Single-phase AC motors

Single-phase AC motors run from an ordinary household alternating-current supply rather than a three-phase industrial supply. This makes them common in domestic appliances, but it also creates a fundamental difficulty: a single alternating current in one stator winding produces a pulsating magnetic field rather than a rotating one. A pulsating field exerts no net starting torque on a stationary rotor, so a plain single-phase induction motor will not start on its own. The various single-phase designs are essentially different solutions to this starting problem.

One approach is the shaded-pole motor, which uses a small copper ring around part of each pole to delay the field there, creating a weak rotating effect that nudges the rotor into motion. It is simple and cheap but low in efficiency and torque, suiting small fans and similar light loads. Other induction designs add a second, auxiliary winding, often with a capacitor, to create the phase difference needed for starting.

The universal motor is a different machine altogether: built like a series-wound DC motor, it runs on either AC or DC and can reach high speeds, which is why it powers vacuum cleaners, drills and food mixers. The repulsion motor uses a wound rotor with brushes and offers high starting torque. Each type trades cost, efficiency, torque and speed differently, so the choice depends on the appliance it must drive.

Frequently asked questions

Why won't a single-phase induction motor start by itself?

A single alternating winding produces a pulsating rather than a rotating magnetic field, which gives no starting torque. An auxiliary winding, capacitor or shaded pole is added to create the rotating effect needed to start.

What is a universal motor?

A universal motor is a series-wound motor that runs on either AC or DC. It can reach high speeds and is used in power tools and small household appliances.

Where are shaded-pole motors used?

Shaded-pole motors are simple and inexpensive but have low efficiency and torque, so they are used for light loads such as small fans and clock mechanisms.