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