SINGLE-PHASE COMMUTATOR MOTORS 415
gets poor at speeds above 150 per cent, of synchronism. With
t = 0.2, or only 20 per cent, of the voltage on the compensating
circuit, the commutation gets perfect at double synchronism.
Best commutation thus is secured by shifting the supply vol-
tage with increasing speed from the compensating to the arma-
ture circuit.
t > 1, or a reverse voltage, — ei, impressed upon the armature
circuit, so still further improves the commutation at very low
speeds.
For high values of t, however, the power-factor of the motor
falls off somewhat.
The impedance of the short-circuited armature coils, chosen
in the preceding example :
Z4 = 7.5 + 10 j,
corresponds to fairly high resistance and inductive reactance in
the commutator leads, as frequently used in such motors.
227. As a further example are shown in Fig. 193 and Fig. 194
curves of a motor with low-resistance and low-reactance com-
mutator leads, and high number of armature turns, that is, low
reduction factor of field to armature circuit, of the constants:
hence:
X4 = 0.373 +0.267 j,
and:
Co = 0.3,
c4 = 0.03,
the other constants being the same as before.
Fig. 193 shows, with the speed as abscissae, the current, torque,
power output, power-factor, efficiency and commutation current,
igt under such a condition of operation, that at low speeds t = 1.0,
that is, the motor is a repulsion jmotor with secondary excita-
tion, and above the speed at which t = 1.0 gives best commuta-
tion (90 per cent, of synchronism in this example), t is gradually
decreased, so as to maintain ig a minimum, that is, to maintain
best commutation.
As seen, at 10 per cent, above synchronism, ig drops below i,
that is, the commutation of the motor becomes superior to that
of a good direct-current motor.
/t
Fig. 194 then shows the commutation factors, fc = -?> of the