Single Phase AC Motors

Most home and business appliances operate on single phase AC power. For this reason, single-phase AC motors are in widespread use. A single-phase induction motor is larger in size, for the same horsepower, than a three-phase motor. When running, the torque produced by a single phase motor is pulsating and irregular, contributing to a much lower power factor and efficiency than that of a polyphase motor. Single-phase AC motors are generally available in the fractional to 10-hp range and all use a solid squirrel-cage rotor.

The single-phase induction motor operates on the principle of induction, just as does a three-phase motor. Unlike three phase motors, they are not self-starting. Whereas a three-phase induction motor sets up a rotating field that can start the motor, a single-phase motor needs an auxiliary means of starting. Once a single-phase induction motor is running, it develops a rotating magnetic field.

However, before the rotor begins to turn, the stator produces only a pulsating, stationary field.

A single-phase motor could be started by mechanically spinning the rotor, and then quickly applying power. How ever, normally these motors use some sort of automatic starting. Single-phase induction motors are classified by their start and run characteristics. The three basic types of single-phase induction motors are the split-phase, split phase capacitor, and shaded-pole.
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