What is the primary function of the starter in a DC motor?

Concept:

The necessity of D.C. Motor Starter:

At starting, when the motor is stationary, there is no back EMF in the armature because the speed is zero.

Consequently, if the motor is directly switched on to the mains, the armature will draw a heavy current (\(I_a=\frac{V-E_b}{R_a}=\frac{V}{R_a}\)) because of small armature resistance.

This heavy starting current may result in:

(i) burning of armature due to excessive heating effect,

(ii) damaging the commutator and brushes due to heavy sparking,

(iii) excessive voltage drop in the line to which the motor is connected.

In order to avoid excessive current at starting, a variable resistance (known as starting resistance) is inserted in series with the armature circuit.

This resistance is gradually reduced as the motor gains speed (and hence Eb increases) and eventually, it is cut out completely when the motor has attained full speed.

By the use of the starter, starting current is limited to 1.25 to 2 times the full-load current.