SINGLE-PHASE COMMUTATOR MOTORS 397
These curves are derived by calculating numerical values in
tabular form, for S = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8,
2.0, 2.2, 2.4.
As seen from Fig. 188, the power-factor, p> rises rapidly, reach-
ing fairly high values at comparatively low speeds, and remains
near its maximum of 90 per cent, over a wide range of speed.
The efficiency, rj} follows a similar curve, with 90 per cent, maxi-
mum near synchronism. The power, P, reaches a maximum of
192 kw. at 60 per cent, of synchronism—450 revolutions with a
four-pole 25-cycle motor—is 143 kw. at synchronism, and van-
ishes, together with the torque, D, at double synchronism. The
torque at synchronism corresponds to 143 -kw., the starting
torque to 657 synchronous kw.
The commutation factor, fc, starts with 1.18 at standstill, the
same value which the same motor would have as series motor,
but rapidly decreases, and reaches a minimum of 0.23 at 70 per
cent, of synchronism, and then rises again to 1.00 at synchron-
ism, and very high values above synchronism. That is, the
commutation of the repulsion is fair already at very low speeds,
becomes very good somewhat below synchronism, but poor at
speeds considerably above synchronism: this agrees with the ex-
perience on such motors.
In the study of the commutation, the short-circuit current
under the commutator brushes has been assumed as secondary
alternating current. This is completely the case only at stand-
still, but at speed, due to the limited duration of the short-circuit
current in each armature coil—the time of passage of the coij.
under the brush—an exponential term superimposes upon the
alternating, and so modifies the short-circuit current and thereby
the commutation factor, the more, the higher the speed, and
greater thereby the exponential term is. The determination of
this exponential term is beyond the scope of the present work,
but requires the methods of evaluation of transient or momentary
electric phenomena, as discussed in " Theory and Calculation of
Transient Electric Phenomena and Oscillations.'7
/
B. Series Repulsion Motor
219. As further illustration of the application of these funda-
mental equations of the single-phase commutator motor, (1) to
(6), a motor may be investigated, in which the four independent
constants are chosen as follows: