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372

ELECTRICAL APPARATUS

as primary or supply circuit. As the result hereof the repulsion
motors, contain a transformer flux, in quadrature position to the
main flux, and lagging behind it, while in the series motors no
such lagging quadrature flux exists, but in quadrature position
to the main flux, the flux either is zero—complete compensation
—or in phase with the main flux—over- or undercompensation.
A. Compensated Series Motors
Series motors give the best power-factors, with the exception
of those motors in which by increasing the lag of the field flux
a compensation for power-factor is produced, as discussed in V.
The commutation of the series motor, however, is equally poor
at all speeds, due to the absence of any commutating flux, and
with the exception of very small sizes such motors therefore are
inoperative without the use of either resistance leads or corn-
mutating poles. With high-resistance leads, however, fair opera-
tion is secured, though obviously not of the same class with
that of the direct-current motor; with commutating poles or coils
producing a local quadrature flux at the brushes good results
have been produced abroad.
Of the two types of compensation, conductive compensation,
1, with the compensating winding connected in series with the
armature, and inductive compensation, 2, with the compensated
winding short-circuited upon itself, inductive compensation nec-
essarily is always complete or practically complete compensa-
tion, while with conductive compensation a reversing flux can
be produced at the brushes by overcompensation, and the com-
mutation thus somewhat improved, especially at speed, at the
sacrifice, however, of the power-factor, which is lowered by the
increased self-inductance of the compensating winding. On the
short-circuit current under the brushes, due to the e.m.f. of alter-
nation, such overcompensation obviously has no helpful effect.
Inductive compensation has the advantage that the compen-
sating winding is not connected with the supply circuit, can be
made of very low voltage, or even of individually short-circuited
turns, and therefore larger conductors and less insulation used,
which results in an economy of space, and therewith an increased
output for the same size of motor. Therefore inductive compen-
sation is preferable where it can be used. It is not permissible,
however, in motors which are required to operate also on direct
current, since with direct-current supply no induction takes place