424

ELECTRICAL APPARATUS

dependent of the wave shape, and thus can be produced whether
the alternating voltage is a sine wave or any other wave.

It is obvious that, instead of shifting the brushes-on the com-
mutator, the magnetic field poles may be shifted, in the opposite
direction, by the same angle, as shown in Fig. 198, A, B, C.

Instead of mechanically shifting the field poles, they can be
shifted electrically, by having each field pole consist of a number
of sections, and successively reversing the polarity of these sec-
tions, as shown in Fig. 199, A, B} C} D.

FIG. 198.—E.m.f. variation by mechanically shifting the poles.

Instead of having a large number of field pole sections, obvi-
ously two sections are sufficient, and the same gradual change
can be brought about by not merely reversing the sections but
reducing the excitation down to zero and bringing it up again in
opposite direction, as shown in Fig. 200, A, B, C, D, E.

A B C D
PIG. 199.—E.m.f. variation by electrically shifting the poles.
In this case, when reducing one section in polarity, the other
section must be increased by approximately the same amount,
to maintain the same alternating voltage.
When changing the direct voltage by mechanically shifting
the brushes, as soon as the brushes come under the field pole
faces, self-inductive sparking on the commutator would result
if the iron of the field poles were not kept away from the brush