370

ELECTRICAL APPARATUS

205. The action of the commutator in an alternating-current
motor, in permitting compensation for phase displacement and
thus allowing a control of the power-factor, is very interesting
and important, and can also be used in other types of machines,
as induction motors and alternators, by supplying these machines
with a commutator for phase control.
A lag of the current is the same as a lead of the e.m.f., and in-
versely a leading current inserted into a circuit has the same ef-
fect as a lagging e.m.f. inserted. The commutator, however,
produces an e.m.f. in phase with the current. Exciting the field
by a lagging current in the field, a lagging e.m.f. of rotation is
produced which is equivalent to a leading current. As it is easy
to produce a lagging current by self-inductance, the commutator
thus affords an easy means of producing the equivalent of a
leading current. Therefore, the alternating-current commutator
is one of the important methods of compensating for lagging
currents. Other methods are the use of electrostatic or electro-
lytic condensers and of overexcited synchronous machines.
Based on this principle, a number of designs of induction
motors and other apparatus have been developed, using the
commutator for neutralizing the lagging ^magnetizing current
and the lag caused by self-inductance, and thereby producing
unity power-factor or even leading currents. So far, however,
none of them has come into extended use.
This feature, however, explains the very high power-factors
feasible in single-phase commutator motors even with consider-
able air gaps, far larger than feasible in induction motors.
VII.- Efficiency and Losses
206. The losses in single-phase commutator motors arc essen-
tially the same as in other types of machines:
(a) Friction losses—air friction or windage, bearing friction
and commutator brush friction, and also gear losses or other
mechanical transmission losses.
(6) Core losses, as hysteresis and eddy currents. These are
of two classes—the alternating core loss, due to the alternation
of the magnetic flux in the main field, quadrature field, and arma-
ture and the rotating core loss, clue to the rotation of the arma-
ture; through the magnetic field. The former depends upon the
frequency, the latter upon the speed.
(c) Commutation losses, as the power consumed by the short-