Shaded pole starting of a single phase induction motor- The single phase induction motors are
The stator of the shaded pole single phase induction motor has projected poles or salient. These poles are shaded by copper band or ring which is inductive in nature. These poles are devided into two unequal halves. The smaller portion carries band and is called as shaded portion of the pole.
Principle: When a single phase supply is given to the stator of shaded pole induction motor an alternating flux is produced. This flux induces emf in the shaded coil. Since this shaded portion is short circuited. The current is produced in it such a direction to oppose the main flux. The flux in shaded pole lags behind the flux in the unshaped pole. The phase different between these two fluxes produces resultant rotating flux. In order to clearly understand the working of shaded pole induction motor we have to consider three regions.
- When the flux changes from zero to nearly maximum positive value.
- When the flux became almost constant at its maximum value.
- When the flux decreases from maximum positive to zero.
Regions 1: In this region the rate of rise of flux and hence whenever there is change in flux emf gets induced. Since the copper band is short circuit the current starts flowing in the copper band due to this induced emf. This current in the copper band produces its own flux.
Region 2: In this region the rate of rise of current hence the flux remains almost constant. So there is very little induced emf in the shaded portion. The flux produced by this induced emf has don’t have any effect on the main flux and for this distribution of flux remains uniform and the magnetic axis lies at the center of the pole.
Region 3: In this region, the rate of decrease in the flux and hence current is very high. According to Faraday’s law when there is change in flux emf will get induce. Since the copper band is short circuit the current starts flowing in the copper band due to this emf induced. This current produces its own flux. Now according to Lenz’s law the direction of the current in copper band opposes its own cause. i.e. decrease in current. This shifting of magnetic axis continues for negative cycle also leads to the production of rotating magnetic field.
Also Read- Losses of 3- phase Induction Motor