7-A] 



POLYPHASE TRANSFORMATION. 



243 



that, if two coils with electromotive forces differing in phase are 

 connected in series, the electromotive force across the two coils 

 is the vector sum (or difference) of the two separate electro- 

 motive forces. A resultant electromotive force of any desired 

 phase can thus be obtained from a polyphase supply by means 

 of two transformers. 



The transformation from 2-phase to 3-phase, or vice versa, is 

 most important on account of the copper economy* in 3-phase 

 transmission and the sometime advantage of 2-phase generation 

 or utilization. 



5. Two-phase to Three-phase Transformation (and vice 

 versa) by T-connection. This method, first published by Mr. 

 C. F. Scott, is shown diagrammatically 

 in Fig. I, in which A and B are the two 

 phases of a 2-phase system ; X, Y and Z 

 represent the three line wires of a 

 3-phase system. 



Let the transformation be from 2- 

 phase to 3-phase. Two transformers are 

 used. One has a primary A A' on phase 

 A of the 2-phase system and has a sec- 

 ondary (XY) wound, let us say, for 100 

 volts with a middle tap at O, dividing the 

 coil into two parts of 50 volts each. 



TWO PHASE 

 PHASE A 



A A' 



MOmOQOOOQOOQ. 



\JT> CO 



I a 



a- < 



FIG. i. Transformation 

 from 2-phase (AB) to 3- 

 phase (XYZ), or vice 



The second transformer has a primary 



BB' on phase B of the 2-phase system and has a secondary 

 (OZ) wound for 86.6 volts (86.6= 100 X 4V3), which has 

 one end connected, as shown, to the middle tap of the first trans- 

 former. It will be found that the three voltages, XY, YZ and 

 ZX, are equal and differ in phase from each other by 120, thus 

 making a 3-phase system. These voltages are represented in 

 Fig. 2. They should be interpreted as in Exp. 6-A; see also 

 'Appendix I. to this experiment. 

 *See Appendix III., Exp. 6-A. 



