Ques. How many watts are lost at the brushes?
Ans. The watt loss is equal to 1.6 to 2. volts for carbon multiplied by the total current carried.
The watt lost on account of friction may be calculated by the formula: ((.3 x 746)/33000) × (P x S) = watts lost by carbon friction, in which P is the total pressure in pounds on the commutator, and S, the rim velocity of the commutator in feet per minute.
The losses due to contact resistance and brush friction are very liable to be greatly increased above the values that may be obtained by the preceding methods, if the commutator and brushes are dirty and rough, or not in good condition.
Figs. 342 to 345.—Various types of brush holder. Fig. 342, arm or lever type; fig. 343, spring arm type; fig. 344, box type; fig. 345, reaction type.
Brush Holders.—These are devices employed to hold the brushes against the commutator with the proper pressure. They differ considerably in various types of machine, hence, no general rules can be given with respect to their construction or use, but any brush holder must fulfill the following requirements:
1. It must hold the brush securely and at the same time feed it forward as it wears away so as to maintain a proper contact;
2. It must hold the brush at the proper contact angle;
3. It must be capable of being raised from the commutator, and held out of contact by some form of catch;
4. It must be so constructed that the brush can be easily removed for cleaning or renewal;