# Auto-Transformer Starting of an Induction Motor

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 xV_{1} 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.

_{1}/Z

_{sc }= xI

_{sc}

_{ }

_{st}.V

_{1}= xV

_{1}(xI

_{sc})

_{st}= x

^{2}I

_{sc}

Thus per phase starting current from Supply Mains has now became x^{2} times that of DOL current. Mind that it has reduced as x is less than 1 so x_{2} 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 x^{2} times.

^{2}time of DOL starting current.

_{1})

^{2}where K is constant of proportionality.

_{1}

^{2}where K is constant of proportionality.

^{2}

^{ }

Thus starting torque with Auto Transformer is less than the starting torque with DOL starting by a factor of x^{2}.

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