SINGLE-PHASE COMMUTATOR MOTORS 371
circuit current under the brush, by arcing and sparking, where
such exists.
(d) i2r losses in the motor circuits—the field coils, the compen-
sating winding, the armature and the brush contact resistance.
(e) Load losses, mainly represented by an effective resistance,
that is, an increase of the total effective resistance of the motor
beyond the ohmic resistance.
Driving the motor by mechanical power and with no voltage
on the motor gives the friction and the windage losses, exclusive
of commutator friction, if the brushes are lifted off the commu-
tator, inclusive, if the brushes are on the commutator. Ener-
gizing now the field by an alternating current of the rated fre-
quency, with the commutator brushes off, adds the core losses
to the friction losses; the increase of the driving power then
measures the rotating core loss, while a wattmeter in the field
exciting circuit measures the alternating core loss.
Thus the alternating core loss is supplied by the impressed
electric power, the rotating core loss by the mechanical driving
power.
Putting now the brushes 'down on the commutator adds the
commutation losses.
The ohmic resistance gives the i2r losses, and the difference
between the ohmic resistance and the effective resistance, calcu-
lated from wattmeter readings with alternating current in the
motor circuits at rest and with the field unexcited, represents
the load losses.
However, the different losses so derived have to be corrected
for their mutual effect. For instance, the commutation losses
are increased by the current in the armature; the load losses are
less with the field excited than without, etc.; so that this method
of separately determining the losses can give only an estimate of
their general magnitude, but the exact determination of the effi-
ciency is best carried out by measuring electric input and me-
chanical output.
VIII. Discussion of Motor Types
207* Varying-speed single-phase commutator motors can be
divided into two classes, namely, compensated series motors and
repulsion motors. In the former, the main supply current is
through the armature, while in the latter the armature is closed
upon itself as secondary circuit; with the compensating winding