THE TRANSFORMER. 



225 



112. Transformation of phases. Transformers in polyphase 

 systems are often arranged so as to change the number of phases 

 and at the same time to produce step- up or step-down transfor- 

 mation. Thus at one of the large power stations at Niagara 

 Falls, two-phase alternators are used, and the step-up transformers 

 which supply the Buffalo transmission line are arranged to change 

 from two-phase to three-phase so to take advantage of the fact 

 that three-phase transmission requires less copper than two-phase 

 transmission, for the same voltage between wires. Also, when 



Fig. 193. 



it is desired to operate, from an #-phase supply, a synchronous 

 converter having m collector rings, the step-down transformers 

 must be arranged to change the number of independent phases. 

 It is impossible by means of transformers to convert a single- 

 phase supply into a polyphase supply, because a transformer can- 

 not convert the pulsating power delivered by a single-phase alter- 

 nator into the steady flow of power required in the polyphase 

 system. A transformer (that is, an ideal transformer) delivers 

 energy at each instant at the same rate that it receives energy. 

 16 



