Auto-transformer Starter in Induction Motor

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.

Auto-transformer Starter
Auto-transformer Starter

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.

Overload protection is provided by thermal overload relays.

Theory of Auto-transformer Starter

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

Theory of Auto-transformer Starter
Theory of Auto-transformer Starter

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;

    \[ I_{st} = \frac{V_1}{Z_{e10}} \]

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

Auto-transformer Starter Circuit Diagram
Auto-transformer Starter Circuit Diagram

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;

    \[ I_{stm} = \frac{x V_1}{Z_{e10}} \]

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

    \[ \frac{I_1}{I_2} = \frac{V_2}{V_1} \]

    \[ V_1 I_1 = V_2 I_2 \]

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

    \[ V_1 I_{st}' = (xV_1) I_{stm} \]

    \[ I_{st}' = x I_{stm} \]

    \[ I_{st}' = x \frac{x V_1}{Z_{e10}} \]

    \[ I_{st}' = \frac{x^2 V_1}{Z_{e10}} \]

(1)   \begin{equation*} I_{st}' = x^2 I_{st} \end{equation*}

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

    \[ T_{st} \propto V_1^2 \]

    \[ T_{st} = K_2 V_1^2 \]

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

    \[ T_{stm} \propto (xV_1)^2 \]

    \[ T_{stm} = K_2 x^2 V_1^2 \]

(2)   \begin{equation*} T_{stm} = x^2 T_{st} \end{equation*}

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.

    \[ x^2 =\frac{1}{3} \]

    \[ x = \frac{1}{\sqrt{3}} = 0.58 \]

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.


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

  • The size of the starter is large.
  • High cost
  • The poor power factor for low HP motors.
  • Need to be installed properly.

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