Question

For a three-phase machine, the three windings are displaced from each other by __________ electrical space degrees along the air-gap periphery.
1. 180
2. 60
3. 90
4. 120

Answer 1

Correct Answer - Option 4 : 120

Three-phase induction motor:

• Three-phase induction motor is self-starting because winding displacement is 120 degrees for each phase and the supply also has a 120 phase shift for 3-phase.
• It results in a unidirectional rotating magnetic field is developed in the air gap which causes a 3-phase induction motor to self-start.

 

Single-phase induction motor:

• The single-phase induction motor is non-self-starting in nature.
• When single-phase AC supply is given to the stator winding of a single-phase motor, it produces alternating flux i.e. which alternates along one space axis only.
• It is not synchronously revolving (or rotating) flux, thus it cannot produce any rotation. Hence single phase induction motor is not self-starting.
• To overcome this problem and to make the motor self-starting, it is temporarily converted into a two-phase motor during starting.
• For this purpose, the stator of a single-phase motor is provided with extra winding known as starting winding in addition to the main winding. These two windings are placed across the single-phase supply.