REGULATING POLE CONVERTERS

431

varying the converter ratio only such higher harmonics can be
used which may exist in the Y voltage without appearing in the
converter terminal voltage or supply voltage.

In general, in an n-phase system an nth harmonic existing in
the star or 7 voltage does not appear in the ring or delta voltage,

FIG. 205.—Superposition of harmonics to change the e.m.f. ratio.

as the ring voltage is the combination of two star voltages dis-

180
placed in phase by----degrees for the fundamental, and thus by

Tit

180°, or in opposition, for the nth harmonic.

Thus, in a three-phase system, the third harmonic can be in-
troduced into the Y voltage of the converter, as in Figs. 205 B
and C, without affecting or appearing in the delta voltage, so
can be used for varying the direct-current voltage, while the fifth
harmonic can not be used in this way, but would reappear and

/, ;

" i
,. -I-