Single Phase Induction Motors Questions and Answers

Answer: A

The two counter-rotating field components give equal and opposite torques at standstill, so nothing starts it, but once turning in either direction the balance is broken and it develops net torque. That is why every practical machine has some starting arrangement.

Answer: B

Each component behaves like the field of a three-phase machine, one giving forward torque and one backward, and their torques cancel at standstill. Once the rotor moves, the slips with respect to the two fields differ and a net torque appears.

Answer: C

The backward field revolves the other way, so the relative speed is the sum rather than the difference, which gives a slip close to two in normal running. The backward torque is therefore small and the forward torque dominates.

Answer: D

At a slip near two it is on the falling part of its characteristic and contributes little torque, but it still drives current in the rotor and heats it. That extra loss is one reason such machines are less efficient than three-phase ones.

Answer: A

Rotation carries rotor conductors across the stator flux and induces a speed emf whose current sets up a flux in quadrature with the main one, and the two together give a revolving resultant. It reaches the same conclusions as the double field theory by a different route.

Answer: B

Correct answer: in whichever direction it was pushed

With only one winding energised the field pulsates rather than revolves, and it can be resolved into two equal fields turning in opposite directions. At standstill their torques cancel exactly. A push breaks the symmetry: the rotor now slips less against the field turning with it, that component produces the greater torque, and the machine accelerates the way it was started.

Answer: C

The auxiliary winding's current leads the main winding's by perhaps thirty degrees, which is enough to make the field elliptical and give a net starting torque. The displacement is small, so the starting torque is modest.

Answer: D

The winding is wound with fine wire and would burn out if left in circuit, so a switch releases at around three quarters of synchronous speed. Failure of that switch to open is a common cause of burnt-out auxiliary windings.

Answer: A

A phase displacement approaching quadrature makes the field much more nearly circular, and the starting torque rises accordingly to perhaps three times full-load value. The capacitor is switched out with the auxiliary winding.

Answer: B

Correct answer: an electrolytic type rated for intermittent duty

A start capacitor needs a large capacitance in a small volume and is in circuit only for a second or two at each start, so an electrolytic type is used. It cannot carry current continuously without overheating, which is precisely why the centrifugal switch must disconnect it once the machine is up to speed.