CHAPTEK XV. 



Polyphase Transformers Phase Transformers and Rotary Converters. 



POLYPHASE TRANSFORMERS. 



116. We have seen that monophase transformers are used for the 

 purpose of transforming alternating electromotive forces from high 

 to low values, or vice versa. 



Polyphase transformers may likewise be used for simply trans- 

 forming the values of the E.M.F.s, or they may be also arranged 

 so as to transform the number of phases of the E.M.F.s. 



There is little to be said in the case of polyphase transformers 

 used simply for transforming E.M.F.s, since the same laws relating 

 to ratio of turns hold good here as in monophase transformers ; 

 that is, the ratio between the primary and secondary E.M.F.s is 

 approximately the same as that between the number of primary 

 and secondary turns. 



We might, of course, in the case of tri-phase currents, employ 

 three single-phase transformers, viz. one in each of the three 

 circuits; but just as it is unnecessary to have three separate 

 circuits with six line-wires for transmission, so is it equally unneces- 

 sary to use three separate transformers. All that is necessary is 

 to have three limbs magnetically short-circuited by common yokes, 

 as shown diagrammatically in Fig. 72, in which PI, P%, PZ represent 

 the primary coils, and Si, $ 2 , $ 3 the secondaries. The phase 

 relationships between the magnetic fluxes in the three cores will 

 be similar to the phase relationships between the three primary 

 currents. 



In the case of di- phase transformers, three cores are again all 

 that is necessary, provided the section of the core wound with the 

 coil which is connected to the common line- wire is \/'2 times the 

 cross-sections of either of the other two, in order that the induction 

 in all three cores may be the same. 



