DC Shunt motor starter

Prior to starting , the handle is in the OFF position.

To start the motor , the handle is moved manually and when it makes contact with the resistance stud 1 it is in the start position.

In this position the field winding receives the full supply voltage, butthe armature current is limited by the graded resistance R(=R1+R2+R3+R4)

The starter handle is then gradually moved from stud to stud, allowing the speed of the motor to build up until it reaches the RUN position.

In this position: (i) motor attains full speed
                      (ii) resistance R is completely cut
The handle is held in RUN position by electromagnetenergised by a no-volt coil (NVC).
 In the event of switching OFFor complete failure of supply the NVC is de-energised.
 This results in the releaseof the handle , which is then pulled back to OFF position by the action of a spring.

 The NVC also provides protection against an open circuitin the field winding which cause a very large current to flow as  a result full line voltage applied across armature.