Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . supposing the two ends of the cylinderof a piston pump were connected by means of a pipeand then, having done away with all the valve move-ments, the pumps were started. At the beginning ofthe stroke, water would be forced out one side of thecylinder around the pipe into the other side of thecylinder, and after the piston had reached the end ofthe stroke and started back, the water would then takea return course back to where it had started. In thiscase the pump could be likened to the dynamo andt
Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . supposing the two ends of the cylinderof a piston pump were connected by means of a pipeand then, having done away with all the valve move-ments, the pumps were started. At the beginning ofthe stroke, water would be forced out one side of thecylinder around the pipe into the other side of thecylinder, and after the piston had reached the end ofthe stroke and started back, the water would then takea return course back to where it had started. In thiscase the pump could be likened to the dynamo andthe pipe to the wires, and the current to the waterflowing back and forth. The form and winding of122 s ELECTRICITY TOR ENGINEERS 123 alternating current dynamos varies considerably, butthey generally follow the plan shown in Fig. 69. Inthe figure the pole pieces are alternately north (N) andsouth (S), while the armature is wound with the samenumber of poles, with the winding so arranged thatthe poles alternate. The fields are excited by directcurrent passing over the field coil windings. This. figure 69. direct current is usually obtained from another sourceaside from the current generated in the armature ofthe alternating current dynamo. A separate dynamo,called an exciter, is employed for supplying the cur-rent to the fields. There are some types of so calledcomposite wound alternating current dynamos inwhich a part of the current from the alternator arma- 124 ELECTRICITY FOR ENGINEERS ture is rectified or commutated by means of a commu-tator mounted on the same shaft with the armature, Armatures are woundin two layers. .__ upper coil. lower » FIGURE 70.
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