
D.C. Motor Drives 123
CONTROL ARRANGEMENTS FOR D.C
DRIVES
The most common arrangement, which is used with only
minor variations over the whole size range from small servo-
type drives up to the largest industrial drives, is the so-called
two-loop control. This has an inner feedback loop to con-
trol the current (and hence torque) and an outer loop to
control speed. When position control is called for, a further
outer position loop is added.
A two-loop scheme for a thyristor d.c. drive is discussed
first, but the essential features are the same in a chopper-fed
drive. Later the simpler arrangements used in low-cost small
drives are discussed. ...