172 



TREATISE ON ALTERNATING CURRENTS. 



Having due regard to these details, the design of a polyphase 

 transformer differs in no respect from that of a monophase 

 transformer. 



PHASE TRANSFORMERS. 



117. Phase transformers are used for the purpose of changing 

 alternating current of any number of phases to alternating current 

 of a different number of phases. 



The problem of producing such phase transformations was first 

 solved by Professor S. P. Thompson in the following manner : 



A ring transformer having a number of coils in closed series 

 was tapped at three equidistant points, and fed thereat by tri- phase 

 currents, with the result that a rotating magnetic field was pro- 

 duced. On further tapping at the extremities of any diameter, 

 single-phase current could be taken from it ; or by tapping at four 

 equidistant points, di-phase currents are obtained. In fact, by 

 making a suitable number of tappings, currents of any number of 

 phases could be obtained. 



Such a phase transformer is really an auto-transformer of 

 special type, the correct phase relationships of the secondary 

 currents being dependent upon the production of a magnetic field 

 rotating with uniform angular velocity. It cannot, therefore, be 

 used for transforming monophase currents to polyphase. 



In the case in which the primary current is tri-phase, the 

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



