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Page 648 | Volume 2 | Chapter 33 | Care of the Commutator and Brushes

  1. Excessive voltage;
  2. Excessive current;
  3. Reversal of polarity of dynamo;
  4. Short circuits or grounds in dynamo, or external circuits.

Ques. What is the indication of excessive voltage?

Ans. It is indicated by the voltmeter, or by the brilliancy of the pilot lamp.

Fig. 724 - Method of smoothing commutator with a stone.
Fig. 724—Method of smoothing commutator with a stone. The proper stone to use made out of white sandstone similar to that uaed for grindstones, but a trifle softer. It is dovetailed into a holder, as shown in the illustration, and held in place by a set screw. When being used, one knob is grasped in one hand and the other knob in the other hand, the stone being moved back and forth along the length of the commutator. As the stone will become coated with copper at first, it must be cleaned frequently by means of coarse sandpaper. The fine dust from the stone will get under the brushes and wear them to a very close fit. After using the stone, finish with fine sandpaper.

Ques. What are the causes of excessive voltage?

Ans. Over excitation of the field magnet or too high speed.

In the former case, resistance should be introduced into the field circuit to diminish the current flowing therein if a shunt machine; or if a series machine, a portion of the current should be shunted across the field coils by means of a resistance arranged in parallel with the series


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