432

ELECTRICAL APPARATUS

cause a short-circuit current in the supply voltage, hence should
be made sufficiently small to be harmless.

235. The third harmonic thus can be used for varying the
direct voltage in the three-phase converter diagrammatically
shown in Fig. 206 A, and also in the six-phase converter with

7

7

FIG. 206.—Transformer connections for varying the e.rn.f. ratio by super-
position of the third harmonic.
double-delta connection, as shown in Fig. 206 B, or double-7
connection, as shown in Fig. 206 C, since this consists of two sepa-
rate three-phase triangles of voltage supply, and neither of. them
contains the third harmonic. In such a six-phase converter
with double-7 connection, Fig. 206 0, the two neutrals, however,