An Auto transformer starter basically uses an auto-transformer to reduce the voltage applied to a motor during its start. As we know that the starting of big motors which are used for prolonged periods exert large current in the circuit. Due to this, a voltage drop occurs which may damage other electrical equipments which are running on the same supply line.
As we know, Voltage is directly proportional to Current, i.e. V∞I. It is therefore, one can understand that if we decrease the voltage supply, the starting current will also decrease. Voltage can be decreased by many ways. One method to decrease the input voltage supplied to the motor is to decrease it with the help of a transformer.
The same reduced voltage is supplied to the motor till the time it picks up its normal running speed. As soon as the motor come to its normal running speed, the step down voltage is replace with the primary full voltage line. However, It should be kept in mind that the transformer which fulfills the purpose in such type of starter is not a normal step down transformer which has different primary and secondary winding. This transformer consists only a single winding which is used for input as well as output.
Such type of transformers have multiple taps for giving outputs. Generally, tapping of auto transformer is located at fifty percent, sixty five percent and eighty percent voltage. These tapping are used to provide various voltage options for starting the motor. These transformers are easy in operation, light in weight and cost less amount of money than a normal transformer with two windings.
Such type of starter does not need complex cabling. This starter is connected to the motor with three terminals only.At the time of the starting of motor, it is fed by the auto transformer’s output. Uses of auto transformer starter reduces the starting current of motor to the square of the voltage ratio of the transformer.