Construction of Single-Phase Induction Motors

Single-phase motors run from a single-phase supply. They are the most common electric motors in homes and light industry, usually in fractional-kW ratings.

Manufacturers have developed many designs to meet these needs. By construction and starting method, single-phase motors fall into three broad classes: induction motors, synchronous motors and commutator motors. This part of the course deals mainly with the first.

The topics covered are: construction and qualitative behaviour (this chapter), principle of operation, double-revolving-field theory and torque, types and starting methods, and the equivalent circuit.

A single-phase induction motor is built much like a three-phase one, with two differences: the stator carries a single-phase distributed winding, and an auxiliary (starting) winding is added.

• Stator: a laminated iron core with slots. It holds two windings, the main and the auxiliary, displaced 90° electrical in space. Both are distributed in slots to give a sinusoidal MMF distribution.

• Rotor: a squirrel cage. The laminated, slotted iron core has aluminium bars cast into the slots and short-circuited at both ends by end rings.

A single-phase supply to the main winding produces an alternating flux. It pulsates along one fixed space axis, unlike the synchronously rotating flux of a two-phase or three-phase winding. A pulsating flux cannot start a stationary cage rotor; only a rotating flux can.

• The pulsating MMF is equivalent to two equal MMF waves rotating in opposite directions. At standstill they produce equal and opposite torques, so the net starting torque is zero.

• If the rotor is given an initial push in either direction, by hand or by a small motor, a net torque develops in that direction and the motor accelerates to its final speed, provided the load torque is not too high.

• The auxiliary winding breaks the symmetry at starting, creating a net torque in the intended direction.

• Single-phase motors are small, fractional-kW machines in widespread domestic and light industrial use.

• The stator has main and auxiliary windings 90° apart in space. The rotor is a squirrel cage.

• A single-phase winding produces a pulsating field with zero starting torque. The auxiliary winding provides starting.

1. MCQ 1. A single-phase winding alone produces
   (a) a rotating field (b) a pulsating field (c) no field (d) a DC field

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   (b) a pulsating field.

2. MCQ 2. The main and auxiliary windings are displaced by
   (a) 45° (b) 90° (c) 120° (d) 180° electrical

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   (b) 90°.

3. MCQ 3. If a single-phase motor is pushed in the reverse direction at start, it will
   (a) not run (b) run in the reverse direction (c) run forward (d) oscillate

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   (b) run in the reverse direction.