SINGLE-PHASE COMMUTATOR MOTORS 353
circuit, during the moment when the armature coils connecting
to it are short-circuited by the brushes; that is, if HI = number of
2
armature turns from brush to brush, the lead is only --•;• of the
<ni
time in circuit, and though excessive current densities in mate-
rials of high resistivity are used, the heating is moderate. In
starting the motor, however, if it does not start instantly, the
current continues to flow through the same resistance leads, and
thus they are overheated and destroyed if the motor does not
start promptly. Hence care has to be taken not to have such
motors stalled for any appreciable time with voltage on.
The most serious objection to the use of high-resistance leads,
therefore, is their liability to self-destruction by heating if the
motor fails to start immediately, as for instance in a railway
motor when putting the voltage on the motor before the brakes
are released, as is done when starting on a steep up-grade to
keep the train from starting to run back.
Thus the advantages of resistance commutator leads are the
improvement in commutation resulting from the reduced short-
circuit current, and the absence of a serious demagnetizing effect
on the field at the moment of starting, which would result from
an excessive short-circuit current under the brush, and such
leads are therefore extensively used; their disadvantage, however,
is that when they are used the motor must be sure to start im-
mediately by the application of voltage, otherwise they are liable
to be destroyed.
It is obvious that even with high-resistance commutator leads
the commutation of the motor can not be as good as that of the
motor on direct-current supply; that is, such an alternating-
current motor inherently is more or less inferior in commutation
to the direct-current motor, and to compensate for this effect
far more favorable constants must be chosen in the motor design
than permissible with a direct-current motor, that is, a lower
voltage per commutator segment and lower magnetic flux per
pole, hence a lower supply voltage on the armature, and thus a
larger armature current and therewith a larger commutator, etc.
The insertion of reactance instead of resistance in the leads
connecting the commutator segments with the armature coils of
the single-phase motor also has been proposed and used for
limiting the short-circuit current under the commutator brush.
Reactance has the advantage over resistance, that the voltage
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