354

ELECTRICAL APPARATUS

consumed by it is wattless and therefore produces no serious
heating and reactive leads of low resistance thus are not liable
to self-destruction by heating if the motor fails to start im-
mediately.

On account of the limited space available in the railway motor
considerable difficulty, however, is found in designing sufficiently
high reactances which do not saturate and thus decrease at
larger currents.

At speed, reactance in the armature coils is very objectionable
in retarding the reversal of current, and indeed one of the most
important problems in the design of commutating machines is to
give the armature coils the lowest possible reactance. There-
fore, the insertion of reactance in the motor leads interferes
seriously with the commutation of the motor at speed, and thus
requires the use of a suitable commutating or reversing flux, that
is, a magnetic field at the commutator brushes of sufficient
strength to reverse the current, against the self-inductance of the
armature coil, by means of an e.m.f. generated in the armature
coil by its rotation. This commutating flux thus must be in
phase with the main current, that is, a flux of overcompensation.
Reactive leads require the use of a commutating flux of over-
compensation to give fair commutation at speed.

Counter E.m.fs. in Commutated Coil
199. Theoretically, the correct way of eliminating the de-
structive effect of the short-circuit current under the commu-
tator brush resulting from the e.m.f. of alternation of the main
flux would be to neutralize the e.m.f. of alternation by an equal
but opposite e.m.f. inserted into the armature coil or generated
therein. Practically, however, at least with most motor types,
considerable difficulty is met in producing such a neutralizing
e.m.f. of the proper intensity as well as phase. Since the alter-
nating current has not only an intensity but also a phase displace-
ment, with an alternating-current motor the production of corn-
mutating flux or commutating voltage is more difficult than with
direct-current motors in which the intensity is the only variable.
By introducing an external e.m.f. into the short-circuited coil
under the brush it is not possible entirely to neutralize its e.m.f.
of alternation, but simply to reduce it to one-half. Several such
arrangements were developed in the early days by Eickemeyerj