Page 630 - Basic Electrical Engineering

Figure 7.18 Starting of a dc motor with a variable resistance connected in the armature circuit

The value of armature resistance for a dc motor is very small. Let us

assume that V = 220 V and R = 0.5 Ω. Then at start,
a

This is a huge current to be allowed to flow through the armature in a small
dc motor. To restrict this high amount of current to flow through the
armature, a variable resistance can be connected in series with the armature

so that eq. (7.3) gets modified as

Once the motor starts rotating, back EMF E starts increasing and the
b
numerator of the expression for I as in eq. (7.5) gets reduced. As the
a
numerator goes on reducing, the denominator of expression (7.5) can be

gradually reduced by reducing the value of variable resistance, R. This
resistance is completely cut out of the circuit, once the motor picks up

sufficient speed.
This variable resistance connected in the armature circuit is called a starter.
Thus, a starter is a variable resistance connected in series with the armature

circuit to limit the initial current drawn by the motor. Once the motor picks
up speed, back EMF E comes into full existence, and automatically the
b
armature current gets reduced even when the extra resistance is cut out.