TRANSFORMERS 



395 



both high- and low -voltage sides. Very few systems are, however, 

 operated on this plan. 



By connecting together the middle points of the low-voltage 

 windings, as shown in Fig. 239, two 100-volt main circuits ac 

 and be are obtained. Also four 70-volt (50 X \/2) side circuits ab, 

 be, cd, and da. 



This method of connection is used when the neutral is to be 

 brought out in connection with Edison three-wire service of rotary 

 converters. If the converter is started from transformer with 



1410- 



-1000- 



WWV 



-1000 



100 



FIG. 238. 



FIG. 239. 



one-third and two-third voltage taps, provision must be made for 

 opening the neutral connection when starting, so as to avoid short- 

 circuit. 



Another two-phase arrangement is shown in Fig. 240, and is 

 commonly called the five-wire system. It is accomplished simply 

 by connecting the low-voltage windings at the middle and bring- 

 ing out an extra wire from these points. 



With the connections shown in Fig. 235 the maximum insula- 

 tion stress in case of a permanent ground is 1000 volts on either 

 phase of the high-voltage side, but a simultaneous grounding of 



