The Current drawn by the armature of motor is given by,
Where, Ia = ( V- Eb ) / Ra
V - Supply Voltage
Eb - Back EMF
Ra - Armature Resistance
· When the motor is at rest, there is no back emf developed in the armature. If now full supply voltage is applied across the stationary armature., it will draw a very large current.
· Because armature resistance is very small.
· This excessive current will blow out the fuse and damage the motor.
· To reduce high stating current, a resistance is connected in series with the armature circuit at the time of starting.
· When the motor speed is increased the back emf is also increased.
· Then Ia value is decrease.
· That time external resistance is cut out.
· Over load Release (O.L.R) or No volt coil
· Hold on Coil
· Thermal Relays
· Fuses (Starting /Running)
· Over load relay
· Two point Starter
· Three point Starter
· Four point Starter