228 



ELEMENTS OF ELECTRICAL ENGINEERING. 



forces generated in a, b and c, Fig. 195, terminate at the apices 

 of an equilateral triangle, then the Y-connected units give sym- 

 metrical three-phase voltages between the mains. This is evident 

 when we consider that the voltage from main I to main 2, Fig. 

 195, is the vector difference a b, that the voltage from main 2 

 to main 3 is the vector difference b c, and that the voltage 

 from main 3 to main i is the vector difference c a. 



In the Scott transformer the point 0, Fig. 196, lies midway 

 between the points / and r as shown in Fig. 197, so that the 



\m 



Fig. 197. 



lines a and b are parallel to each other and at right angles to 

 the line c. Then the voltage c may be produced by the single 

 secondary of a transformer supplied from phase B of the two- 

 phase system, and the voltages a and b may be produced by 

 two secondary coils (one for a and one for b) of a transformer 

 supplied from phase A of the two-phase system, the three 

 secondaries a, b and c being ^connected or, as it is some- 

 times called, ^-connected, to the three-phase mains as shown 

 in Fig. 198. The one condition that is necessary is that the coils 

 a and b be related to each other as halves of one continuous 

 winding. 



Example of a Scott transformer. The similar primary coils 

 of two transformers have 1,000 turns of wire each and they are 

 connected to the respective phases of a 1,000 volt two-phase 



