3 POINT STARTER OF DC MOTOR

A 3 point starter is a device that helps in the starting and running of a DC shunt motor or compound wound DC motor.
Now the question arises that why these DC motors requires the assistance of the starter. It is because of Back EMF, it plays an important role. Back EMF is zero during starting and hence the armature current is very high during the starting of the motor.
So the main function of starter is to limit the high inrush starting current which can severely damage the motor.
Starter basically connects the resistance in series with the armature circuit at the time of starting and when the motor starts to attain the speed, resistance is cut gradually. 

CONSTRUCTION OF 3 POINT STARTER

Figure below shows the connection diagram of 3 point starter.

Construction of 3 point starter

3 point starter consists of a resistance which is divided into several sections and includes three terminals namely L, A and F. 

Here,  L( line terminal) is connected to the positive supply, 
         A (armature terminal) is connected to the armature winding, 
and    F (field terminal) is connected to the field winding.

Apart from these, OLR (Overload release) and NVC (No Volt Coil) are also provided in the motor. These two acts as a protecting device for the motor.

NO VOLT COIL (NVC)

Field winding is connected with No Voltage Coil. So when field current flows, the NVC is magnetized. Now when the handle is in the ‘RUN’ position, a soft iron piece is connected to the handle and because of flow of current through it, it gets attracted by the magnetic force produced by NVC,  The NVC is designed in such a way that it holds the handle in ‘RUN’ position against the force of the spring as long as supply is given to the motor. Thus NVC holds the handle in the ‘RUN’ position.

Now in case of any supply failure, current will stop flowing through NVC and NVC will get demagnetized. Under the action of spring force, handle will move back to OFF position and motor circuit will be open. Thus protecting the motor.

OVERLOAD RELEASE

Overload Release is just a small electromagnet. As the name suggests, it is used to give protection to motor in case of overload. Under the normal condition, OLR carries the armature current and this current is not sufficient to pull the lever (as shown in fig.). But when the armature current exceeds the normal rated value, it attracts that lever and in turn, the triangular iron closes the contact PP. Thus, NVC gets short-circuited and it results in the release of handle, which returns to OFF position and supply to the motor gets cut off.

WORKING OF 3 POINT STATER

For starting the motor, handle is in the OFF position when the supply to the DC motor is switched on. Then handle is slowly moved against the spring force to make contact with stud No. 1. At this point, field winding of the shunt or the compound motor gets supply through the parallel path provided to starting the resistance, through No Voltage Coil. The high starting armature current thus gets limited because at starting entire resistance of the starter comes in series with the armature.

As the handle is moved further, it goes on making contact with studs 2, 3, 4, etc., thus gradually cutting off the series resistance from the armature circuit as the motor gathers speed. Finally, when the starter handle is in ‘RUN’ position, the entire starting resistance is eliminated, and the motor runs with normal speed.

This is because as the speed increases, back emf is developed consequently to counter the supply voltage and reduces the armature current.

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