Fig. 338.—Holzer Cabot multiple brush holder. Each brush is fastened securely to a machined surface by one or two machine screws, making a positive contact. Several strips of flexible copper of ample section to carry the current are interposed between the part of the holder carrying the brush and the portion clamping the stud, no sliding contact or spring being therefore required to carry any current. The brushes are proportioned for a carrying capacity of not more than 25 amperes per square inch of brush surface. The brush can be adjusted to any degree of tension during the operation of the machine if necessary. Each holder is insulated in such a manner that no short circuit can occur if the holder be accidentally tipped backward while the operator is changing a brush or cleaning the commutator during a run.
Ques. For what class of machine are carbon brushes specially adapted?
Ans. For machines furnishing a small current at high pressure.
When carbon brushes are used, it is desirable that the current be small, because, on account of the low conductivity of the carbon, more contact area is necessary than with copper for equal current transmission. For fixed lead and fluctuating currents, carbon brushes should be used.
Ques. For what class of machine are copper brushes especially adapted?
Ans. For machines furnishing large current at low pressure, as in [fig. 328].
Size of Brushes.—The number of brush sets depends upon the number of poles of the machine, but there may be several brushes in each set. It is usual, except in the smallest machines, to place at least two brushes exactly similar side by side instead of one broad brush, thus allowing one brush to be removed for trimming or renewal while the machine is running. Moreover, better contact is secured by this sub-division, because a slight elevation in the commutator surface at one point may slightly raise one brush of a set at each revolution without much harm, while with one broad brush, the entire brush would be lifted, causing bad sparking.
Ques. What determines the number of brushes in each set?
Ans. It depends upon the current capacity, size of machine, and judgment of the designer.