SURGING OF SYNCHRONOUS MOTORS

297

External sources of negative, P2, may be, for instance, the
magnetic field of a self-exciting, direct-current generator, driven
by the synchronous motor. With decrease of speed, this field
decreases, due to the decrease of generated voltage, and increases
with increase of speed. This change of field strength, however,
lags behind the exciting voltage and thus speed, that is, during
decrease of speed the output is greater than during increase of
speed. If this direct-current generator is the exciter of the
synchronous motor, the effect may be intensified.
The change of power input into the synchronous motor, with
change of speed, may cause the governor to act on the prime
mover driving the generator, which supplies power to the motor,
and the lag of the governor behind the change of output gives a
pulsation of the generator frequency, of 60, which acts like
a negative power, P2. The pulsation of impressed voltage,
caused by the pulsation of /3, may give rise to a negative,
P2, also.
An internal cause of a negative term, P2, is found in the lag
of the synchronous motor field behind the resultant m.m.f. In
the preceding discussion, e is the "nominal generated e.m.f."
of the synchronous machine, corresponding to the field excita-
tion. The actual magnetic flux of the machine, however, does
not correspond to e, and thus to the field excitation, but corre-
sponds to the resultant m.m.f. of field excitation and armature
reaction, which latter varies in intensity and in phase during the
oscillation of /3. Hence, while e is constant, the magnetic flux
is not constant, but pulsates with the oscillations of the machine.
This pulsation of the magnetic flux lags behind the pulsation of
m.m.f., and thereby gives rise to a term in b in equation (28).
If PQ, ft e} e0, Z are such that a retardation of the motor increases
the magnetizing, or decreases the demagnetizing force of the
armature reaction, a regative term, Pi, appears, otherwise a
positive term.
P2 in this case is the energy consumed by the magnetic cycle
of the machine at full frequency, assuming the cycle at full fre-
quency as the same as at frequency of slip, s.
Or inversely, e may be said to pulsate, due to the pulsation of
armature reaction, with the same frequency as /?, but with a
phase, which may either be lagging or leading. Lagging of the
pulsation of e causes a negative, leading a positive, P2.
P2, therefore, represents the power due to the pulsation of e