Shaded pole motor operating principle

A shaded-pole motor is a simple single-phase induction motor in which the starting rotating field is produced by a short-circuited copper ring (the shading ring) placed on part of each salient pole.

Construction

• Stator: salient poles with a concentrated exciting winding fed from the single-phase supply. Each pole is slotted, and about one-third of the pole face is surrounded by a heavy copper ring (shading coil).
• Rotor: ordinary squirrel-cage rotor.
• No capacitor, no centrifugal switch, no auxiliary winding.

        +----------------------+
        |  unshaded  | shaded  |
        |    part    | [ring]  |  pole face
        +----------------------+
               ( rotor )
   field moves: unshaded ---> shaded

Principle of operation

The flux in the shaded part is made to lag the flux in the unshaded part. Consider one half cycle of the current:

1. Current rising quickly (near zero): the rapidly changing flux induces a large current in the shading ring. By Lenz's law it opposes the change, so most flux passes through the unshaded part. Flux axis is at the unshaded part.
2. Current near peak (little change): almost no emf is induced in the ring, so flux is spread uniformly over the pole. Flux axis is at the centre of the pole.
3. Current falling: the ring current now opposes the decrease, so flux is concentrated in the shaded part. Flux axis moves to the shaded part.

Thus the flux axis sweeps from the unshaded to the shaded part every half cycle. This moving field (two fluxes displaced in space and in time) behaves like a weak rotating field and induces currents in the cage rotor, producing torque in the direction unshaded → shaded.

Characteristics

• Very low starting torque (about 40–50% of full-load torque)
• Low power factor and low efficiency (5–35%) because of copper loss in the ring
• Direction fixed by construction; cannot be reversed electrically
• Speed fairly constant; ratings up to about 1/20 kW (some up to 1/4 HP)

Applications

Table and ceiling fans of small size, exhaust fans, hair dryers, record players, small blowers, toys, advertising displays and electric clocks.