Study Unit 4

Question 1

How does a synchronous motor work?

Answer 1

1. Constant speed is achieved by the interaction between a constant and rotating magnetic field. The rotor produces a constant magnetic field. The stator produces a rotating magnetic field.
2. The field coil of the stator is excited by a three phase AC supply which produces a revolving magnetic field which rotates at synchronous speed.
3. The rotor is either excited by a DC power supply so it acts like a permanent magnet or it can be a permanent magnet.
4. When the rotor and stator poles are of the same polarity, they exercise a repulsive force on one another and then a moment later, an attractive force. The alternation between these two forces causes the rotor and stator to spin continuously, causing a continuous flow of electricity.

Question 2

How does an induction motor work?

Answer 2

1. When a three phase AC supply passes through the windings of the stator, magnetic flux is produced.
2. An emf is induced in the stator windings, according to Faraday’s law.
3. The emf produces a current through the stator.
4. The current produces another magnetic flux, within the rotor this time. This flux lags behind the stator flux causing the rotor to experience torque. The torque allows the rotor to continuously rotate in the magnetic field’s direction, causing a continuous flow of electricity.