Question

Why is the starting current high in a DC motor? Explain the working of a four-point starter for a DC machine.

Answer 1

In a DC motor, back EMF will not be generated in the armature at the time of start. The armature has very low resistance. Because of this there will high starting current in a DC motor.

Starters are used in order to limit this starting current in DC motor.

Working of 4-point starter:

The internal structure of the four-point starter will be like this.

The main difference between the 3point starter and 4point starter is the additional resistance R.

L, N, F, A are the four terminals of the 4-point starter.

L is the line terminal and connected to the positive supply voltage.

A is the Armature terminal and it is connected to the armature winding of the motor.

F is the Field terminal and it is connected to the field winding of the motor

N is connected to No Voltage coil of the motor

During normal operating conditions, the starter will be in ON position.

At this time, the HC (Holding coil) will be in series with the starting resistance and with the additional resistance R.

The main function of the additional resistance R is to protect the circuit from short circuit

In this 4-point starter in addition to the L, F, A terminals the N terminal will also be connected to the supply voltage via the terminal N.

OFF, 1, 2, 3, 4, 5, RUN are called studs and these are the contact points.

The handle of the starter will be moving across these studs. In addition, it regulates the starting current there by controlling speed.

Let the supply be given to the circuit and now the handle is taken onto the stud 1

Now the circuit will be closed and then the line current will start to flow through the starter.

Now, the current is divided into 3 parts and will be flowing through the 3 different points.

The first part of the current will be flowing through the starting resistance (R1,R2,R3,R4,R5) and then through the armature.

The second part of the current will be flowing through field winding F.

The third part of the current will be flowing through the no voltage coil in series with the resistance R.

This unique configuration does not result in the modification of the shunt field circuit in the no voltage coil because the two circuits are independent of each other.

It effectively implies that the electromagnet pull applied to the handle 's soft iron bar by the no-voltage coil at all times should be sufficiently strong to hold the handle at its RUN location, or otherwise to avoid the spring force from returning the handle at its original OFF location, regardless of whether the field rheostat is placed.