Shaded Pole Motor

SHADED POLE MOTOR

The shaded‒pole motor is very popular for ratings below 0.05 H.P. (~40W) because of its extremely simple construction. It has salient poles on the stator excited by single‒phase supply and a squirrel‒cage rotor as shown in Fig. 5.31. A portion of each pole is surrounded by a short‒circuited turn of copper strip called shading coil.

Working

The working of the motor can be understood by referring of Fig. 5.30. which shows one pole of the motor with a shading coil.

1. During the portion OA of the alternating‒current cycle [Ref Fig. 5.32. (i)], the flux begins to increase and an e.m.f. is induced in the shading coil. The resulting current in the shading coil will be in such a direction (Lenz's law) so as to oppose the change in flux. Thus the flux in the shaded portion of the pole is weakened while that in the unshaded portion is strengthened as shown in Fig. (5.32) (ii).

2. During the portions AB of the alternating‒current cycle, the flux has reached almost maximum value and is not changing. Consequently, the flux distribution across the pole is uniform [Ref Fig. 5.32. (iii)] since no current is flowing in the shading coil. As the flux decreases (portion BC of the alternating cycle), current is induced in the shading coil so as to oppose the decrease in current. Thus the flux in the shaded portion of the pole is strengthened while that in the unshaded portion is weakened as shown in Fig. 5.32. (iv).

3. The effect of the shading coil is to cause the field flux to shift across the pole face from the unshaded to the shaded portion. This shifting flux is like a rotating weak field moving in the direction from unshaded portion to the shaded portion of the pole.

4. The rotor is of the squirrel‒cage type and is under the influence of the moving field. Consequently, a small starting torque is developed. As soon as this torque starts to revolve the motor, additional torque is produced by single‒phase induction motor action. The motor accelerates to a speed slightly below the synchronous speed and runs as a single‒phase induction motor.

Characteristics

(i) The salient features of this motor are extremely simple construction and absence of centrifugal switch.

(ii) Since starting torque, efficiently and power factor are very low, these motors are only suitable for low power applications

Applications

• Small fans

• Toys

• Hair dryers

• Desk fans etc.

The power rating of such motors is upto 30 W