Chapter 16 - Induction Motors

Question 1

Explain the operation of a three phase induction motor?

Answer 1

• A Magnetic Field moving passed a Conductor induces an E.M.F in the Conductor.
• Installing various conductors around the rotor would cause differing E.M.F’s to be induced at different times
• Shorting out various conductors with end rings will cause current to flow within the conductors
• We now have current flowing in a conductor sitting in the magnetic field. Hence we have motion and the rotor will rotate. Has per Fleming’s Left Hand rule.

Question 2

Explain the construction of a squirrel cage rotor?

Answer 2

• The Squirrel Cage Rotor consists of a number of Copper or Aluminium Conductors Short Circuited at each end via End Rings

Question 3

State why the rotor speed can never equal the synchronous speed in a Three phase induction motor?

Answer 3

The rotor must rotate at a speed lower than that of the rotating magnetic field if relative motion is to exist between the two The rotor rotates at less than synchronous speed, this percentage difference between the speed of the rotating stator field and the speed of the rotor is called “Slip”

Question 4

State the Advantages & Disadvantages of a Double Caged Rotor?

Answer 4

Advantages

1. High Starting torque.
2. Reduced starting current.

Disadvantages

1. Max torque is less than single cage rotor of the same size.
2. Copper losses in the high resistance cages at startup may cause burnout if frequent starts attempted

Question 5

What do the terms synchronous & Asynchronous mean?

Answer 5

• Synchronous mean to be in sync (Spining at the same speed an in the same location)
• Asynchronous means they are not in sync (Not Spinning at the same speed or location)

Question 6

Describe a Rotating Flux?

Answer 6

• The Rotating Flux is the magnetic flux field created in the stator and has current is passed through the conductors this creates a magnetic flux.
• With a 3 phase motor there would be a red phase conductor a Blue phase conductor and a yellow phase conductor Current flow happens at different time due to the phase shift of 120 Degrees.
• This creates a rotating flux which causes the rotor to turn by inducing an E.M.F in its embedded short circuited conductors.