1. Necessity of Starter
A Squirrel cage induction motor just before starting is similar to a poly phase transformer with a short-circuited secondary. If normal voltage is applied to the stationary motor then, as in the case of a transformer, a initial current, to the turn of 5 to 6 times the normal current or the normal rated current, will be drawn by the motor from the mains. This initial excessive current is objectionable, because it will produce line voltage (The potential difference or the voltage across two phases) drop, which in turn will affect the operation of the other electrical equipments and the lights connected to the same line.
The initial rush of current is controlled by applying a reduced voltage to the stator (motor) winding during the starting period and then the full normal voltage is applied when the motor has run up to speed. For the small capacity motors say up to 3H.P, full normal voltage can be applied at the start. How ever to start and stop the motor and to protect the winding from the over load current and low voltages, a starter is required in the motor circuit. In addition to this the starter may also reduce the applied voltage to the motor at the time of starting.
Types of Starter as invented
Following are the different types of starter used for starting of the Squirrel cage induction motor.
1. Direct on line Starter(DOL Starter)
2. Star-Delta Starter(Hand operated)
3. Auto Star-Delta Starter
4. Step down transformer starter
5. Auto-transformer starter
In the above starters, except for the Direct on-line starter, reduce voltage is applied to the stator (motor) winding of the squirrel cage induction motor at the time of starting and regular voltage is applied once the motor pick up the speed.
The entire above invented starter can not be used as multipurpose operation.
Many factors must be considered when selecting starting equipment. These factors include starting current, the full load current, voltage rating of motor, voltage (line voltage) drop, cycle of operation, types of load, motor protection and also safety of the operator.