TRANSFORMERS 



419 



the increased voltage safely. If the secondary windings are made 

 up of two distinct sections, which is not, however, standard prac- 

 tice, the connections may be made as in Fig. 262. The latter 

 connection is, however, somewhat complicated and when three- 

 phase operation with full output is desired and without change of 

 voltage, the double-delta connection is generally preferable. 



Double-delta. For the double-delta connection two inde- 

 pendent low-voltage coils are required for each transformer, as 

 shown in Fig. 263. The second set are all reversed, and then con- 



To 61 k rfjle /j 



FIG. 263. 



nected in a similar manner to the first set, so that the two deltas 

 are displaced 180. 



The high-voltage windings should preferably be connected 

 delta, as it permits the system to be operated with only two 

 transformers, in case one should be damaged. 



The current in each coil for double-delta is equal to / = 



output in watts 



- and the current in each line equals I X 1.73. 

 delta voltage X 2X3 



Full output, three-phase may also be obtained by connecting 

 as shown in Fig. 262. 



Double-delta connection cannot be used with Edison three- 

 wire service, as it has no neutral, and in such cases separate auto 

 transformers would be required. 



Double-Y. Like the double-delta, this system requires two 

 sets of low-voltage coils, displaced 180, as shown in Fig. 264. 



The high-voltage windings may be either delta- or Y-connected 

 even with regulating pole converters, but in this case the two low- 



