The main difference in the construction of these motors is in how the second phase is produced. Some of the different types of single phase motors are the shaded pole motor, the split phase motor, the permanent split capacitor motor (also called the single value capacitor motor), and two value capacitor motor. This phase is often called the start phase or the auxiliary phase. Most of these involve producing a second phase to help produce a rotating magnetic field in the stator. Through the years engineers have come up with clever ways to start single phase motors. However, once the rotor begins to rotate it will continue to rotate as a result of the oscillating magnetic field in the stator. Because of this a true single phase motor has zero starting torque. A single phase sets up an oscillating magnetic field that goes back and forth rather than a rotating magnetic field (see bottom figure).

Single phase motors work on the same principle as 3 phase motors except they are only run off one phase. One important thing to note for 3 phase motors is that because they run off of three phases that are offset from each other, they are self-starting. The interaction between the stator and rotor magnetic fields causes the rotor to rotate. The current in the rotor sets up its own magnetic field. This rotating magnetic field induces a current in the bars of the rotor. The magnetic field rotates due to the 120° phase offset in each phase of the power supply. The current in the stator of a three phase motor (the stationary coils in the motor) sets up a rotating magnetic field.

In order to understand how an AC induction single phase motor operates, it is helpful to understand the basics of a three phase induction motor.

10 hp single phase motor starter