HOW INDUCTION MOTOR WORKS??

Basic learning of induction motor for additional knowledge.

Induction motors in practice

What controls the speed of an AC motor?

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.

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).

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.
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.

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