Question

Which of the following is NOT a method of starting Single Phase Induction Motor?
1. Direct switching
2. Phase splitting
3. Pole shading
4. Repulsion start

Answer 1

Correct Answer - Option 1 : Direct switching

Concept:

• The Single-phase induction motor is not self-starting. Hence it requires starting circuit
• It will rotate in the direction of the magnetic rotation provided by starting or auxiliary winding and capacitor.
• When  AC is supplied to the stator of a single-phase induction motor, it produces a flux of magnitude ϕm.
• According to the double field theory, this alternating flux ϕm is divided into two components of magnitude ϕm / 2 each and rotates in the opposite direction, with the synchronous speed NS.
• The resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at that particular instant.
• At starting both the flux components are exactly opposite to each other and are equal in magnitude. So, they cancel each other and hence net torque produces by the rotor at the starting condition is zero and the single-phase induction motor is not self-starting.
• Hence, to start the motor starting windings are used.

​Direct switching is not a method of starting a Single Phase Induction Motor as it draws more and more current and burns the machines.