
120 Electric Motors and Drives
a b c
Figure 4.6 Stages in motor reversal using a single-converter drive
e.m.f.
changes sign (as the motor reverses) the converter volt-
age again becomes positive and greater than E, as shown in
Figure 4.6(c). The converter is then rectifying, with power
being fed into the motor, and the system is operating in
quadrant 3.
Schemes using reversing contactors are not suitable where
the reversing time is critical, because of the delay caused by
the switchover time, which may easily amount to
200-400ms. Field reversal schemes operate in a similar
way, but reverse the field current instead of the armature
current. The ...