Split-phase induction motors

The motor of Figure 3.5a is called a split-phase induction motor. A set of starting windings has been added on poles that are rotated 90° from the running windings. Compared to the running windings, the starting windings have fewer turns of higher resistance wire. Because of the lower resistance and higher inductance of the starting windings, the current in the starting windings is displaced in phase from the running windings (see Figure 3.5b). Consequently, a clockwise-rotating magnetic field is established that starts the rotor into clockwise motion. A centrifugal switch in series with the starting windings opens when the motor has attained sufficient speed and the motor then continues to run as an ordinary induction motor. If the leads are brought out to the terminal housing, the direction of rotation can be reversed by reversing the leads, A and B, of the starter-windings circuit. The split-phase motor is generally available in sizes from 0.04 to 0.25 kW (1 /20 to 1 /3 hp). Compared to other single-phase induction motors, it has higher starting current and lower starting torque. It is best suited to fans and other machines that do not start under heavy load.

Figure 3.5 – Illustration of a split-phase induction motor