TRANSFORMERS 409 



1 73 



transformer must also here be -f = 1.155 or 15.5 per cent greater 



l.o 



than one-half the group rating. 



Phase Transformation. Of the connections for transforming 

 one polyphase system into another with a different number of 

 phases, the following are the most commonly used: 



Two- or three-phase to single-phase. 

 Two-phase to six-phase. 

 Three-phase to two-phase. 

 Three-phase to six-phase. 



Two- or Three-phase to Single-phase. 1 It is practically impos- 

 sible to transform from polyphase to single-phase by means of 

 static transformation with balanced conditions. Various schemes 

 have been proposed and investigated, but none of the combina- 

 tions give better results than can be obtained by simply using a 

 transformer across on phase. 



The reason for this is explained by Dr. Steinmetz (A.I.E.E. 

 1892) to be as follows: 



u Single-phase power changes from a maximum to zero and back 

 to maximum every half cycle, while polyphase power is delivered 

 at a constant rate. Therefore, any system capable of transform- 

 ing from balanced polyphase current to single-phase current must 

 be capable of storing energy during the interval of time when the 

 power delivered to the single-phase side is less than the power 

 received from the three-phase side. The transformer is incapable 

 of fulfilling this requirement." 



Nevertheless, it is desirable to know the best method of 

 taking single-phase power from a three-phase system and often 

 ingenious although complicated connections are proposed with the 

 idea of more uniformly distributing a single-phase load. Most 

 of these schemes do not present a single feature that is superior 

 to the placing of the single-phase load directly across two wires. 

 When there is one feature which is apparently superior, there are 

 generally undesirable features which more than offset it. The 

 four schemes shown in Figs. 251 to 254 are ones commonly sug- 

 gested and Table XL VII gives the characteristics of these con- 

 nections and shows that they are inferior to straight single-phase 



1 Three papers on Single-phase Power Service from Polyphase Systems 

 appeared in A.I.E.E. Proceedings for October, 1916. 



