POLYPHASE POWER 



403 



to change from a high-voltage two-phase system to a low- 

 voltage three-phase system (or vice versa), it may be done 

 providing only that the secondary voltage of A is 86.6% 

 that of B. 



Three-phase Transformation. Three-phase transforma- 

 tion may be accomplished by one three-phase transformer 

 or by three single-phase transformers. In special cases 

 two single-phase transformers may be used for three-phase 

 transformation. 



The ratio in which the voltage is changed depends not 

 only upon the ratio of the transformers themselves but 



Transformer A 

 Ratio 1 -.866 



FIG. 264. Connection of Transformers for Changing Two-phase 

 Power to Three-phase Power or Vice Versa. 



also upon the way in which they are interconnected. Sup- 

 posing the transformers themselves have a ratio of one, 

 the ratio of the transformation is as given below: 



Primaries 

 Connected. 



Y 

 Y 



A 

 A 



The significance of the terms A and Y and the method 

 for making the connections was explained when discussing 

 the armature windings of an alternator. 



