CHAPTER XVIII
SURGING OF SYNCHRONOUS MOTORS
166. In the theory of the synchronous motor the assumption
is made that the mechanical output of the motor equals the power
developed by it. This is the case only if the motor runs at
constant speed. If, however, it accelerates, the power input is
greater; if it decelerates, less than the power output, by the power
stored in and returned by the momentum. Obviously, the
motor can neither constantly accelerate nor decelerate, without
breaking out of synchronism.
If, for instance, at a certain moment the power produced by
the motor exceeds the mechanical load (as in the moment of
throwing off a part of the load), the excess power is consumed by
the momentum as acceleration, causing an increase of speed.
The result thereof is that the phase of the counter e.m.f., e,
is not constant, but its vector, e, moves backward to earlier time,
or counter-clockwise, at a rate depending upon the momentum.
Thereby the current changes and the power developed changes
and decreases. As soon as the power produced equals the load,
the acceleration ceases, but the vector, e, still being in motion,
due to the increased speed, further reduces the power, causing
a retardation and thereby a decrease of speed, at a rate depend-
ing upon the mechanical momentum. In this manner a periodic
variation of the phase relation between e and e0, and correspond-
ing variation of speed and current occurs, of an amplitude and
period depending upon the circuit conditions-and the mechanical'
momentum.
If the amplitude of this pulsation has a positive decrement,
that is, is decreasing, the motor assumes after a while a constant
position of e regarding eQ, that is, its speed becomes uniform.
If, however, the decrement of the pulsation is negative, an
infinitely small pulsation will continuously increase in amplitude,
until the motor is thrown out of step, or the decrement becomes
zero, by the power consumed by forces opposing the pulsation,
as anti-surgirig devices, or by the periodic pulsation of the syn-
chronous reactance, etc. If the decrement is zero, a pulsation
Oftft