TRANSFORMERS 



417 



that a connection given in Fig. 260 is superior, but where the 

 two-phase load predominates, the T connection is preferable. 



Three-phase to Six-phase. In transforming from three- to six- 

 phase, there are four different connections, which may be used, 

 namely : 



Diametrical. 

 Double-delta. 

 Double-Y. 

 Double-T. 



Diametrical. The diametrical connection, as represented in 

 Fig. 261, is the most commonly used of any three-phase to six- 

 phase transformations, and there is very little reason for using 



5 6 



Collector Rings 



FIG. 261. 



any other connection for the operation of six-phase converters. 

 It requires only one low-voltage coil on each transformer which 

 are connected to diametrically opposite points on the armature 

 windings. It furthermore gives the simplest arrangement of 

 switches, transformer taps and connections for starting six-phase 

 converters from the alternating current side, while on the other 

 hand it is possible to operate a six-phase converter at reduced 

 capacity with one transformer out of service, leaving. the other 

 two connected across their respective diameters. 



With diametrically connected low-voltage windings, the high- 

 voltage windings should preferably be connected in delta so as 

 to avoid the triple frequency harmonics of the e.m.f., as described 

 under Y-Y connection on page 403. With regulating pole con- 

 verters, however, the high-voltage windings must be connected Y 



