M6-S7: AC Induction Motor

  • Function: motor converts electrical energy into mechanical energy
  • AC induction motor is different to conventional DC motor because it does not use the motor effect to generate mechanical energy; instead it makes use of Faraday and Lenz’s Law.

  • Stator provides the external magnetic field that is fixed in position
    • Consists of paired electromagnets connected to AC power supply. AC is essential as each electromagnet must alternate in direction throughout a cycle.
    • The external magnetic field is turned on sequentially such that only opposing electromagnets (pair) are on at any given instance. The sequence of magnetic activation creates a magnetic field that changes orientation in a rotating manner.
    • Each pair of electromagnets is referred to as a single phase. Typically, AC induction motors use three phases, if not more.

 

  • Rotor (squirrel cage) is a free-to-move conductor located within the stator
    • Activation of electromagnets in the manner described above creates a changing magnetic field. By Faraday’s law of electromagnetic induction, eddy currents are induced in the squirrel cage.
    • By Lenz’s law, these eddy currents’ accompanying magnetic field opposes the changing magnetic flux by spinning the rotor in the same direction as the changing magnetic field. In essence, it attempts to ‘catch up’ to the external magnetic field in order to restore the orientation of magnetic field it was previously experiencing.
    • Consequently, this transforms electrical energy initially in the electromagnet into mechanical energy in the form of rotor’s spinning motion.
    • Rotational frequency is equivalent to frequency of AC used.
  • The rotational speed of the rotor is always slower than the ‘movement’ 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.
  • When the induction motor is connected to a load, the speed is further reduced.

 

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

 

Investigating the Mechanism of AC Induction Motor in Laboratory

Equipment:

  • Metal disc suspended from a retort stand; usually can be made from simple aluminium foil
  • Permanent magnet
  • Electric drill
Method:
  • The permanent magnet is brought to close proximity to the metal disc
  • The permanent magnet is made to rotate using the electric drill (analogous to the stator in an AC induction motor)
  • The relative movement of the magnet causes a change in magnetic flux experienced by the metal disc. As a result, by Faraday’s law, a current is created in the disc. By Lenz’s law, this current will generate a force to oppose the field of the permanent magnet.
  • Observation
    • The metal disc will move in the same direction as the spinning magnet as the electrons attempt to restore the number of magnetic flux it was originally experiencing
    • This creates a rotating movement, simulating the rotation of the rotor in an AC induction motor

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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