How does an AC induction motor work?

How does an AC induction motor work?

Here’s a little animation to summarize things and hopefully make it all clear:

1. Two pairs of electromagnet coils, shown here in red and blue, are energized in turn by an AC supply (not shown, but coming in to the leads on the right). The two red coils are wired in series and energized together and the two blue coils are wired the same way. Since it’s AC, the current in each coil doesn’t switch on and off abruptly (as this animation suggests), but rises and falls smoothly in the shape of a sine wave: when the red coils are at their most active, the blue coils are completely inactive, and vice-versa. In other words, their currents are out of step (90° out of phase).
2. As the coils are energized, the magnetic field they produce between them induces an electric current in the rotor. This current produces its own magnetic field that tries to oppose the thing that caused it (the magnetic field from the outer coils). The interaction between the two fields causes the rotor to turn.
3. As the magnetic field alternates between the red and blue coils, it effectively rotates around the motor. The rotating magnetic field makes the rotor spin in the same direction and (in theory) at almost the same speed.

Induction motors in practice

What controls the speed of an AC motor?

Photo: A variable-frequency motor. Photo by Warren Gretz courtesy of NREL.

In synchronous AC motors, the rotor turns at exactly the same speed as the rotating magnetic field; in an induction motor, the rotor always turns at a lower speed than the field, making it an example of what’s called an asynchronous 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’s the “phase” of an AC motor?

We don’t necessarily have to drive the rotor with four coils (two opposing pairs), as illustrated here. It’s possible to build induction motors with all kinds of other arrangements of coils. The more coils you have, the more smoothly the motor will run. The number of separate electric currents energizing the coils independently, out of step, is known as the phase of the motor, so the design shown above is a two-phase motor (with two currents energizing four coils that operate out of step in two pairs). In a three-phase motor, we could have three coils arranged around the stator in a triangle, six evenly spaced coils (three pairs), or even 12 coils (three sets of four coils), with either one, two, or four coils switched on and off together by three separate, out-of-phase currents.

Animation: A three-phase motor powered by three currents (indicated by the red, green, and blue pairs of coils), 120° out of phase.

Advantages and disadvantages of induction motors

Advantages

The biggest advantage of AC induction motors is their sheer simplicity. They have only one moving part, the rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free. DC motors, by contrast, have a commutator and carbon brushes that wear out and need replacing from time to time. The friction between the brushes and the commutator also makes DC motors relatively noisy (and sometimes even quite smelly).

Artwork: Electric motors are extremely efficient, typically converting about 85 percent of the incoming electrical energy into useful, outgoing mechanical work. Even so, there is still quite a bit of energy wasted as heat inside the windings—which is why motors can get extremely hot. Most industrial-strength AC motors have built-in cooling systems. There’s a fan inside the case attached to the rotor shaft (at the opposite end of the axle that’s driving whatever machine the motor is attached to), shown here in red. The fan sucks air into the motor, blowing it around the outside of the case past the heat ventilating fins. If you’ve ever wondered why electric motors have those ridges on the outside (as you can see in the top photo on this page), that’s the reason: they’re cooling the motor down.

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.

Who invented the induction motor?

Artwork: Nikola Tesla’s original design for the AC induction motor. It works in exactly the same way as the animation up above, with two blue and two red coils alternately energized by the generator over on the right. This artwork comes from Tesla’s original patent deposited at the US Patent and Trademark Office, which you can read for yourself in the references below.

Nikola Tesla (1856–1943) was a physicist and prolific inventor whose many amazing contributions to science and technology have never been fully acknowledged. After he arrived in the United States at the age of 28, he began working for the famous electrical pioneer Thomas Edison. But the two men fell out disastrously and soon became bitter rivals. Tesla firmly believed that alternating current (AC) was far superior to direct current (DC), while Edison thought the opposite. With his partner George Westinghouse, Tesla championed AC, while Edison was determined to run the world on DC and dreamed up all kinds of publicity stunts to prove that AC was too dangerous for widespread use (inventing an electric chair, to prove that AC could be lethal, and even electrocuting Topsy the elephant with AC to show how deadly and cruel it was). The battle between these two very different visions of electric power is sometimes known as the War of the Currents.

Despite Edison’s best (or worst) efforts, Tesla won the day and AC electricity now powers much of the world. That’s largely why many of the electric motors that drive the appliances in our homes, factories, and offices are AC induction motors, powered by rotating magnetic fields, which Nikola Tesla designed in the 1880s (his patent, illustrated here, was granted in May 1888). An Italian physicist named Galileo Ferraris independently came up with the same idea at around the same time, but history has treated him even more cruelly than Tesla and his name is now all but forgotten.