CHAPTEE XIV. 



Polyphase Currents Generators Combination of Currents Production of 

 Rotary Magnetic Fields Intensity of Rotary Field Induction Motors 

 Starting Devices for Induction Motors Structure and Performance of 

 Induction Motors. 



POLYPHASE CURRENTS. 



94. In the last chapter we saw that an ordinary alternating- 

 current machine can be driven as a motor, provided that its speed 

 is first brought to synchronism with the alternating current which 

 feeds it. In other words, a synchronous motor such as is described 

 in the preceding chapter is not self-starting, but requires an auxiliary 

 machine to first bring it into synchronism with the current. 



This disadvantage caused electrical engineers to seek some new 

 type of alternating- current motor which would start without 

 mechanical aid. Such a motor was at length devised whose 

 working depends upon a suitable combination of alternating 

 currents differing in phase from each other so called Poly- 

 phase Currents. 



Before describing the motors themselves, we will consider how 

 polyphase currents can be generated, and investigate the electrical 

 and magnetic effects due to their combined action. 



The only systems of polyphase currents in practical use are 

 (1) Di-phase Currents, or two alternating currents of the 

 same strength and periodicity, but differing in phase by a quarter 

 of a period, and (2) Tri-phase Currents, or three alter- 

 nating currents of the same strength and periodicity, but mutually 

 out of phase with each other by one-third of a period. 



DI-PHASE CURRENTS. 



95. Suppose that, instead of tapping the armature of a 

 dynamo- electric machine at two points 180 apart only, as in a 



