400 ELECTRICAL MACHINERY 



Hence the three-phase line requires only three-quarters 

 as much copper as the single-phase line. 



It may be proved that of all polyphase systems the three- 

 phase requires less weight of copper than any other, and 

 as it has just been shown to require less copper than the 

 single-phase, the three-phase system is practically always 

 used for power transmission systems. 



127. Polyphase Machinery Compared to Single-phase 

 Machinery. The first cost of a polyphase machine of a given 

 kw. output is much less than that of a single-phase machine 

 of equal output, and the weight of the polyphase machine 

 is considerably less than that of the single-phase machine. 



The reason for this may be seen by considering an alter- 

 nator, of a given number of armature coils, etc. Let the 

 safe current capacity of the coils be / and the voltage gen- 

 erated by all the coils when connected in series (single 

 phase winding) be EI. The capacity of the alternator 

 will evidently be equal to E\I. 



If now the coils are connected in three separate groups 

 to form a three-phase winding the current capacity of each 

 group will be 7, as before, and the voltage of one group 



will be 



The possible output of the machine will be 



Hence the three phase winding, made up of the same 

 coils used to form the single-phase winding, has a capacity 

 which is about 30% greater tha that of the single-phase 

 winding. 



If the coils were made up into a six-phase winding, 



