404 ELECTRICAL APPARATUS

to field circuit, that is:

number of field turns ' nnn

___________

number of effective short-circuit turns'

hence a large quantity.

The absolute value of the short-circuit current, therefore, is:

hence a minimum for that value of t, where :

/ = Co2 _|_ £2 (1 _ t (Co2 + 52)) 2 = minimum, or
= 1 - t (Co2 + S2) == 0, hence,

and:

S •=

That is, t = ~~ = „ .-—i gives minimum short-circuit cur-
' e 02 + Co2

rent at speed, S, and inversely, speed S = \ T — c02, gives
, V £
minimum short-circuit current at voltage ratio, £.
For t = 1, or the repulsion motor with secondary excitation,
the short-circuit current is minimum at speed, S = \/l — c02, or
somewhat below synchronism, and is ^'4 = — , while in the re-
^
pulsion motor with primary excitation, the short-circuit current
is a minimum, and equals zero, at synchronism S = 1.
The lower the voltage ratio, t = -2, the higher is the speed, S,
@
at which the short-circuit current reaches a minimum.
The short-circuit current, /'4, however, is of far less importance
than the commutation current, Jg.
(6) Commutation Current
222. While the value, l'g = /'4 + /i, or the current change in
the armature coils while entering commutation, is of minor irn-
portance, of foremost importance for good commutation is that
the current change in the armature coils, when leaving the short-
circuit under the brushes:
7, = /i- /'« (103)
is zero or a minimum.