CHAPTER VI 

 THE POLYPHASE SYSTEM 



48. The single-phase system and the polyphase system. A 



brief reference was made in Chapter I to the distinction between 

 the single-phase alternator and the polyphase alternator, and, 

 although the importance of the polyphase system cannot be fully 

 appreciated until the induction motor and the rotary converter 

 are discussed, still it is necessary to outline here some of the 

 general features of the polyphase system inasmuch as this system 

 enters into every division of our subject, alternators, transmission 

 lines, transformers, synchronous motors, rotary converters, and 

 induction motors. 



The single-phase system consists essentially of a simple alter- 

 nator delivering an alternating current to a single circuit. 



The polyphase system consists essentially of two or more 

 electrically distinct (but not mechanically distinct) alternators 

 delivering distinct alternating currents over distinct transmission 

 lines to electrically distinct receiving circuits. 



49. The two-phase alternator. A two -phase alternator is an 

 alternator upon whose armature two distinct windings are placed 

 each with its own pair of collector rings, the two windings being 

 arranged so that the electromotive force generated in one wind- 

 ing is in quadrature with the electromotive force generated in the 

 other winding. A clear idea of the essential features of the two- 

 phase alternator may be obtained from Fig. 90 which represents 

 two distinct armatures A and B mounted on one shaft and 

 revolving inside of the same crown of field magnet poles. The 

 armature B is shown smaller than the armature A for the sake 

 of clearness. The two armatures are arranged so that the groups 

 of winding slots in one armature are midway between the field 

 magnet poles when the groups of winding slots in the other 



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