Shaded pole induction motor is more or less similar to single-phase induction motor. The stator poles of this motor are wound only with main winding, so as to make them electromagnets. The pole has a slot cut at one third for one end for housing high reactance but low resistance copper bars enclosing a part of the pole. This part is called the shaded part and the other is called the unshaded part. The rotor of such motor is similar to squirrel cage rotor.
Fig. 1. Shaded Pole Induction Motor.
Shaded pole induction motor is the simplest and inexpensive type of motor similar to single-phase induction motor. The stator poles of this motor are wound only with main winding, so as to make them electromagnets. The pole has a slot cut at one third of one end for housing high reactance but low resistance copper bars enclosing a part of the pole. This part is called the shaded part and the other is called the unshaded part. The rotor of such motor is similar to squirrel cage rotor.
Working Principle of Shaded Pole Induction Motor
The required phase-split for making self-start is obtained through induction principle (transformer principle) when a shaded pole motor is supplied with a single-phase A.C supply. This single-phase current produces two pole alternating flux which is as explained follows.
Working of Shaded Pole Induction Motor
Consider a positive half cycle with different time instants i.e t1, t2, t3 as shown in figure (2). Consider one electromagnet in which the current flowing is positive half.
Along Time 0t1 :
During this time, as the exciting current is increasing, there will be an e.m.f generated in the shaded coil and hence large current is set-up. The current flows in a direction so as to oppose its cause and thus reduces the flux under the shaded pole. So most of the flux is concentrated under the unshaded part, thus moving the magnetic axis as shown in figure 3(1).
Along Time t1t2 :
During this time, there is no change in the exciting current and hence the flux will be uniform throughout the pole. This moves the magnetic axis to center of pole as shown in figure 3(ii).
Along Time t2t3 :
During this time, the exciting current is decreasing thus, inducing e.m.f in the shaded coil. The currents flowing in the shading coils are in a direction so as to oppose the exciting current and the flux produced will be adding with the main field flux and thus the magnetic axis moves under the shaded pole as shown in figure 3(iii). This motion of magnetic axis from unshaded part to the shaded part (this effect) is similar to that as if the real poles are actually sweeping in space. So, the rotor starts rotating in the direction of magnetic axis (unshaded part to shaded part). As the torque is very small, this type of motors are generally used in toys, hair driers, desk fans, electric clocks etc.
Torque-Speed Characteristics of Shaded Pole Induction Motor
A shaded pole motor is a self start single-phase induction motor. This property of self start is achieved by splitting the phase by induction principle using shaded ring. So, this motor is thereby named as shaded pole motor. From torque-speed characteristics, it is known that with the increase in speed initially the torque increases and then decrease.
Disadvantages of Shaded Pole Induction Motor
The following are the limitations of shaded pole motors.
- Compact size
- Less power rating
- Poor starting torque
- Less power factor
- Less efficiency due to I2R and copper losses in the shading ring
- Complexity in speed reversal.
Applications of Shaded Pole Induction Motor
Shaded pole motors have the characteristics of low starting torque, low power factor, high losses and hence low efficiency. These are preferred for applications requiring low power ratings in the order of 40 W. Some of the applications of shaded pole motor are, relays, table fans, exhaust fans, hair driers, fans for refrigerators, air conditioning equipments, toys, film projectors, photo copying machines, record players, tape recorders, single-phase synchronous timing motors etc.
Q1. What will be the direction of rotation of a shaded pole single phase induction motor?
Whenever the exciting winding of a single phase induction motor is excited by a single phase A.C supply, the magnetic axis will shift from the unshaded part of the pole to the shaded part of the pole. This shifting is similar to the actual rotation of poles. So, the rotor starts rotating in the direction of magnetic axis. Hence, the direction of rotation of a shaded pole single phase induction motor will be from unshaded part to the shaded part.
Q2. State the method of reversal of rotation of shaded pole motor.
There are two common methods of reversing the rotation of shaded pole motor. They are,
- By using two shaded poles and one main winding
- By using one shaded pole and two main windings.
Figure shows reversing the direction of rotation of a shaded pole meter. In the first method, the switch is moved to the other position, connecting the alternate winding and the previous winding which was being used is disconnected. Due to this other side of the main winding is connected to the shaded pole which causes the rotor to move in opposite direction.
In the second method the two main windings are wounded in the slots such that shaded pole is positioned on the opposite side of each winding. As rotor will move towards the shaded pole, this will cause the rotor to rotate in reverse direction.