Two Point Starter for DC Series Motor
Two point starter is used for limiting the starting current of DC series motor.The circuit of two point starter show in fig.1
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| Fig.1 Two point starter for DC series motor |
The starting resistance is divided into number of sections by means of brass studs.
The handle is pivoted and it can be moved against the spring tension.
No volt coil:
(NVC) is connected in series with the starting resistance and motor circuit.
When motor is to be switched ON, the DC supply is made ON and handle is moved in clockwise direction against the spring tension. The motor current flows in following path positive terminal of supply starter resistance -- NVC - series field winding - motor armature negative terminal of supply. Thus entire starter resistance comes in series with armature and high starting current is limited to a safe value.
✒The handled is pushed in clockwise direction till it's soft iron piece (SIP) rests on NVC.
✒ The NVC holds the handle in run position due to its magnetic field.
✒ On no load or at light load condition, the motor runs at a dangerously high speed.
But due to NVC is prevented. The magnetic field of NVC is weak at no load condition
therefore handle is thrown back to off position due to spring tension. Thus motor is protected from over speeding.
✒When supply fails, the NVC is de-energized i.e. it losses the magnetism and handle is thrown back to OFF position. This prevents the motor from a condition-starting without any starter resistance.
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