340

ELECTRICAL APPARATUS

ductance, but does not pass through the armature conductors,
and so does no work; that is, it lowers the power factor, just as
over compensation would do. . The distribution curve of the

armature winding can, however, be
made equal to that of the compen-
sating winding, and therewith local
complete compensation secured, by
using a fractional pitch armature
winding of a pitch equal to the pole
arc. In this case, in the space be-
tween the pole corners, the currents
are in opposite direction in the
upper and the lower layer of con-
ductors in each armature slot, as
shown in Fig. 160, and thus neutralize
magnetically; that is, the armature
reaction extends only over the space
of the" armature circumference covered
by the pole arc, where it is neutralized
by the compensating winding in the pole face.

To produce complete compensation even locally, without im-
pairing the power-factor, therefore, requires a fractional-pitch

FIG. 159.—Completely
distributed compensating
winding. "

FIG. 160.—Fractional pitch arma-
ture winding.

FIG. 161.—Repulsion motor with
massed winding.

armature winding, of a pitch equal to the field pole arc, or some
equivalent arrangement.
Historically, the first compensated single-phase commutator
motors, built about 20 years ago, were Prof. Elihu Thomson's
repulsion motors. In these the field winding and compensating