor is fully utilized. hence much copper is saved. There is no skin effect in dc. No inductance. What are the advantages of high voltage dc system over high voltage ac system?  It requires only tow conductors for transmission and it is also possible to transmit the power through only one conductor by using earth as returning conductor.

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