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.

Therefore, per phase starting current of Motor = xV1/Zsc = xIsc
 
Here Isc is the current through the stator during direct switching of motor.
 
Thus we observe that starting current of Motor has reduced and is x times that of current during DOL (Direct Online) starting.
 
Again,
 
Input VA of Auto-Transformer = Output VA of the Auto-Transformer
 
Ist.V1= xV1(xIsc)
 
Therefore,
 
Per phase starting current from the Supply Mains Ist = x2Isc

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.

Note that starting current is the motor winding is x times while the starting current from the supply mains has became x2 time of DOL starting current.
 
Now we will have a look at Torque of Induction Motor.
 
As the torque of an Induction Motor is directly proportional of square of applied voltage at the stator terminals, therefore
 
Starting Torque with Auto-Transformer Test = K(xV1)2 where K is constant of proportionality.
Starting Torque with DOL Test = KV12 where K is constant of proportionality.
Therefore,
 
Test with Auto-Transformer/ Test with DOL = x2
 

Thus starting torque with Auto Transformer is less than the starting torque with DOL starting by a factor of x2.

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