Operation of AC Induction Motors

How Does an AC Induction Motor Work?

This video analyses the operation of AC induction motors.

 

Operation of AC Induction Motors

AC induction motors are different to conventional DC motors because they do not use the motor effect to generate mechanical energy; instead it makes use of Faraday and Lenz’s Law.

AC induction motors are split in to two sections: the stator (stationary parts) and the rotor (rotating parts). The stator consists of 3 pairs of electromagnets that are connected to an AC power supply. The electromagnets are out of phase by 120° relative to each other. This is why the name 'three-phase induction motor' is commonly used to refer to an AC induction motor. 

The graph below illustrates the 3 phase-nature of each electromagnet pair. Because of their voltages being out of phase, the magnetic fields produced by each electromagnet will also be out of phase. The superposition of these out-of-phase magnetic fields gives the effect of a rotating magnetic field.

 

The rotor consists of bars attached to two end rings, as shown below.

 

 

This structure closely resembles a squirrel cage, as is why the rotor is commonly called a 'squirrel cage rotor'. The rotating magnetic field causes the rotor to experience a change in magnetic flux and thus an induced emf. This induced emf causes eddy currents to run along the bars. 

 

By Lenz’s law, these eddy currents are oriented to oppose the change in magnetic flux. The effect is that the rotor will spin in the same direction as the changing magnetic field. In essence, it attempts to ‘catch up’ to the rotating external magnetic field in order to restore the flux it was previously experiencing.

 

The rotational speed of the rotor is always slower than that of the external magnetic field. This is because if it moves at the same rate as the magnetic field, there would be no change in magnetic flux experienced by the rotor. As a result, moving at the same speed leads to no induced current and thus movement.

 

Analysis of AC Induction Motor

Advantages

Disadvantages

·      Rotation of squirrel cage is friction-free as it is not physically in contact with any other component. Therefore, it is associated with less maintenance and repair   ·      No need for brush and commutators hence no induction of sparks when the commutators ‘scrape’ past the brush. As a result, less energy loss; AC induction motor is more energy efficient   ·      More reliable because the rotational speed can be controlled by manipulating the electromagnets. Unlike back EMF in a DC motor, the induced current in an induction motor is beneficial.

·      Limited rotational speed/frequency leads to limited applications   ·      Complex set-up mainly due to the need for at least three phases of electromagnets instead of just one in conventional motors     ·      Low starting torque. The rotor needs to ‘warm-up’ gradually to desired speeds

 

Practice Question 1

Explain how an AC induction motor functions. (4 marks)

 

Practice Question 2 

Compare DC motor and AC induction motor in terms of their:

(a) Function (1 mark)

(b) Components (2 marks)

(c) Underlying physics principles (2 marks)

(d) Applications (2 marks)

 

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