4-15
Figure 4-13.—Shaded poles as used in shaded-pole ac induction motors.
The motion of the field back and forth between shaded and unshaded portions produces a weak
torque to start the motor. Because of the weak starting torque, shaded-pole motors are built only in small
sizes. They drive such devices as fans, clocks, blowers, and electric razors.
Q13.
Why are shaded-pole motors used to drive only very small devices?
Speed of Single-Phase Induction Motors
The speed of induction motors is dependent on motor design. The synchronous speed (the speed at
which the stator field rotates) is determined by the frequency of the input ac power and the number of
poles in the stator. The greater the number of poles, the slower the synchronous speed. The higher the
frequency of applied voltage, the higher the synchronous speed. Remember, however, that neither
frequency nor number of poles are variables. They are both fixed by the manufacturer.
The relationship between poles, frequency, and synchronous speed is as follows:
where n is the synchronous speed in rpm, f is the frequency of applied voltage in hertz, and p is the
number of poles in the stator.
Let’s use an example of a 4-pole motor, built to operate on 60 hertz. The synchronous speed is
determined as follows:
