Mod 6 AC induction motors (1 Viewer)

[NexusRich]

Can someone please explain how AC induction motors work ? I get the part where they use rotating magnetic fields to induce a current in the conductor bars of the rotor. But I dont really understand how the 3 - phase induction works ( its graph and cycle).
The textbook said that the 3 - phase produced the rotating magnetic field, does it mean that with only one set of magnets in an induction motor, it would not work ? so all induction motors are 3 - phase ?

 

[ezOolong]

in short:
- both single-phase and 3-phase induction motors will work
- the graph and cycle is just how the name sounds like (these three are 120˚ out of phase, so that when you consider the magnetic field produced by these electromagnets, it will be rotating around right)

And from a student's perspective, you aren't really expected to know how exactly the 3-phase generator/motor functions like you do with the DC motors; just need to know a rough description of it so that you can write something for a long response question in the HSC.



excerpt from jacaranda:

Household electric motors are single-phase motors. This is because houses are usually supplied with only one phase of the three phases that are produced in power stations. It is not important to understand how the rotating magnetic field is achieved in single-phase AC induction motors; therefore, this topic will concentrate on the three-phase motor, as its workings are easier to visualise.

In both single- and three-phase AC induction motors, the stator sets up a rotating magnetic field that has a constant magnitude. The stator of a three-phase induction motor usually consists of three sets of coils that have iron cores. The stator is connected to the frame of the motor and surrounds a cylindrical space in which it sets up a rotating magnetic field. In three-phase induction motors, this is achieved by connecting each of the three pairs of field coils to a different phase of the mains electrical supply. The coils that make a pair are located on opposite sides of the stator and they are linked electrically. The magnetic field inside the stator rotates at the same frequency as the mains supply; that is, at 50 Hz. A cutaway diagram of a stator is shown in Figure 7.33.

The magnetic field rotates at exactly the same rate as the electromagnet in the power station generator that provides the AC electricity. Each pair of coils in the stator of the generator supplies a corresponding pair of coils in the stator of the motor. Therefore, the magnetic field in the motor rotates at exactly the same rate as the electromagnet in the generator. This is represented in Figure 7.34.