The speed of a fractional-horsepower motor is determined by the number of poles the motor has and the frequency of the current supplied to it. This equation is used to determine the synchronous speed of an ac motor: where “f” is the frequency (cycles per second) of the power supply, and p is the number of poles.
The shaded-pole motor shown here has four poles. Therefore, assuming a line frequency of 60 Hz, its synchronous speed is 1,800 rpm.
However, because a motor usually operates under a load, its rotor does not rotate at exactly the synchronous speed. The difference between the synchronous speed and the lower actual speed is called slip, and is usually about 3 to 4% for fractional-horsepower motors.