How does an AC motor work?

Unlike toys and flashlights, most homes, offices, factories, and other buildings aren’t powered by little batteries: they’re not supplied with DC current, but with alternating current (AC), which reverses its direction about 50 times per second (with a frequency of 50 Hz). If you want to run a motor from your household AC electricity supply, instead of from a DC battery, you need a different design of motor.

In an AC motor, there’s a ring of electromagnets arranged around the outside (making up the stator), which are designed to produce a rotating magnetic field. Inside the stator, there’s a solid metal axle, a loop of wire, a coil, a squirrel cage made of metal bars and interconnections (like the rotating cages people sometimes get to amuse pet mice), or some other freely rotating metal part that can conduct electricity. Unlike in a DC motor, where you send power to the inner rotor, in an AC motor you send power to the outer coils that make up the stator. The coils are energized in pairs, in sequence, producing a magnetic field that rotates around the outside of the motor.

Photo: The stator makes a magnetic field using tightly wound coils of copper wire, which are known as the windings. When an electric motor wears out, or burns out, one option is to replace it with another motor. Sometimes it’s easier to replace the motor windings with new wire—a skilled job called rewinding, which is what is happening here. Photo by Seth Scarlett courtesy of US Navy.

How does this rotating field make the motor move? Remember that the rotor, suspended inside the magnetic field, is an electrical conductor. The magnetic field is constantly changing (because it’s rotating) so, according to the laws of electromagnetism (Faraday’s law, to be precise), the magnetic field produces (or induces, to use Faraday’s own term) an electric current inside the rotor. If the conductor is a ring or a wire, the current flows around it in a loop. If the conductor is simply a solid piece of metal, eddy currents swirl around it instead. Either way, the induced current produces its own magnetic field and, according to another law of electromagnetism (Lenz’s law) tries to stop whatever it is that causes it—the rotating magnetic field—by rotating as well. (You can think of the rotor frantically trying to “catch up” with the rotating magnetic field in an effort to eliminate the difference in motion between them.) Electromagnetic induction is the key to why a motor like this spins—and that’s why it’s called an induction motor.

Photo: An efficient AC induction motor. Photo by Al Puente courtesy of NREL.

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