380 Electrical Power Systems Technology
Figure 14-23 shows the operational principle of the repulsion motor.
This motor has a wound rotor that functions similarly to a squirrel-cage
rotor. It also has a commutator/brush assembly. The brushes are shorted
together to produce an effect similar to the shorted conductors of a squir-
rel-cage rotor. The position of the brush axis determines the amount of
torque developed and the direction of rotation of the repulsion motor.
In position 1, Figure 14-23A, the brush axis is horizontally aligned
with the stator poles. Equal and opposite currents are now induced into
both halves of the rotor. Thus, no torque is developed with the brushes in
this position. In position 2 (Figure 14-23B), the brushes are placed at a 90°
angle to the stator field poles. The voltages induced into the rotor again
counteract one another, and no torque is developed. In position 3 (Figure
14-23C), the brush axis is shifted about 60° from the stator poles. The cur-
rent flow in the armature now causes a magnetic field around the rotor.
The rotor field will now follow the revolving stator field in a clockwise
direction. As might be expected, if we shift the brush axis in the opposite
direction, as shown in position 4 (Figure 14-23D), rotation reversal will
result. Thus, magnetic repulsion between the stator field and the induced
rotor field causes the rotor to turn in the direction of the brush-shift.
Repulsion-start induction motors, and some similar types of modi-
fied repulsion motors, have very high starting torque. Their speed may
be varied by varying the position of the brush axis. However, the me-
chanical problems inherent with this type of motor have caused it to be-
come obsolete.
Figure 14-22. Illustration of the operational principle of the shaded-pole AC in-
duction motor: (A) A single-phase AC alternation, (B) Time t 1 (C) Time t 2 , (D)
Time t 3 , (E) Time t 4