The dc shunt motor is started by a three-point starter consisting of the resistance, handle, studs no-volt release coil and overload release coil.
When armature is connected to the supply, the current to the armature goes through the starting resistance and the field winding is connected directly to the mains through the no-volt release coil. When the motor attains speed, some back emf is induced in the armature. The starting current is gradually cut off and thus the starting current is reduced to a low value.
Parts of Three-Point Starter and their Function
The resistance is put in series of the armature to protect the motor from the heavy rush of current at the time of starting. The tapping is taken at several places and connected to studs. The resistance is reduced by gradually moving the handle from left to right from one stud to another.
The handle is hand operated and insulated. It carries a soft iron piece which is directly attached to No-volt release coil when it reaches running position.
The brass arc is provided below and along the series resistance stud. It is provided to supply full voltage to shunt field from the process of starting to running position.
No-volt Release Coil
No-volt release coil is safety device provided in the starter. The no-volt coil consists of a thin wire of many turns and is connected in series with the field winding of the motor. This coil is magnetized when current flows through the shunt field winding. Its function is to attract the handle of the starter and keep it in on position. It releases the handle and goes to the OFF position by the spring tension due to failure of supply and in this way disconnects the motor from the supply.
Overload Release Coil
Its function is to demagnetize the no-volt release coil in the case abnormal condition (such as a fault or overload) of the machine. It consists of a few turns of thick wire and is connected in series to the armature. This coil will only be sufficiently magnetized when the excessive current flows through the armature due to overload of some fault. Now coil attract the tripping plunger which short circuited the terminal of the no-volt coil. The no-volt release coil will be demagnetized and release the starting handle which will come to the OFF position at once due to spring tension. Thus motor stops