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Wednesday, 13 November 2019

Shaded Pole Motor Working Principle

In this type of motor, the rotor is of squirrel cage and the stator salient pole type. A part of the pole is cut from the pole face and a copper ring is slipped into it as shown in Fig.

When the pole is excited by the field winding by a single phase supply, current flowing through the field winding produces an alternating flux. The part of the flux passing through the copper ring sinks with it inducing e.m.f. in the copper ring. Since the ring is a closed circuit like that of a coil, and due to induced e.m.f. in the ring, a current flows through the ring which in turn produces flux in the core. The direction of flux due to the current in the copper ring will oppose the main flux resulting in lagging of the flux with respect to the flux in the unshaded portion of the pole face.

Consider the flux produced in Fig. In Fig. (a) when the e.m.f. in the main pole rises from zero, e.rn.f. is induced in the ring which sets up large current. The flux produced by this current opposes the main flux and hence most of the flux shifts to the unshaded portion. In Fig. (b) when the e.m.f. in the main pole is maximum no e.m.f. is induced in the ring and the flux in the shaded and unshaded portion will be equal. The magnetic axis is shifted from the unshaded portion to the centre of the pole. In Fig (c) the e.m.f. in the main pole is decreasing from maximum value. It again induces e.m.f. in the ring.

The current in the ring produces flux which aid the flux in the shaded portion due to'which the flux is strengthened in the shaded portion. The magnetic axis is again shifted from the centre of the pole to the shaded portion of the pole face. The ,total effect correlates to the rotating magnetic field. A starting torque is developed and the rotor starts rotating in a fixed direction. The motor accelerates and runs at a speed lesser than the synchronous speed. The direction of rotation depends upon the position of the shading coil on the pole, since the rotation depends upon the position of the shading coil on the pole. Since the starting torque is less than the capacitor start motors, it is used in clocks, phonographs, instruments, hair dryers, small fans etc. Shaded pole motors are built commercially for small capacity varying from 1/250 h.p. to 1/6 h.p. These motors are simple in construction, rugged, reliable and cheap. The disadvantages in this type of motors are :-

a. it has low starting torque
b. it has very little overload capacity
c. its efficiency is low

A typical torque speed curve of shaded pole motor is shown in Fig.

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