Stator resistance starting

 Stator resistance starting

In this method, external resistances are connected in series with each phase of stator winding during starting. 

This causes voltage drop across the resistances so that voltage available across motor terminals is reduced and hence the starting current. 

The starting resistances are gradually cut out in steps (two or more steps) from the stator circuit as the motor picks up speed. 

When the motor attains rated speed, the resistances are completely cut out and full line voltage is applied to the rotor see Figure.

This method suffers from two drawbacks. 

First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. 

Secondly a lot of power is wasted in the starting resistances.