4-15Figure 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 weaktorque to start the motor. Because of the weak starting torque, shaded-pole motors are built only in smallsizes. 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 MotorsThe speed of induction motors is dependent on motor design. The synchronous speed (the speed atwhich the stator field rotates) is determined by the frequency of the input ac power and the number ofpoles in the stator. The greater the number of poles, the slower the synchronous speed. The higher thefrequency of applied voltage, the higher the synchronous speed. Remember, however, that neitherfrequency 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 thenumber of poles in the stator.Let’s use an example of a 4-pole motor, built to operate on 60 hertz. The synchronous speed isdetermined as follows: