CHAPTER IX. 

 THE POLYPHASE CIRCUIT. 



Generation of 2 and 3 phase Currents. If the armature of an 

 alternating-current generator has conductors situated at 

 equal distances apart round its circumference, the phase 

 of the electromotive forces induced in them by the field 

 will differ. Only those conductors which are situated in 

 similar positions with regard to the poles at any instant 

 will be identical in phase, while conductors situated 

 midway between these will be intermediate in 

 phase. As already explained, it is usual to connect in 

 series coils which are formed of conductors lying in 

 several slots, and which are, therefore, not identical in phase, 

 although differing but slightly. In order to produce an 

 alternating current, generally only those conductors can 

 be advantageously joined together which are approxi- 

 mately in phase with each other, being similarly situated 

 under similar poles, or which are 180 out of phase with each 

 other, the latter being situated in corresponding positions 

 under opposite poles. There are thus portions of the 

 armature between the conductors which are joined in 

 this way which cannot be advantageously wound with 

 conductors connected to the same circuit. 



Two-phase Current. If an exactly similar set of conduc- 

 tors is wound in the intermediate positions between the 

 first set, and these are independently connected together, 

 they will give an alternating electromotive force similar in 

 voltage and periodicity to that induced in the first winding, 

 and only differing from it in phase. 



An alternator having two armature windings with the 

 conductors of one winding situated in advance of the 

 other by half the pitch of the magnet poles, i.e., the coils 

 of one exactly half-way between the coils of the other, 

 will supply a 2-phase current at the four terminals with 

 which the armature is then provided. 



Thus, a 2-phase current consists of two independent 

 alternating currents, each carried by a single pair of 

 conductors. The two circuits have the same voltage and 

 periodicity, and a fixed relative difference of phase of a 



