It can easily be seen from the slip torque characteristics of an induction motor that, there is some finite torque when the slip s=1 i.e. speed is zero. This simply means that Induction Motor is a self-starting motor and begins to rotate on its own when connected to a 3 phase supply.
At the instant of starting, a three phase Induction Motor behaves like a Transformer with its secondary winding shorted. Therefore, Induction Motor during starting takes a high current from the supply mains. To limit this high starting current of Induction Motor, different starting methods are used. In this post we will have a look at the Auto-Transformer Starting Method of Induction Motor.
The main philosophy of starting any Induction Motor is to start it at a reduced voltage and as soon as the motor reaches its rated speed, full supply voltage is applied to the terminals of Induction Motor. A schematic diagram for Auto-Transformer Starting of an Induction Motor is given below.
It shall be observed that, using Auto-Transformer we are only applying a reduced voltage xV1 to the Stator terminal of Induction Motor. Here x is less than 1. As soon as the Induction Motor reaches its rated speed, full supply voltage is applied to the terminals of stator.
Thus per phase starting current from Supply Mains has now became x2 times that of DOL current. Mind that it has reduced as x is less than 1 so x2 will be much less than 1. Thus the main advantage of using Auto-Transformer is that it reduces the starting current from the Supply Mains by x2 times.
Thus starting torque with Auto Transformer is less than the starting torque with DOL starting by a factor of x2.