248 PHASE CHANGERS, ETC [Exp. 



commonly used for obtaining a low starting voltage for 3-phase 

 motors and converters. The taps y and s can be located where 

 desired. It is to be noted that the voltage is changed, but not 

 the phase. A third auto-transformer, 7Z, 

 might be used, with a tap at O. Although 

 better for continuous operation, this would 

 have the disadvantage of requiring an addi- 

 "J A tional auto-transformer; furthermore, this ar- 

 FIG. 6. Auto- rangement could not give less than half vol- 

 transformers on 3 - tagej anc j WO uld make a reversal of phase in 



phase circuit. 



changing from low to high (starting to run- 

 ning) voltage, which is not desirable in starting a synchronous 

 machine from the alternating current end. 



17. Two-phase to Six-phase Transformation. The most 

 practical method for this consists in transforming from a 2-phase 

 primary circuit to two sets of T-connected secondaries, one set 

 being inverted; (thus T and X). Two or four transformers can 

 be used. It is not necessary to make this transformation in the 

 laboratory. (Detailed connections are given in McAllister's 

 Alternating Current Motors; see also 27, Exp. 6-A.) 



APPENDIX I. 

 MISCELLANEOUS NOTES. 



1 8. Further Interpretation of T-connection. A general discussion 

 of the vector combination of electromotive forces is given in Exp. 

 6-A (particularly Appendix I.), and the general principles there given 

 can be applied to the T-connection. The following is a more detailed 

 discussion of this particular case. 



The electromotive force of any alternating current coil may be repre- 

 sented by a vector in a certain direction. If this coil is the secondary 

 of a transformer connected to a secondary line, as in the present case, 

 the electromotive force impressed upon this line will be represented 

 by the same vector. If in connecting the coil to the line the termi- 

 nals are reversed, the vector representing the electromotive force 



