Starter is required because
Basic equation of armature current in the motor is as below:
Ia = V – Eb/ Ra
Ia = Armature current
V = terminal voltage
Eb = Back emf of motor
Ra = armature resistance
now at the starting back emf will be zero, so equation becomes V/Ra and it becomes too high and may damage the armature winding even it can burn i out because at starting it will draw 5 to 7 times of load current. that is why the starter is needed for gradually decreasing the resistance.
Starter is used for a motor to limit the high starting current. During starting of a dc motor high current of upto 4-5 times the full load current occurs which is avoided by use of high resistance slowly cut out of the circuit.
- Back emf create critical role in governing the operation of dc motor.Back emf is generated as the motor armature start to rotate in presence of magnetic field and It is counter to the supply voltage. The back emf at the starting is zero and develops as the motor gradually speed up.We know that general emf equation E = Eb + laRa. At starting Eb = 0 so la = E/Ra,which indicates that current will be dangerously high at starting(As Armature resistance Ra is small).Hence it is necessary to use starter to limit the starting current to allowable lower value.