The performance of a motors with various ratings vary as follows: slip varies directly as the output; starting torque and starting current vary inversely with the output. Frequent starting, reversing or heavy starting conditions may necessiate selection of a motor with an output rating higher than that normally required for continuous duty. The normal design criterion is that the motor shall be capable of two starts in succession at rated load torque, inertia and temperature, followed by a cooling period of 30 minutes before attempting a further starting sequence. At rated voltage, the motor can withstand 1.25 to 1.5 times the rated current for two minutes without suffering damage or permanent deformations.
While selecting motors for a particular application the torque requirements, speed, run-up time and starting current are the most important features which are to be considered. The standard motors can withstand a starting up time of 5 to 7 seconds for two consecutive starts from the cold condition. The following table shows the starting characteristics of different induction motors with various combinations of starting devices:
|Motor type||Starter||Starting Torque (pu)||Starting Current (pu)|
|Squirrel cage||DOL||1.5 – 2.5||5.0 – 7.5|
|Star Delta||0.3 – 0.6||1.3 – 2.5|
|Slip Ring||Rotor Resistance||1.0 – 2.0||1.3 – 2.5|
Table of Contents
Enclosures for Motors
Various types of enclosures are provided for the motors to withstand ambient conditions. The categorization of industrial protection (enclosure) is as below: The enclosures for the electrical equipments (motors, generators and contactors) are classified according to their capability to withstand the entry of solid particles and liquids. There are six classes of protection against solid particles given as ‘A’ and eight classes of protection against liquids given as ‘B’. The category will be read as IPAB. A stands for ingress of solid particles and B for ingress of liquids.
|First Character Numeral||‘A’ Protection against ingress of solids||Second Character Numeral||‘B’ Protection against ingress of liquid|
|1||52.5 mm thick particle||1||Drop of condensed water|
|2||12.5 mm thick particle||2||Drop of liquid|
|3||2.5 mm thick particle||3||Rain|
|4||1.0 mm thick particle||4||Splashing|
|5||Harmful deposit of dust||5||Water jets|
|6||Complete ingress of dust||6||Splashing on a deck of a ship|
|8||Indefinite immersion in water under specified pressure|
Normally for motors IP 44, IP 54, and IP 55 classes are available. The International Electro Technical Commission have standardized certain dimensions for a range of motor capacities. The IEC frame no denotes the center height in mm from the base of the motor to the center of the shaft. The suffix S/M/L indicates short/medium/long frame length respectively.
Selection of Protective Devices
For the starting / speed control of slip-ring motor, the resistance of the controller should be designed correctly, so that sufficient starting torque can be obtained. Since the cooling effect of the fan is reduced as the speed decreases, torque and output must be reduced in accordance with the following table, if the motor runs at reduced speed for prolonged periods, otherwise a large motor should be selected. If ventilated separately say be ducting etc, the motor may be capable of delivering the full load torque at any speed.
The rated current of a given contactor depends on the duty to be performed by the contactor which are: AC1 (Resistive), AC2 (Rotor resistance switching), AC3 (Normal cage motor starting and stopping on load), AC4 (Inching). AC3 duty requires about six times the rated current to be made at the rated voltage. The magnetic coil rating required shall also be checked while buying a contactor. When a contactor is to be used for frequent starting and stopping AC4 duty ratings are to be taken.
The bi-metallic overload relays used in conjunction with the contactors are normally suitable for 10 to 15 switching cycles per hour. However the switching cycle is limited to 2 to 3 due to the limitation of allowed switching frequency of the motors. The adjustable relay shall have the motor full load current at the midpoint of the overload relay range. The relays protect themselves against the overloads approximately upto 10 times the maximum setting. Beyond this i.e. in the short circuit zone, the relay must be protected either by fuses or circuit breakers.
The time-current characteristics of the overload relay and the fuses must be matched for the different protection system. The fuse has to be selected so that when its time current characteristics is superimposed on that of the overload relay, it takes over for fault current beyond the safe limit of the overload relay, as shown in the figure below:
For currents lower than the take-over point, the overload device will operate leaving the fuse intact; and for higher current, the fuse will operate. Since the starting currents are different for different motors with direct-on-line and assisted starts, the recommended backup fuse ratings are also different. Usually 6 to 7 times the full load current for 10 seconds is assumed in the case of DOL start motors and about 3 times for 20 seconds for assisted start motors.
