There are various methods of starting electric motors which are given below.
1. Full voltage starting:
In this type of starter full line voltage to the motor terminals is given. This is also called 'direct-on-line-starting'. D.C. motors up to 2 kW and squirrel cage induction motors as well as certain small synchronous motors up to 4 or 5 kW are usually direct on line-started.
2. Reduced voltage starting:
In order to avoid heavy starting current and the consequent voltage dip in the supply lines majority of motors are started by applying a reduced voltage to their terminals and subsequently increasing it to its normal value.The starting of a dc motor is, often, accomplished by the addition of suitable external resistance in the armature circuit and the starting controller is arranged so that this resistance is short-circuited in steps as the motor comes up to speed. Reduced voltage starting of induction motors is achieved by (I) stator resistance starting, (ii) stator reactor starting, (iii) star-delta starting, and (iv) autotransformer starting. .
The disadvantage of above methods is that the torque at starting, in all the above methods, gets reduced.
3. Increased torque starting:
With a wound rotor induction motor, resistance can be added in the rotor circuit so as to decrease the starting current while increasing the starting torque, even, up to the value of maximum torque that can be developed by the motor.
4. Starting by means of smooth variation of voltage or frequency:
With ac motor-de generator sets, dc motors can be started by smooth variation of applied voltage and with variable frequency sources both induction and synchronous motors can be started by smooth variation of supply frequency, simultaneously varying proportionally the applied voltage to the motors.
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