REGULATING POLE CONVERTERS

433

must not be connected together, as the third harmonic voltage
exists between the neutrals. In the six-phase converter with
diametrical connections, the third harmonic of the F voltage ap-
pears in the terminal voltage, as the diametrical voltage is twice
the Y voltage. In such a converter, if the primaries of the sup-

If

FIG. 207.—Shell-type transformers.

ply transformers are connected in delta, as in Fig. 206 D, the
third harmonic is short-circuited in the primary voltage triangle,
and thus produces excessive currents, which cause heating and
interfere with the voltage regulation, therefore,, this arrangement

FIG. 208.—Core-type transformer.
Is not permissible. If, however, the primaries are connected in
Y, as in Fig. 206 E, and either three separate single-phase trans-
formers, or a three-phase transformer with three independent
magnetic circuits, is used, as in Fig. 207, the triple-frequency
voltages in the primary are in phase with each other between