Shaded Pole Induction Motor - Construction, Working Principle & Applications

As we know that a single-phase induction motor is not a self-starting motor. In order to make it self starting, changes are to be done to the construction. Let us see the construction and working principle of the shaded pole induction motor.

Construction of Shaded Pole Induction Motor :

The construction of a shaded type induction motor is similar to a normal single-phase induction motor, except for its stator pole. The stator poles of a shaded pole motor are divided into two parts.

One part of the pole consists of a short-circuited coil made up of copper which is known as the shading coil (shaded part) and the remaining part is known as the unshaded part of the pole, hence the motor is known as a shaded pole induction motor.

The construction of the rotor is of a normal squirrel cage type of a 3-phase induction motor. The below figure shows the single-phase two-pole induction motor with a shading coil on both the stator poles. Check out this animated video for more detailed explanations.

Working Principle of Shaded Pole Induction Motor :

When a single-phase a.c. supply is given to the stator winding. The poles of the motor generate a magnetic field Φ_m, but there is one more magnetic field Φ_s produced by shading coils either in the same or opposite direction.

The combination of these fields makes a rotating magnetic field which makes the rotor rotate. Let us consider three different cases at different time instants t₁, t₂, and t₃ on a positive half cycle of an ac supply.

Case-1 (At instant t = t₁) :

As seen from the waveform, at instant t = t₁. The current in the coil increases, which in turn increases the flux Φ_m produced by the coil. Now due to supply is alternating, the rate of change of flux will tends to cause an emf induced in a shading coil of that pole.

As the shading coil is short-circuited, currents will flow through it. Simultaneously causes to produces another flux Φ_s by the shading coil, which in the opposite direction to the main flux as shown below.

Now due to the opposition of these two fluxes produced by the shading coil and main winding. The net flux across the area of the shading coil will be zero. Therefore, the magnetic axis of the net flux will be at the center of the unshaded part.

Case-2 (At instant t = t₂) :

Now, at instant t = t₂ i.e., the maximum or peak value of the cycle or current. Here further there will be no increase in the current (no change of current). So it decreases the rate of change of flux, which also decreases the emf induced in the shading coil. At this point, the flux Φ_s produced by the shading coil will be almost negligible.

Hence the flux Φ_m produced by the main winding will be uniformly distributed along the pole. Therefore, the magnetic axis of the pole will be at the center of the whole pole (with shaded and unshaded parts) as shown below.

Case-3 (At instant t = t₃) :

At this instant, the rate of change of current will be in decreasing. As the current changes, there will be an induced emf in the shading coil due to the change of flux.

But here the direction of the fluxes Φ_m and Φ_s produced by both the main winding and shading coil will be in the same direction. Also there will be crowding of flux in the shaded part as compared to the unshaded part. Due to this, the net magnetic axis of the pole will be at the center of the shading part of that pole, as shown below.

This sequence of instants keeps on repeating for the negative half cycle also. As it will result in the production of a rotating magnetic field. By which the motor tends to start on its own. The starting torque produced by this type of motor will be 50% to 60% of the full-load torque.

Advantages of Shaded Pole Induction Motor :

1. Simple in construction.
2. Cheap.
3. Extremely rugged.
4. No centrifugal switch is required.

Disadvantages of Shaded Pole Induction Motor :

1. Starting torque is poor.
2. Efficiency is very low due to copper losses in the shading ring.
3. Very limited over-load capacity.

Speed Reversal :

The speed reversal is very difficult. To reverse the direction of rotation. two sets of shading rings are to be provided on both portions of the poles. By opening any ring and closing the other ring particular direction of rotation can be achieved. But the method is complicated and expensive.

Applications of Shaded Pole Induction Motor :

These are used in small fans, toys, hair, dryers film projectors. advertising displays etc., where starting torque required is very low.