Nikola Tesla's AC motor was a game-changer. But how does it work?

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The two main types of AC motors are induction motors and synchronous motors. The induction motor (asynchronous motor) always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the rotor AC winding. 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.

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.

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.

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What controls the speed of an AC motor?

The theoretical speed of the rotor in an induction motor depends on the frequency of the AC supply and the number of coils that make up the stator and, with no load on the motor, comes close to the speed of the rotating magnetic field. In practice, the load on the motor (whatever it's driving) also plays a part—tending to slow the rotor down. The greater the load, the greater the "slip" between the speed of the rotating magnetic field and the actual speed of the rotor. To control the speed of an AC motor (make it go faster or slower), you have to increase or decrease the frequency of the AC supply using what's called a variable-frequency drive. So when you adjust the speed of something like a factory machine, powered by an AC induction motor, you're really controlling a circuit that's turning the frequency of the current that drives the motor either up or down.

What are the disadvantages?

Since the speed of an induction motor depends on the frequency of the alternating current that drives it, it turns at a constant speed unless you use a variable-frequency drive; the speed of DC motors is much easier to control simply by turning the supply voltage up or down. Though relatively simple, induction motors can be fairly heavy and bulky because of their coil windings. Unlike DC motors, they can't be driven from batteries or any other source of DC power (solar panels, for example) without using an inverter (a device that turns DC into AC). That's because they need a changing magnetic field to turn the rotor.