Shaded Pole Motor

SHADED POLE MOTOR

Construction

Shaded pole motor is a split phase type single phase induction motor. It has salient poles on the stator excited by single phase supply and a squirrel cage rotor is shown in fig.3.82. A portion of each pole is surrounded by a short circuited turn of copper strip called shading coil. It has no commutator, brushes, collector rings, contactors, capacitors or moving switch parts and it is relatively cheaper, simpler and extremely rugged in construction and reliable.

Operation

Fig.3.83 shows one pole of the motor with a shading coil.

(i) During the portion OA of the alternating current cycle, the flux begins to increase 210115 and an emf is induced in the shading coil. The resulting current in the shading ni be coil will be in a direction as to oppose change in flux. Thus the flux in the shaded portion of the pole is weakened and the unshaded portion is strengthened is shown in fig (ii)

(ii) During the portion A to B of the alternating current, the field flux has reached almost maximum value and is not changing. Consequently the flux distribution across the pole is uniform since no current is flowing in the shading coil is shown in fig (iii)

(iii) As the flux decreases (ie) portion B to C of the alternating current, 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 and the unshaded portion is weakened is shown in fig (iv)

The effect of the shading coil is to cause the flux to shift across the pole face from the unshaded to the shaded portion. This shifting of flux is like a rotating weak field moving in the direction from unshaded portion to the shaded portion of the pole. The rotor is squirrel cage type. A small starting torque is developed.

This torque starts to resolve the rotor, 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.

Torque-speed characteristics

Fig.3.84 shows the torque-speed characteristics of a shaded pole motor.

The main disadvantages are

i) Low efficiency

ii) Low power factor

iii) Very low starting torque

Applications:

i) Fans

ii) Blowers

iii) Turntables

iv) Hair driers

v) Motion picture projectors

Characteristics

(i) The rated starting torque is 40% to 60%.

(ii) The rated breakdown torque is unto 140%.

(iii) The power factor of the motor is in the range of 0.25 to 0.4.

(iv) The efficiency is 15% to 40%.

 (v) The power rating is upto 40W.