CHAPTER XIII 

 POLYPHASE POWER 



125. Polyphase Power Compared to Single-phase Power, 



The polyphase system, whether two-, three- or six-phase, 

 is evidently more complicated than the single-phase system, 

 yet practically all electrical power systems are three-phase. 

 Points of Superiority of Polyphase Power. There are 

 three important reasons for this fact. Polyphase appa- 

 ratus is cheaper to luild than single-phase apparatus of the 

 same capacity; polyphase machinery (induction motors, 

 synchronous converters, etc.,) has better operating character- 

 istics than single-phase machinery; three-phase power 

 transmission lines of a given kw. capacity and a specified 

 efficiency of transmission, require only three-quarters as much 

 copper for the line as would be required by a single -phase 

 line. 



126. Polyphase Power for Transmission Lines. If a 

 given amount of power is to be transmitted, the two factors 

 which are generally fixed are the voltage and the efficiency 

 of transmission. We will compare the single-phase line with 

 a three-phase line, the same amount of power to be trans- 

 mitted over each and the power lost in each to be the same. 



Let P = the power to be transmitted; 



E ihe voltage of the line, either single-phase or 



three-phase; 



7i = the current, single-phase line; 

 Ri =the resistance per wire of the single-phase line; 

 /3 = the current, three-phase line; 

 Rs = the resistance per wire of the three-phase line. 



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