The current in the stator of a three phase motor (the stationary coils in the motor) sets up a rotating magnetic field. The magnetic field rotates due to the 120° phase offset in each phase of the power supply. This rotating magnetic field induces a current in the bars of the rotor. The current in the rotor sets up its own magnetic field. The interaction between the stator and rotor magnetic fields causes the rotor to rotate. 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. (See top figure.)
How it “rotates”
Single phase motors work on the same principle as 3 phase motors except they are only run off one phase. A single phase sets up an oscillating magnetic field that goes back and forth rather than a rotating magnetic field (see bottom figure). Because of this a true single phase motor has zero starting torque. However, once the rotor begins to rotate it will continue to rotate as a result of the oscillating magnetic field in the stator.
Through the years engineers have come up with clever ways to start single phase motors. Most of these involve producing a second phase to help produce a rotating magnetic field in the stator. This phase is often called the start phase or the auxiliary phase.
Single Phase Motor Types
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. The main difference in the construction of these motors is in how the second phase is produced. The shaded pole and the split phase motors dont use a capacitor while the permanent split capacitor (PSC) and two value capacitor motors do. The split phase and the two value capacitor motors may use a centrifugal switch to cut out the start phase once the motors get up to speed while the shaded pole and PSC motors dont have a switch.
Each of these motors has different performance trade-offs, as well. Shaded pole motors are very simple motors and usually inexpensive but they have poor efficiency and are generally for low powered applications. Split phase motors are generally low cost motors but they have low starting torque and high starting current. PSC motors offer higher starting torque and higher efficiency than motors with no capacitor.