444 Electrical Power Systems Technology
Dynamic Braking
When a motor is turned off, its shaft will continue to rotate for a
short period of time. This continued rotation is undesirable for many ap-
plications. Dynamic braking is a method used to bring a motor to a quick
stop whenever power is turned off. Motors with wound armatures utilize
a resistance connected across the armature as a dynamic braking method.
When power is turned off, the resistance is connected across the armature.
This causes the armature to act as a loaded generator, making the mo-
tor slow down immediately. This dynamic braking method is shown in
Figure 16-18.
AC induction motors can be slowed down rapidly by placing a DC
voltage across the winding of the motor. This DC voltage sets up a con-
stant magnetic field, which causes the rotor to slow down rapidly. A circuit
for the dynamic braking of a single-phase AC induction motor is shown in
Figure 16-19.
Universal Motor Speed Control
An important advantage of a universal AC/DC motor is the ease of
speed control. The universal motor has a brush/commutator assembly,
with the armature circuit connected in series with the field windings. By
varying may be varied from zero to maximum.
The circuit used for this purpose, shown in Figure 16-20 uses a gate-
controlled triac. The triac is a semiconductor device whose conduction
may be varied by a trigger voltage applied to its gate. A silicon-controlled
rectifier (SCR) could also be used as a speed-control device for a universal
motor. Speed-control circuits like this one are used for many applications,
Figure 16-17. Control circuit for the forward and reverse operation of a three-
phase AC induction motor