Other motor protection devices include single phasing preventors, thermistor motor protection relays (which senses the overheating of motor winding and passes a signal to trip the starters.).
The various tables for selection of various devices are as below:-
|HP Rating||Frame size||GD2 of rotor (kgf.m²)||Rated speed (rpm)||Rated Current (A)||Rated torque (kgf⋅m)||Efficiency (%)||Power factor||Starting current (pu)||Starting torque (pu)||Pullout torque (pu)|
|HP Rating||Frame size||GD2 of rotor (Kgf.m2)||Rated speed (rpm)||Rated Current (A)||Rated torque (Kgf.m)||Efficiency (%)||Power factor||Rated rotor current (A)||Rotor voltage at stand still||Pullout torque (pu)|
|HP Rating||Full load current (A)||Per Phase current (A)||D.O.L. STARTING||STAR DELTA STARTING|
|Contactor rating (A)||O/L relay range (A)||HRC fuse (A)||Contactor rating (A)||O/L relay range (A)||HRC fuse (A)|
|1||1.9||1.10||10||1.0 – 2.5||6||10||1.5 – 2.5||6|
|1.5||2.5||1.45||10||1.0 – 2.5||6||10||2.5 – 4.0||10|
|2||3.4||1.97||10||1.5 – 2.5||6||10||2.5 – 4.0||15|
|3||4.8||2.77||16||2.5 – 4.0||6||10||4.0 – 6.5||15|
|4||6.4||3.70||16||2.5 – 4.0||10||10||6.0 – 10.0||20|
|5||7.8||4.50||16||4.0 – 6.5||10||10||6.0 – 10.0||20|
|7.5||11.6||6.70||16||6.0 – 10.0||15||16||10.0 – 16.0||25|
|10||14.4||8.31||16||6.0 – 10.0||15||16||10.0 – 16.0||35|
|12.5||17.3||10.00||16||6.0 – 10.0||20||25||13.0 – 21.0||50|
|15||21.1||12.18||16||10.0 – 16.0||25||25||20.0 – 25.0||50|
|20||28.0||16.17||16||10.0 – 16.0||35||32||20.0 – 32.0||63|
|25||35.0||20.20||32||20.0 – 32.0||63||40||28.0 – 40.0||80|
|30||41.0||23.67||32||20.0 – 32.0||63||40||28.0 – 40.0||100|
|35||48.0||27.71||32||20.0 – 32.0||80||63||45.0 – 63.0||120|
|40||55.0||31.76||40||28.0 – 40.0||80||63||45.0 – 63.0||120|
|45||62.0||35.80||40||28.0 – 40.0||100||63||45.0 – 63.0||160|
|50||69.0||39.84||40||28.0 – 40.0||100||125||60.0 – 100.0||160|
|60||83.0||47.92||63||45.0 – 63.0||100||125||60.0 – 100.0||200|
|70||97.0||56.00||63||45.0 – 63.0||125||125||90.0 – 125.0||200|
|80||110.0||63.58||125||60.0 – 100.0||160||125||90.0 – 125.0||200|
|90||123.0||71.02||125||60.0 – 100.0||160||125||90.0 – 125.0||250|
|100||136.0||78.52||125||60.0 – 100.0||160||300||120.0 – 200.0||320|
|125||171.0||98.73||125||60.0 – 100.0||200||300||120.0 – 200.0||320|
|150||200.0||115.47||125||90.0 – 150.0||200||300||120.0 – 300.0||350|
|175||231.0||133.37||125||90.0 – 150.0||250||300||180.0 – 300.0||400|
|200||263.0||151.85||300||120.0 – 200.0||300||300||180.0 – 300.0||500|
|225||293.0||169.36||300||180.0 – 300.0||500|
|250||324.0||187.28||400||263.0 – 400.0||500|
|300||385.0||225.54||400||263.0 – 400.0||500|
|350||449.0||259.53||630||376.0 – 630.0||500|
|400||505.0||291.91||630||376.0 – 630.0||600|