CHAPTER XX

SINGLE-PHASE COMMUTATOR MOTORS
I. General

189. Alternating-current commutating machines have so far
become of industrial importance mainly as motors of the series
or varying-speed type, for single-phase railroading, and as con-
stant-speed motors or adjustable-speed motors, where efficient
acceleration under heavy torque is necessary. As generators,
they would be of advantage for the generation of very low fre-
quency, since in this case synchronous machines are uneconom-
ical, due to their very low speed, resultant from the low frequency.

The direction of rotation of a direct-current motor, whether
shunt or series motor, remains the same .at a reversal of the im-
pressed e.m.f., as in this case the current in the armature circuit
and the current in the field circuit and so the field magnetism
both reverse. Theoretically, a direct-current motor therefore
could be operated on an alternating impressed e.m.f. provided
that the magnetic circuit of the motor is laminated, so as to fol-
low the alternations of magnetism without serious loss of power,
and that precautions are taken to have the field reverse simul-
taneously with the armature. If the reversal of field magnetism
should occur later than the reversal of armature current, during
the time after the armature current has reversed, but before the
field has reversed, the motor torque would be in. opposite direc-
tion and thus subtract; that is, the field magnetism of the alter-
nating-current motor must be in phase with the armature cur-
rent, or nearly so. This is inherently the case with the series
type of motor, in which the same current traverses field coils
and armature windings.

Since in the alternating-current transformer the primary and
secondary currents and the primary voltage and the secondary
voltage are proportional to each other, the different circuits of
the alternating-current commutator motor may be connected
with each other directly (in shunt or in series, according to the
type of the motor) or inductively, with the interposition of a

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