Construction of Shaded Pole Induction Motor
A shaded pole induction motor is a simple type of self-starting single-phase induction motor. It consists of a stator and a cage-type rotor.
The stator is made up of salient poles. Each pole is slotted on the side, and a copper ring is fitted on the smaller part. This part is called the shaded pole. The ring is usually a single-turn coil and is known as shading coil. Shading band or ring is a loop of conductor placed around the part of pole, which is then called Shaded pole.
When single phase supply is applied across the stator winding, an alternating field is created. The flux distribution is non-uniform due to shading bands on the poles. The shading band acts as a single turn coil and when links with alternating flux, emf is induced in it. The emf circulates current as it is simply a short circuit.
The current produces the magnetic flux in the shaded part of pole to oppose the cause of its production which is the change in the alternating flux produced by the winding of motor. Now consider three different instants of time t1, t2, t3 of the flux wave to examine the effect of shading band as shown in the figure.
At instant t1: The flux is positive and rising, hence the shading band current producesits own flux to oppose the rising main flux. Due to this opposition, the net flux in shaded portion of pole is lesser than that in unshaded portion. Thus the magnetic axis lies in the unshaded portion and away from shaded portion.
At instant t2: The flux is maximum, the rate of change of flux is zero. So the shading band emf and current are zero. Thus the flux distribution among shaded and unshaded portion is equal. The magnetic axis lies in the centre of the pole.
At instant t3: The flux is positive but decreasing, hence according to Lenz’s rule, the shading band emf and current try to oppose the fall in the main flux. So the shading band current produces its own flux which aids the main flux. Since shading band produces aiding flux in shaded portion, the strength of flux in shaded portion increases and the magnetic axis lies in the shaded portion.
Thus it is seen that as time passes, the magnetic neutral axis shifts from left to right in every half cycle, from non-shaded area of pole to the shaded area of the pole. This gives to some extent a rotating field effect which is sufficient to provide starting torque to squirrel cage rotor and rotor rotates.