[Electric engineering.] . it is necessary to allowample contact surface between brushes and about 1 square inch of brush contact surface isnecessary for every 30 to 40 amperes collected, and in thecase of low-tension incandescent-lighting dynamos this callsfor a large commutator. Notwithstanding the fact thatcarbon brushes require a much larger and more expensivecommutator than copper brushes, they operate so muchbetter that the increased first cost is warranted. Thecarbon brushes run with much less sparking and do not cutthe commutator, as copper brushes are apt to do, unle
[Electric engineering.] . it is necessary to allowample contact surface between brushes and about 1 square inch of brush contact surface isnecessary for every 30 to 40 amperes collected, and in thecase of low-tension incandescent-lighting dynamos this callsfor a large commutator. Notwithstanding the fact thatcarbon brushes require a much larger and more expensivecommutator than copper brushes, they operate so muchbetter that the increased first cost is warranted. Thecarbon brushes run with much less sparking and do not cutthe commutator, as copper brushes are apt to do, unlessthey are very carefully looked after. Most incandescentdynamos have bar-wound armatures, i. e., the conductorson the armatures are in the shape of copper bars rather thanwires, because a large cross-section of conductor is necessaryin order to carry the current. The ends of these bars lead-ing to the commutator are seen at M, Fig. 50. Themachine here shown has a capacity of 75 kilowatts and runs 86 ELECTRIC LIGHTING. 16. §16 ELECTRIC LIGHTING. 87 at a speed of 275 revolutions per minute. The speed of thelarger sizes of direct-connected machine is correspondinglylower, and in case they are direct connected to large, slow-speed Corliss engines, the speed is usually from 75 to100 revolutions per minute. ALTERNATING-CURRENT DYNAMOS. 94. Alternators may be constructed with either the arma-ture or field as the revolving member, or they may be madewithout any revolving wire whatever, the only revolvingpart being a mass of iron, called an inductor. We mayhave, then, a comparatively large number of different typesof alternators to choose from as compared with direct-current machines. When alternating current first cameinto use for lighting work, machines with revolving arma-ture and stationary field were the only kind used, and thesemachines were almost invariably belt driven. The fre-quency employed was usually from 125 to 133 cycles persecond, and the machines were often provided w
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