A starter is used to control starting current of the motor.
I = ( V – Eb ) / R
I=field winding current,
During starting Eb = 0, and R is very small resistance.
so,magnitude of the starting current is very high.The high current passes through the winding, there is chance for winding to burn out.
To avoid this we need to reduce the voltage (V) or increase
the winding resistance. Since the reduction of supply voltage is difficult to attain, a starter is
used, which will push high resistance during starting and gradually reduces and fully removed from the circuit,when the motor attains the full speed.
Because when the motor attains the normal speed it generate the back emf,which enough to reduce the winding current.
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