Why is Starter used for starting Dc motor? - Electrical4u

 Why is Starter used for starting Dc motor?

At the time of starting of motor , the speed of the motor is zero hence the back emf ( or induced emf) across the armature is zero ( since , Eb directly proportional N). So the motor will draw a very large current due to a very small armature resistance when it is connected  to related supply voltage .

Let us consider a 400 D.C Shunt motor with full load current of 50A and armature resistance of 0.2 ohms . At the time of starting , This motor will draw V / Ra = 400 / 0.2 =  2000A , which is 40 times that of full load current of motor. Similarly , a 220V series motor with full load current of 20A and armature and series field resistance of 0.3 ohms and 0.2 ohms respectively will draw.

V(Ra + Rf ) = 220 / (0.3 + 0.2) = 440A

Which is 22 times of the full load current . This excessive current will cause 

1. the burning of armature winding 
2. reduction of life period of armature due to mechanical shock with large sudden torque 
3. Damage of commutator and brush gear due to heavy sparking 

For the above mentions reasons , All Motor except small frictional-kW Motors , must be started with a device in which there will be an arrangement of external resistance include in armature circuit to limit the starting current to safe values. the device is called stater and the stater resistance is gradually cut out as the motor gains the speed.