Page 632 - Basic Electrical Engineering

7.10.3 Armature Control Method

By putting an extra variable resistance in the armature circuit the speed can
be reduced as

is a variable resistance

It can be seen here that the speed can only be reduced because the numerator
will get reduced.

7.11 STARTER FOR A DC MOTOR

We have mentioned earlier that a starter is a variable resistance connected in
series with the armature circuit during starting to reduce the starting current.

This resistance is gradually cut out as the motor starts running. Two types of
starters, namely a three-point starter and a four-point starter are described

below.

7.11.1 Three-point Starter

A three-point starter circuit is described as follows. To start the motor, the
starter arm, as shown in Fig. 7.19 (a) is moved in the clockwise direction.

The arm will touch the point 1 of the starting resistance R. The whole of the
resistance will appear in the armature circuit. The field winding will also get

full supply through the coil of the NVR. The motor will develop torque and
start rotating with full starting resistance in the armature circuit. The

resistance will be cut in succession by moving the starter arm in the
clockwise direction and will be brought to RUN position. In the RUN

position, the soft iron piece fixed on the starter arm will face the NVR
magnet piece and remain attracted. The starter arm, therefore, will stay in the
RUN position against the spring tension, and the operator can remove his

hand from the arm. In case of supply failure, the NVR electromagnet will get
de-energized, and the starter arm will automatically return to OFF position

due to the spring pressure. In case the motor is over loaded, the armature will