REACTION MACHINES 261
is zero, and the only e.m.f. existing in the alternator is the e.m.f.
of self-induction; that is, the e.m.f. induced by the alternating
current upon itself. If, however, the synchronous reactance is
constant, the counter e.m.f. of self-induction is in quadrature
with the current and wattless; that is, can neither produce nor
consume energy.
In the synchronous motor running without field excitation,
always a large lag of the current behind the impressed e.m.f.
exists; and an alternating-current generator will yield an e.m.f.
without field excitation only when closed by an external circuit
of large negative reactance; that is, a circuit in which the current
leads the e.m.f., as a condenser, or an overexcited synchronous
motor, etc.
148. The usual explanation of the operation of the synchronous
machine without field excitation is self-excitation by reactive
armature currents. In a synchronous motor a lagging, in a
generator a leading armature current magnetizes the field, and in
such a case, even without any direct-current field excitation, there
is a field excitation and thus a magnetic field flux, produced by the
m.m.f. of the reactive component of the armature currents. In
the polyphase machine, this is constant in intensity and direc-
tion, in the single-phase machine constant in direction, but pul-
sating in intensity, and the intensity pulsation can be reduced
by a short-circuit winding around the field structure, as more
fully discussed under "Synchronous Machines.7'
Thus a machine as shown diagrammatically in Fig. 124, with
a polyphase (three-phase) current impressed on the rotating
armature, A, and no winding on the field poles, starts, runs up
to synchronous and does considerable work as synchronous
motor, and under load may even give a fairly good (lagging) power-
factor. With a single-phase current impressed upon the arma-
ture, A, it does not start, but when brought up to synchronism,
continues to run as synchronous motor. Driven by mechanical
power, with a leading current load it is a generator.
However, the operation of such machines depends on the
existence of a polar field structure, that is a structure having a
low reluctance in the direction of the field poles, P — P, and a
high reluctance in quadrature position thereto. Or, in other
words, the armature reactance with the coil facing the field poles
is high, and low in the quadrature position thereto.
In a structure with uniform magnetic reluctance, in which