Operation of AC Induction Motors

AC Induction Motor Explained (Video)

This video analyses the components and operation of AC induction motors in terms of underlying electromagnetism principles.

 

How Do AC Induction Motors Work?

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). 

Stator

The stator of an AC induction motor consists of multiple pairs of electromagnets that are connected to an AC power supply. In a classic three-phase AC induction motor, there are three pairs of electromagnets which are switched on at different times throughout the motor's operation. These electromagnets are referred to as 'out of phase'.

 

 

The following graph below illustrates the output of electromagnets in a three phase AC induction motor.

 

 

The magnitude of each electromagnet is determined by its electrical output. Since the outputs are deliberately made out of phase, their corresponding magnetic fields are also out of phase. The three pairs of electromagnets are switched on and off in a sequential manner to produce a rotating magnetic field. The same phenomenon occurs in AC induction motors that use more than three phases.

Rotor

The rotor of an AC induction motor consists of multiple conductive 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 produced by the stator causes the rotor to experience a change in magnetic flux. By Faraday's Law, emf is induced in the bars.

By Lenz’s Law, the direction of the resultant induced current are oriented to oppose the change in magnetic flux. This in turn produces force (Motor Effect) and torque on the rotor, allowing it to rotate 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 minimise the changes in magnetic flux.

Since the induced emf and current are resulted from the changes in magnetic flux experienced by the rotor bars, the speed of the rotor should always be slower than the speed of the magnetic field to generate torque. The small delay between the magnetic field speed and the rotor speed is referred to as the slip.

In a real induction motor, 0% slip is physically impossible while generating torque. If the rotor were to spin at the exact same speed as the magnetic field (synchronous speed), there would be no relative motion between the rotor bars and the magnetic field lines. Thus, no magnetic flux would be cut by the conductors, and no emf and current would be induced. In the absence of induced current, zero force and torque would be produced.

Analysis of AC Induction Motor

Advantages

• 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

• Induction motors do not use brushes. 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.

Disadvantages

• 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 pair of magnets in conventional motors.

• When the induction motor is starting, the slip is at its greatest (rotor is stationary, and magnetic field is rotating). This causes induction motors to have low starting torque, which gradually increases as the rotor increases in speed (slip decreases). 

Test Your Understanding

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)

 

Previous section: Operation of Simple DC and AC Generators

Next section: Lenz's Law in DC Motors and Magnetic Braking

 

RETURN TO MODULE 6: ELECTROMAGNETISM