Page 635 - Basic Electrical Engineering

Figure 7.19 (b) A four-point starter for a dc motor

7.11.2 Four-point Starter

The disadvantage of a three-point starter is that when a large value resistance
is connected in the field circuit to increase the speed of a motor, the field

current gets reduced. Since the field winding and the coil of the NVR are
connected in series, the current flowing through the coil of the NVR will also

get reduced. The attractive force of the NVR magnet to hold the starter arm in
the RUN position against the spring tension may not be sufficient. The
holding magnet may release the arm of the starter during normal running of

the motor when current flowing through its coil becomes too small. The
effect of this will be that the motor will stop, which may not be desirable.

In a four-point starter the NVR coil is connected independently across the
supply voltage instead of connecting it in series with the motor field winding.
Thus, in a four-point starter there will be three parallel circuits connected

across the supply voltage as has been shown in Fig. 7.19 (b). When the starter
arm is brought to the ON position, current will flow through the armature

circuit through the starter resistance. Current will flow from the supply via
the starting arm and the starter resistance. This will limit the starting current

to a large extent. Simultaneously, the field circuit will also get full supply