In case of split phase induction motor we use resistance for creating phase difference but here we use capacitor for this purpose.We are familiar with this fact that the current flowing through the capacitor leads the voltage.
So the shaded ring flux opposes the main flux, which leads to the crowding of flux in non shaded part of stator and the flux weaken in shaded part.We know that the running winding is inductive in nature.Our aim is to create the phase difference between the two winding and this is possible if the starting winding carries high resistance. We know that for highly resistive winding the current is almost in phase with the voltage and for highly inductive winding the current lag behind the voltage by large angle.This non uniform distribution of flux causes magnetic axis to shift in the middle of the non shaded part.When the flux remains almost constant at its maximum value- In this region the rate of rise of current and hence flux remains almost constant.
Design of single phase capacitor start induction motor
The single phase induction motors are made self starting by providing an additional flux by some additional means.Now depending upon these additional means the single phase induction motors are classified as: In addition to the main winding or running winding, the stator of single phase induction motor carries another winding called auxiliary winding or starting winding.So, in capacitor start inductor motor and capacitor start capacitor run induction motor we are using two winding, the main winding and the starting winding. With starting winding we connect a capacitor so the current flowing in the capacitor i.e I.When the flux decreases from maximum positive value to zero - In this region the rate of decrease in the flux and hence current is very high.
Since the copper band is short circuit the current starts flowing in the copper band due to this induced emf. Now according to Lenz's law the direction of the current in copper band is such that it opposes its own cause i.e decrease in current.Hence there is very little induced emf in the shaded portion.The flux produced by this induced emf has no effect on the main flux and hence distribution of flux remains uniform and the magnetic axis lies at the center of the pole.In order to clearly understand the working of shaded pole induction motor consider three regions- When the flux changes its value from zero to nearly maximum positive value – In this region the rate of rise of flux and hence current is very high.According to Faraday's law whenever there is change in flux emf gets induced.