SINGLE-PHASE COMMUTATOR MOTORS

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the compensating winding, however, is constant in the space
from pole corner to pole corner, as shown by C in Fig. 158, and
since the total m.m.f. of the compensating winding equals that
of the armature, the armature m.m.f. is higher at the brushes,
the compensating m.m.f. higher in front of the field poles, as
shown by curve R in Fig. 158, which is the difference between
A and C; that is, with complete compensation of the resultant
armature and compensating winding, locally undercompensation
exists at the brushes, overcompensation in front of the field

FIG. 158.—Distribution, of m.m.f. in compensated motor.

poles. The local undercompensated armature reaction at the
brushes generates an e.m.f. in the coil short-circuited under the
brush, and therewith a short-circuit current of commutation
and sparking. In the conductively compensated motor, this can
be avoided by overcompensation, that is, raising the flat top of
the compensating m.m.f. to the maximum armature m.m.f., but
this results in a lowering of the power-factor, due to the self-
inductive flux of overcompensation, and therefore is undesirable.
193. To get complete compensation even locally requires the
compensating winding to give the same distribution curve as the
armature winding, or inversely. The former is accomplished by
distributing the compensating winding around the entire cir-
cumference of the armature, as shown in Fig. 159. This, how-
ever, results in bringing the field coils further away from the
armature surface, and so increases the magnetic stray flux of the
field winding, that is, the magnetic flux, which passes through
the field coils, and there produces a reactive voltage of self-in-

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