THE TRANSFORMER. 



the 72-phases being transmitted over n wires, then a 2n-ring con- 

 verter may be supplied, if the n-phases be kept entirely separate on 

 the secondary side of the step-down transformers, that is, if each of 

 the n secondary coils delivers current to the converter over a 

 separate pair of wires. This is exemplified by what is said above 

 concerning the four-ring converter. It is also exemplified by the 

 supply of currents to a six-ring converter from a three-phase 

 system, the three phases being separate and each delivered to the 



AAAAA/y' AAAAAA 2 " 



Fig. 199. 



converter over two wires. To supply a six-ring converter from 

 a two-phase system it is necessary to transform to three-phase. 



The simplest method of connecting a six-ring converter to a 

 three-phase (six-wire) system is to connect each of the three 

 phases to opposite collector rings of the six-ring converter. This 

 is called the diametrical connection and it is shown in Fig. 199. 



There is some confusion as to the exact meaning of the term n-phase system, as is 

 evident from the above discussion which shows the possibility of operating a four-ring 

 converter from a two-phase system and the possibility of operating a six-ring converter 

 from a three-phase system. Indeed one should speak of an w-phase alternator as one 

 which has n armature windings displaced i/w of 1 80 electrical degrees* from each 

 other. Thus, a two-phase alternator is an alternator which has two armature wind- 

 ings 90 electrical degrees apart, and a three-phase alternator is an alternator having 

 three armature windings 60 electrical degrees apart ; and the electromotive forces of 

 two-phase and three-phase alternators may be most properly represented by the full 



*One electrical degree is 1/180 of the angular distance between adjacent north 

 and south field magnet poles center to center. 



