INDUCTION-MOTOR REGULATION

187

induction machines, the phenomenon is frequently observed that
the machine starts at moderate voltage, but does not run up to
synchronism, but stops at an intermediary speed, in the neighbor-
hood of half speed, and a considerable increase of voltage, and
thereby of motor torque, is required to bring the machine beyond
the dead point, or rather "dead range," of speed and make it
run up to synchronism. In this case, however, the phenomenon
is complicated by the effects due to varying magnetic reluctance
(magnetic locking), inductor machine effect, etc.
Instability of such character as here described occurs in elec-
tric circuits in many instances, of which the most typical is the
electric arc in a constant-potential supply. It occurs whenever
the effect produced by any cause increases the cause and thereby
becomes cumulative. When dealing with energy, obviously
the effect must always be in opposition to the cause (Lenz's
Law), as result of the law of conservation of energy. When
dealing with other phenomena, however, a$ the speed-torque
relation or the volt-ampere relation, etc., instability due to the
effect assisting the cause, intensifying it, and thus becoming
cumulative, may exist, and frequently does exist, and causes
either indefinite increase or decrease, or surging or hunting, as
more fully discussed in Chapters X and XI, of " Theory and
Calculation of Electric Circuits."
4. GENERATOR REGULATION AND STABILITY
85. If the voltage at the induction-motor terminals decreases
with increase of load, the maximum torque and output are de-
creased the more the greater the drop of voltage. But even if
the voltage at the induction motor terminals is maintained con-
stant, the maximum torque and'power may be reduced essen-
tially, in a manner depending on the rapidity with which the
voltage* regulation at changes of load is effected by the generator
or potential regulator, which maintains constancy,of voltage, and
the rapidity with which the motor speed can change, that is,
the mechanical momentum of the motor and its load.
This instability of the motor, produced by the generator
regulation, may be discussed for the case of a load requiring
constant torque at all loads, though the corresponding pheno-
menon may exist at all classes of load, as discussed under 3,
and may occur even with a load proportional to the square of
the speed, as ship propellors.