. Electrical world. 5255-82253-15125-45 142147144145146 I 1,,8001,8001,800 50985 only two or four poles, but rotating armatures, although satisfactoryfor 500 to 2,000 volts, have not been found suitable for the highervoltages of and 10,000 which are now common, and thereforerotating fields and armatures have been adopted in many ofthe recent alternators. For continuous-current dynamos the sameremarks apply, only here sparklcss commutation has to be providedfor. Carbon brush blocks cannot be used,
. Electrical world. 5255-82253-15125-45 142147144145146 I 1,,8001,8001,800 50985 only two or four poles, but rotating armatures, although satisfactoryfor 500 to 2,000 volts, have not been found suitable for the highervoltages of and 10,000 which are now common, and thereforerotating fields and armatures have been adopted in many ofthe recent alternators. For continuous-current dynamos the sameremarks apply, only here sparklcss commutation has to be providedfor. Carbon brush blocks cannot be used, as at these speeds thebrushes are apt to vibrate, and so diminish the intimacy of contactand cause heating and undue wear. The result is that it has beenfound best to form the brushes of wire, gauze or foil, preferably ofbrass, and these must be sufficiently flexible so as to maintain agood contact with the commutator over the whole section of thebrush. It follows, therefore, that the properties of the carbon brush. giving more ampere-turns per pair of poles than is usual. In alter-nators this gives no trouble at all, as all that has to be provided is FIG. 5.—2,000-KW PARSONS blocks in giving sparkless commutation without alteration of thelead of the brushes, cannot in turbine-driven dynamos be utilized,and other means must be adopted to secure sparkless commutationat varying loads. One way is to shift the brushes automaticallyaccording to the change of load, and this can be effected by con-necting the brush gear to a steam cylinder controlled by a spring andsupplied with steam from the point where the steam enters the tur-^bine. At this point the pressure of the steam is proportional to theload of the dynamo, and therefore the piston in the steam cylinderbeing controlled by a spring takes up a position proportional to theload and thus shifts the brushes to the point of sparkless commu-tation. Another method is to provide commutating poles as proposedby Prof. Ryan
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectelectri, bookyear1883
