Effect of Shifting Brushes From Neutral

Author: E.E. Kimberly

If the brushes be shifted slightly backward from their neutral position against the direction of rotation, a curve like (b) in Fig. 11-4 may be obtained. When the brushes are shifted backward, the conductors between them cut both the main flux and the flux of one interpole. Since the interpoles are opposite in polarity to the main poles paired with them between brushes, their fluxes have the effect of weakening the main-pole fluxes. Furthermore, the conductors lying in the angles as 2α in Fig. 10-21 (c) produce demagnetizing ampere-turns that weaken the main field.

Fig. 11-4. Shunt-Motor Speed Regulation

As explained on page 133, a weakening of the main-field flux causes the speed to rise, and so the differential action of the interpoles and the demagnetizing mmf causes the usual droop of curve (a) to be approximately compensated as shown by (b). Too much backward shift of the brushes may over-accentuate the characteristic of curve (b) and may cause unstable speeds at loads greater than one-third or one-half of the rated load. Any improvement in speed regulation obtained by this method is gained at the expense of good commutation.