[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