A shaded pole induction motor is a type of self-starting 1-phase induction motor. It consists of a stator and a squirrel cage type rotor. The stator of the shaded pole motor is made up of salient poles and each pole is slotted on a side and a copper ring is fitted on the smaller part of the slot as shown in the figure. The part with copper ring is known as shaded pole and the copper ring is known as shading coil which is usually a single turn coil.
The main winding surrounds the entire pole and is given the 1-phase AC supply. The poles of a shaded pole motor are always laminated to minimise the iron losses.
In order to understand that how the shading coil helps the shaded-pole induction motor to set up rotating magnetic field. To do so, consider the changes in the magnetic flux produced by the main winding at three intervals, which are given as follows:
When the magnetic flux is increasing from zero to maximum.
When the flux is maximum.
When the flux is decreasing from maximum to zero.
Any change in the magnetic flux in each pole of the motor induces an EMF in the shading coil and this induced EMF circulates a current in it. This induced current always sets up a magnetic flux that opposes the change in the flux in the shaded portion (according to Lenz’s law).
Refer the waveform of the AC supply, during the portion OA of the waveform, the magnetic flux in the pole is increasing and so is the current induced in the shading coil. The shading coil produces a magnetic flux that opposes the increase in the magnetic flux linking the coil. Consequently, most of the magnetic flux passes through the unshaded portion of the pole as shown in the figure below. Therefore, the magnetic axis of the flux is the centre of the unshaded portion of the pole.
At point A of the current waveform, the magnetic flux has reached at its maximum value. Therefore, the rate of change of the flux is zero. Thus, the induced EMF and the current in the shading coil being zero. As a result of it, the distribution of the magnetic flux across the pole is uniform as shown in the figure below. Consequently, the magnetic axis of the flux shifts to the centre of the pole.
During the portion AB of the current waveform, the magnetic flux produced by the main winding begins to decrease. The current induced in the shading coil reverses the direction in order to oppose the decrease in the flux produced by the main winding i.e. the shading coil produces a magnetic flux which passes through the shaded portion of the pole as shown in the figure below. Therefore, the magnetic axis of the flux is shifted to the centre of the shaded portion of the pole.
From the about discussion it is clear that the presence of the shading coil forces the magnetic axis to shift from unshaded to shaded part of the pole. This shifting flux is just like rotating magnetic field which rotates from unshaded to shaded portion of the pole.
As the rotor of the shaded pole induction motor is of the squirrel cage type and is under the influence of rotating magnetic field. As a result, a small starting torque is developed. As soon as the starting torque starts to revolve the rotor, an additional torque is produced by the single-phase induction motor action. Hence, the motor accelerates to a speed slightly below the synchronous speed and runs as a 1-phase induction motor.
The characteristics of a shaded pole induction motor are given below −
The construction of the shaded pole induction motor is very simple.
These motors have low starting torque, efficiency and power factor.
The reversal of the direction of rotation in the shaded pole motors is not possible.
The shaded pole motors are very cheap.
The shaded pole motors are built only in small sizes of the power rating of the order of 40 W or less.
As the shaded pole induction motors have low starting torque, efficiency and power factor, so these motors are only suitable for low power applications such as −
To drive the loads which require low starting torque.
In table fans
In exhaust fans
Fans for refrigeration & air-conditioning equipment
In electronic equipment, etc.