Electrical Engineering ⇒ Topic : Construction and principle of shaded pole motor

Fig (a) shows the general construction and principle of shaded pole motor.

Construction. Each of the laminated poles of the stator has a slot cut across the laminations about one-third the distance from one edge. Around the smaller of the two areas formed by this slot is placed a heavy copper short-circuited coil, called a shading coil ; the iron around which the shading coil is placed is called the shaded part of the pole, while the free portion of the pole is the unshaded part. The exciting coil surrounds the entire coil.

Principle of Operation. When the exciting winding is connected to an A.C. source of supply,the magnetic axis will shift from the unshaded part of the pole to the shaded part of the pole. This shift in the magnetic axis is, in effect equivalent to an actual physical motion of the pole ; the result is that the squirrel cage rotor will rotate in a direction from the unshaded part to the shaded part.The shifting of flux is explained below

Refer Fig. (b). When the flux in the field poles tend to increase, a short-circuit current is induced in the shading coil, which by Lenz's law opposes the force and the flux producing it. Thus, as the flux increases in each field pole, there is a concentration of flux in the main segment of each pole, while the shaded segment opposes the main field flux.

                                                            General construction and principle shaded pole motor.

At point (c) shown in Fig.(e), the rate of change of flux and of current is zero, and there is no voltage induced in the shaded coil. Consequently, the flux is uniformly distributed across the poles [Fig. (c)].

When the flux decreases, the current reverses in the shaded coil to maintain the flux in the same direction. The result is that the flux crowds in the shaded segment of the pole [Fig.(d)].