Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . figure 28. running is best for the purpose of preventing the com-mutator from cutting. Should the commutatorbecome rough, it should be smoothed with sandpaper,never using emery cloth, because emery cloth is con-ductor of electricity, and the particles of emery areliable to lodge themselves between the commutatorbars in the mica and short circuit the two bars, thereby ELECTRICITY FOR ENGINEERS 57 burning a small hole wherever such a particle of emery-has lodged itself. The emery will also work into
Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . figure 28. running is best for the purpose of preventing the com-mutator from cutting. Should the commutatorbecome rough, it should be smoothed with sandpaper,never using emery cloth, because emery cloth is con-ductor of electricity, and the particles of emery areliable to lodge themselves between the commutatorbars in the mica and short circuit the two bars, thereby ELECTRICITY FOR ENGINEERS 57 burning a small hole wherever such a particle of emery-has lodged itself. The emery will also work into thebrushes and copper bars and wear them down; it beingalmost impossible to remove all the emery. In the end-on carbon brushes, Fig. 30, the contactsurface of the brushes should be occasionally cleanedby taking a strip of sandpaper, with the smooth sideof the paper to the commutator, and the sanded sidetoward the contact surface of the brush, and then by. FIGURE 29. leaving the tension of the brush down on the sand-paper, it is an easy matter to move the sandpaper toand fro and throughly clean off the glazed and dirtysurface from the carbon, leaving it with a concave thatwill exactly fit the commutator. The advantages of carbon brushes are the cardinal points are: The armature mayrun in either direction without it being necessary toalter the brushes; the carbon can be manufactured witha quantity of graphite in its construction, thereby 58 ELECTRICITY FOR ENGINEERS lowering the mechanical friction of the brushes on thecommutator; they do not cut a commutator so muchby sparking; the commutator has a longer life, thewear being more evenly distributed. Carbon brushes, due to their rather high resistance,will often heat up considerably, but, although thisheat is objectionable, their resistance tends to cutdown the sparking. The brushes are sometimes coatedwith copper to reduce their resistance. Often a car-
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