[Part 1 begins with an overview of the operation of thyristor-fed DC motor drives. Part 2 continues with a look at converter output impedance, four-quadrant operation, single- and double-converter reversing drives and power factor and supply effects. Part 3 examines the operation of a standard dc drive system with speed and current control.]
CHOPPER-FED D.C. MOTOR DRIVES
If the source of supply is d.c. (for example in a battery vehicle or a rapid transit system) a chopper-type converter is usually employed. The basic operation of a single-switch chopper was discussed in Chapter 2, where it was shown that the average output voltage could be varied by periodically switching the battery voltage on and off for varying intervals. The principal difference between the thyristor-controlled rectifier and the chopper is that in the former the motor current always flows through the supply, whereas in the latter, the motor current only flows from the supply terminals for part of each cycle.
A single-switch chopper using a transistor, MOSFET or IGBT can only supply positive voltage and current to a d.c. motor, and is therefore restricted to quadrant 1 motoring operation. When regenerative and/or rapid speed reversal is called for, more complex circuitry is required, involving two or more power switches, and consequently leading to increased cost.
Many different circuits are used and it is not possible to go into detail here, though it should be mentioned that the chopper circuit discussed in Chapter 2 only provides an output voltage in the range 0 < E, where E is the battery voltage, so this type of chopper is only suitable if the motor voltage is less than the battery voltage. Where the motor voltage is greater than the battery voltage, a 'step-up' chopper using an additional inductance as an intermediate energy store is used.
Performance of chopper-fed d.c. motor drives
We saw earlier that the d.c. motor performed almost as well when fed from a phase-controlled rectifier as it does when supplied with pure d.c. The chopper-fed motor is, if anything, rather better than the phase-controlled, because the armature current ripple can be less if a high chopping frequency is used.
Typical waveforms of armature voltage and current are shown in Figure 4.13(c): these are drawn with the assumption that the switch is ideal. A chopping frequency of around 100 Hz, as shown in Figure 4.13, is typical of medium and large chopper drives, while small drives often use a much higher chopping frequency, and thus have lower ripple current. As usual, we have assumed that the speed remains constant despite the slightly pulsating torque, and that the armature current is continuous.
Figure 4.13 Chopper-fed d.c. motor. In (a) the transistor is 'on' and armature current is flowing through the voltage source; in (b) the transistor is 'off' and the armature current freewheels through the diode. Typical armature voltage and current waveforms are shown at (c), with the dotted line representing the current waveform when the load torque is reduced by half.
The shape of the armature voltage waveform reminds us that when the transistor is switched on, the battery voltage V is applied directly to the armature, and during this period the path of the armature current is indicated by the dotted line in Figure 4.13(a). For the remainder of the cycle the transistor is turned 'off ' and the current freewheels through the diode, as shown by the dotted line in Figure 4.13(b). When the current is freewheeling through the diode, the armature voltage is clamped at (almost) zero.
The speed of the motor is determined by the average armature voltage, (Vdc), which in turn depends on the proportion of the total cycle time (T) for which the transistor is 'on'. If the on and off times are defined as Ton = kT and Toff = (1 - k)T, where 0 < k < 1, then the average voltage is simply given by
Vdc = kV (4:3)
from which we see that speed control is effected via the on time ratio, k.