6-A] GENERAL STUDY. 211 



cuit, the potential diagram of which forms a hexagon. Its only 

 use is in connecting transformer secondaries to 6-phase syn- 

 chronous converters.* The usual and best method for obtaining 

 a 6-phase circuit is by means of the diametrical-conneciion, as 

 follows. Three transformers have primaries connected to a 

 3-phase circuit. The six wires of the 6-phase circuit may be 

 represented by the apices of a hexagon; the three transformer 

 secondaries, Fig. 2 (a), are connected so as to form diagonals 

 or diameters of the hexagon. The three neutral or middle points 

 of the secondaries may, or may not, be interconnected. Connect 

 transformers in this manner, with the neutrals interconnected, and 

 test with a voltmeter; for present purposes this one test will be 

 sufficient. 



If each transformer has two separate secondaries of equal 

 voltage, these six coils can be used as a 6-phase supply by a ring- 

 or mesh-connection (each coil forming diagrammatically one side 

 of a hexagon) ; or, a 6-phase supply can be obtained, by a 

 double T or double delta, one T or delta being reversed with 

 respect to the other. A double F-connection is the same as the 

 diametral-connection. One advantage of the diametral-con- 

 nection is that it gives a neutral which may be used as a " derived 

 neutral " for a 3-wire system on the direct current service from 

 the converter; this is particularly useful in lighting systems. 



PART IV. 



28. Equivalent Single-phase Quantities. Polyphase quan- 

 tities are sometimes reduced to equivalent single-phase values for 



* (273). A 3-phase converter may be increased in rating 40 or 50 per 

 cent, with no increased losses and with a corresponding higher efficiency 

 when changed to 6-phase by the addition of three more collector rings and 

 (if necessary) an extension of the commutator. A most valuable paper 

 on this subject is one by Woodbridge (A. I. E. E., February 14, 1908), who 

 states that of 1,000,000 K. W. of railway converters, one third are 6-phase; 

 above 500 K. W. one company makes all converters 6-phase. See also 

 Chap. XL, Alternating Current Motors, by A. S. McAllister, where 6-phase 

 transformer connections are given in detail. 



