414

ELECTRICAL APPARATUS

Hence, substituting into the preceding equations:

(90) ZK = Z3 - jScoZ - X4c0Z (c0 - jS) + X2Z'

= (0.160 + 0.975 S) + j (0.590 - 0.187 S),

ff\c\\ T __ ^ :L (o* c\ f/» /i Qf\ _i_ \ i

(yz) /i-^-^u-OA4ico-j-a;-i-\»i

' = ^ + ^? {(- 0.031 + 0.035 S - 0.179 S2}

ZlJ\. Z/JL\.

- j (- 0.007 + 0.072 S + 0.087 S2)},

(91) /2 = /x (0.969 + 0.007;) + et (0.010 - 0.096 j)

(93)

_e(Q.Q72+Q.Q35fi+Set{(Q.Q16-Q.Q72S)-j 0.045+0.035 £)j

/4" z^c

etc.

226. As seen, these four curves are very similar to each other
and to those of the repulsion motor, with the exception of the

commutation current, ig

and commutation factor, k = -?•

I/

The commutation factor of the compensated series motor,
that is, the ratio of current change in the armature coil while
leaving the brushes, to total armature current, is constant in the
series motor, at all speeds. In the series repulsion motors, the
commutation factor, fc, starts with the same value at standstill,
as the series motor, but decreases with increasing speed, thus
giving a superior commutation to that of the series motor, reaches
a minimum, and then increases again. Beyond the minimum
commutation factor, the efficiency, power-factor, torque and out-
put of the motor first slowly, then rapidly decrease, due to the
rapid increase of the commutation losses. These higher values,
however, are of little practical value, since the commutation is
bad.

The higher the voltage ratio, t, that is, the more voltage is
impressed upon the compensating circuit, and the less upon the
armature circuit, the lower is the speed at which the commuta-
tion factor is a minimum, and the commutation so good or perfect.
That is, with t — 1, or the repulsion motor with secondary ex-
citation, the commutation is best at 70 per cent, of synchronism,
and gets poor above synchronism. With t = 0.5, or a series
repulsion motor with half the voltage on the compensating, half
on the armature circuit, the commutation is best just above syn-
chronism, with the motor constants chosen in this instance, and

'.'I IL