# Auto-transformer Starter in Induction Motor

An auto-transformer starter is used in induction motors and it is suitable for both star and delta-connected motors.

In an auto-transformer starter, the starting current is limited by using a three-phase auto-transformer to reduce the initial stator supply voltage.

The auto-transformer is provided with tapings. The schematic diagram of the auto-transformer starter is shown in the figure below.

To obtain the most suitable starting voltage, the stator is connected to one particular tapping.

A double-through switch is used to connect the auto-transformer in a circuit to start the motor.

When the handle H of the switch S is in the START position, the primary winding of the auto-transformer is connected to the supply line and the secondary winding is connected to the motor.

When the motor picks up the speed (about 80% of rated speed), the handle H is quickly moved to the RUN position.

The auto-transformer is disconnected from the circuit and a motor is directly connected to supply lines. Now, the motor gets full rated voltage.

The handle is held in the RUN position by the under-voltage relay.

If the supply voltage fails or decreases below a certain value, the handle is released and returns to the OFF position.

## Theory of Auto-transformer Starter

The below figure shows the motor is directly connected with the supply.

Ze10 = Equivalent standstill impedance per phase of motor referred to as stator side

V1 = supply voltage per phase

When full load voltage V1 per phase is given to the motor, the starting current drawn from the supply is;

The below figure shows the motor is connected with supply lines via an auto-transformer.

When the motor is connected via auto-transformer, the voltage per phase is xV1 (where x is a transformation ratio of auto-transformer).

Hence, the starting current of motor is given by;

In transformer, the current ratio is inversely proportional to the voltage ratio (assumed the no-load current is neglected).

If the current taken from the supply by auto-transformer is Ist’,

(1)

The torque developed is proportional to the square of the applied voltage, the starting torque with direct switching is;

Similarly, the starting torque when the motor is connected via auto-transformer;

(2)

From the above equation, with an autotransformer, the starting current and starting torque of an induction motor is reduced to x2 times their corresponding values with direct on-line starting.

In star-delta starter, the reduction factor is 1/3. Hence, if we take x2 is equal to 1/3, the auto-transformer is equivalent to the star-delta starter.

The star-delta starter is cheaper than an auto-transformer starter and is most commonly used for both small and medium size motors.

The advantages of an auto-transformer starter are listed below.

• Starting is simple.
• Number taps are available in the auto-transformer. Hence, we can choose any suitable tap to be connected to.
• No power loss at starting compared to stator resistance starter.
• Suitable for high HP rating motor above 25 HP.
• A long starting period can be selected.
• Suitable for both star and delta-connected motor.