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CHAPTER XIII
SYNCHRONOUS INDUCTION GENERATOR
113. If an induction machine is driven above synchronism,
the power component of the primary current reverses, that is,
energy flows outward, and the machine becomes an induction
generator. The component of current required for magnetiza-
tion remains, however, the same; that is, the induction generator
requires the supply of a reactive current for excitation, just as
the induction motor, and so must be connected to some apparatus
which gives a lagging, or, what is the same, consumes a leading
current.
The frequency of the e.m.f. generated by the induction gen-
erator, /, is lower than the frequency of rotation or speed, /<>,
by the frequency, /i, of the secondary currents. Or, inversely,
the frequency, /i, of the secondary circuit is the frequency of
slip—that is, the frequency with which the speed of mechanical
rotation slips behind the speed of the rotating field, in the induc-
tion motor, or the speed of the rotating field slips behind the
speed of mechanical rotation, in the induction generator.
As in every transformer, so in the induction machine, the
secondary current must have the same ampere-turns as the
primary current less the exciting current, that is, the secondary
current is approximately proportional to the primary current,
or to the load of the induction generator.
In an induction generator with short-circuited secondary,
the secondary currents are proportional, approximately, to the
e.m.f. generated in the secondary circuit, and this e.m.f. is pro-
portional to the frequency of the secondary circuit, that IB,
the slip of frequency behind speed. It so follows that the slip
of frequency in the induction generator with short-circuited
secondary is approximately proportional to the load, that is,
such an induction generator does not produce constant syn-
chronous frequency, but a frequency which decreases slightly
with increasing load, just as the speed of the induction motor
decreases slightly with increase of load.
Induction generator and induction motor so have also been