38 ELECTRIC DISCHARGES, WAVES AND IMPULSES,
with the armature current, that is, is instantaneous. The arma-
ture reaction, however, is the m.m.f. of the armature current in its
reaction on the m.m.f. of the field-exciting current* That in, that
part #2 = &o — #1 of the synchronous reactance which corresponds
to the armature reaction is not a true reactance at all, consumes
no voltage, but represents the consumption of field ampere turns
by the m.m.f. of the armature current, and the corresponding
change of field flux. Since, however, the field flux represents
stored magnetic energy, it cannot change instantly, and the arma-
ture reaction thus does not appear instantaneously with the arma-
ture current, but shows a transient which is determined essentially
by the constants of the field circuit, that is, is the counterpart of
the field transient of the machine,
If then an alternator is shortnsireuitod, in the firnt moment only
the true self-inductive part Xi of the synchronous reactance exists,
and the armature current thus is i\ = ;-°f where <?« is the induced
#i
e.m.f., that is, the voltage corresponding to the magnetic-field
excitation flux existing before the short circuit. Gradually the,!
armature reaction lowers the field flux, in the manner an repre-
sented by the synchronous reactance #Q, and the whort-circuit cur-
rent decreases to the value io ** -•"' *
• • ^o
The ratio of the momentary short-circuit current to the perma-
nent short-circuit current thus is, approximately, the ratio v -> * %
i« x\
that is, synchronous reactance to Rolf-indwtivo reactance, or
armature reaction plus armature Belf-induction, to armature
self-induction. In, machines of relatively low Helf-induetion
and high armature reaction, the momentary Hhort-circnut cur-
rent thus may be many times the permanent Hhort-mrmtit
current.
The field flux remaining at short circuit m that giving the volt-
age consumed by the armature self-induction, while the decrease
of field flux between open circuit and short circuit corresponds to
the armature reaction, The ratio of the open-circuit field flux to
the short-circuit field flux thus is the ratio of armature reaction
plus self-induction, to the self-induction; or of the synchronous
reactance to the self-inductive reactance: « *