REGULATING POLE CONVERTERS

449

Choosing pL = 0.04, or 4 per cent, loss of current, equation
(25) gives, for the three-phase and for the six-phase converter:

(a) no brush shift (n = 0) :

*3° = 0.467, (30)

*6° = 0.123;

that is, in the three-phase converter this would require a third
harmonic of 46.7 per cent., which is hardly feasible; in the six-
phase converter it requires a third harmonic of 12.3 per cent.,
which is quite feasible.

(b) 20° brush shift (rb = 20) :

= 1 - 0.533

= i - 0.877

COS

COS Ta
COS (ra +

for -

COS Ta

= 0, or no flux shift, this gives:

£300 = 0.500,
Z6°° = 0.176.

(31)

(32)

COS (ra + TV,)

Since ——-----L <

COS Ta

1 for brush shift in the direction of

armature rotation, it follows that shifting the brushes increases
the third harmonic required to carry out the voltage regulation
without increase of converter heating, and thus is undesirable.

It is seen that the third harmonic, t, does not change much
with the flux shift, ra, but remains approximately constant, and
positive, that is, voltage raising.

It follows herefrom that the most economical arrangement
regarding converter heating is to use in the six-phase converter
a third harmonic of about 17 to 18 per cent, for raising the vol-
tage (that is, a very large pole arc), and then do the regulation
by shifting the flux, by the angle, raj without greatly reducing the
third harmonic; that is, keep a wide pole arc excited.

As in a three-phase converter the required third harmonic is
impracticably high, it follows that for variable voltage ratio the
six-phase converter is preferable, because its armature heating
can be maintained nearer the theoretical minimum by propor-
tioning t and T^

29

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