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j

430

ELECTRICAL APPARATUS

which is not compensated by the armature reaction of the power
component of the alternating current. It is interesting to note
that in this case, in regard to heating, output based thereon, etc.,
the converter equals that of one of normal voltage ratio.

B. Variable Ratio by Change of Wave Shape of the Y Voltage

234. If in the converter shown diagrammatically in Fig. 204
the magnetic flux disposition and the pitch of the armature
winding are such that the potential difference between the point,

a, of the armature and the neutral 0,
or the 7 voltage, is a sine wave, Fig.
205 A, then the voltage ratio is
normal. Assume, however, that
the voltage curve, a, differs from
sine shape by the superposition of
some higher harmonics: the third
harmonic in Figs. 205 B and C;
the fifth harmonic in Figs. 205 D
and E. If, then, these higher
harmonics are in phase with the
fundamental, that is, their maxima
coincide, as in Figs. 205 B and D,
they increase the maximum of the
alternating voltage, and thereby the
direct voltage; and if theseharmonics
are in opposition to the funda-
mental, as in Figs. 205 C and E, they decrease the maximum
alternating and thereby the direct" voltage, without appreciably
affecting the effective value of the alternating voltage. For in-
stance, a higher harmonic of 30 per cent, of the fundamental
increases or decreases' the direct voltage by 30 per cent., but
varies the effective alternating voltage only by \/l + 0.32 =
1.044, or 4.4 per cent.

The superposition of higher harmonics thus offers a means of
increasing as well as decreasing the direct voltage, at constant
alternating voltage, and without shifting the angle between the
brush position and resultant magnetic flux.

Since, however, the terminal voltage of the converter does not
only depend on the generated e.m.f. of the converter, but also
on that of the generator, and is a resultant of the two e.m.fs. in
approximately inverse proportion to the impedances from the
converter terminals to the two respective generated e.m.fs., for

FIG. 204.—Variable ratio con-
verter by changing wave shape
of the Y. e.m.f.