1. Split phase motor:
The main winding has low resistance but high reactance and starting winding has high resistance but low reactance.
Current in the auxiliary winding called starting winding lags the supply voltage by lesser angle. The current in the main winding being highly inductive lags the supply voltage with greater angle. Now there are two currents which are not in phase with each other. As shown in the figure Im is the current in the main winding lagging the voltage by angle φm and Is, the current in the starting winding lagging the voltage by angle φs. The angle between the two currents is θ as shown in the figure. The torque produced is such that it gives circular movement to the rotor and is proportional to the sine of angle θ. A centrifugal switch which is normally closed is incorporated in series with the starting winding. When the motor comes up to speed about 75% of synchronous speed it opens automatically with the help of centrifugal force and puts the starting winding out of circuit.
Typical torque speed characteristics is shown here.
Capacitor Start Motor:
The starting winding has a capacitor in series with it. The motor gives high starting torque due to large value of C.
Torque speed characteristics is shown below.
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