A shaded-pole motor is one of the simplest and cheapest of manufactured motors. It is essentially an induction machine, since its squirrel-cage rotor receives power in much the same way as does the rotor of the polyphase induction motor. There is however, one extremely important difference between the two. Whereas the poly-phase induction motor creates a true revolving field, in the sense that it is constant in magnitude and rotates at synchronous speed completely round the entire core, the field of the shaded-pole motor is not constant in magnitude but merely shifts from one side of the pole to the other. Because the shaded-pole motor does not create a true revolving field, the torque is not uniform but varies from instant to instant.
Fig. 8 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. 8 (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 afflux in the main segment of each pole, while the shaded segment opposes the main field flux.
- At point (c) shown in Fig. 8 (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. 8 (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. 8 (d)]
A typical torque-speed characteristic is shown in Fig. 9.
- Shaded pole motors are built up to about 40 W.
i. Rugged construction
ii. Cheaper in cost
iii. Small in size
iv. Requires little maintenance
v. Its stalling locked-rotor current is only slightly higher than its normal rated current, so that it can remain stalled for short periods without harm .
i. Very low starting torque
ii. Low efficiency
iii. Low power factor.
Uses. Its low starting torque limits its application to phone-motors or turntables, motion picture projectors, small fans and blowers, bending machines, rotating store-window display tables and relatively light loads.
Reversing shaded-pole motor. Fig. 10 shows a reversing shaded-pole motor with the switch in the clockwise rotation position. This places 1 and 3 actively in the circuit, being short-circuited by
Fig. 10. Reversing shaded pole motor.
the reversing switch. Coils 2 and 4 remain in series with each other, but arc on open circuit and so are inactive, The main-field flux is shown increasing vertically downward, and the arrows on the shading coils show the current in them. It is then seen that the shading coil must be connected in such a manner as not to have their induced voltages in opposition, or there may not be any current in them, and hence no flux lag.