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196

ELECTRICAL APPARATUS

machine generates more power as alternator, if the direction of
rotation of the secondary revolving field is in opposition to the
speed. In the former case, the primary frequency equals speed
minus secondary impressed frequency: / = /o — /i; in the latter
case, the primary frequency equals the sum of speed and sec-
ondary impressed frequency: / = /o + fi, and the machine is a
frequency converter or general alternating-current transformer,
with the frequency, /i, as primary, and the frequency, /, as
secondary, transforming up in frequency to a frequency, /,
which is very high compared with the impressed frequency,
so that the mechanical power input into the frequency con-
verter is very large compared with the electrical power input.
The synchronous-induction generator, that is, induction gen-
erator in which the secondary frequency or frequency of slip is
fixed by an impressed frequency, so can also be considered as a
frequency converter or general alternating-current transformer.
115. To transform from a frequency, /i, to a frequency ;/2, the
frequency, /i, is impressed upon the primary of an induction
machine, and the secondary driven at such a speed, or fre-
quency of rotation, /0, -that the difference between primary
impressed frequency, /i, and frequency of rotation, /o, that is,
the frequency of slip, is the desired secondary frequency, /2.
There ar.e two speeds, /0, which fulfill this condition: one
below synchronism: /0 =/i — /2, and one above synchronism:
/o = fi + /2. That is, the secondary frequency becomes /2,
if the secondary runs slower than the primary revolving field
of frequency, fi, or if the secondary runs faster than the primary
field, by the slip, /2.
In the former case, the speed is below synchronism, that is,
the machine generates electric power at the frequency, /2, in the
secondary, and consumes electric power at the frequency, /],
in the primary. If /2 < /i, the speed /0 = fi — /2 is between
standstill and synchronism, and the machine, in addition to
electric power, generates mechanical power, as induction motor,
and as has been seen in the chapter on the " General Alternating-
current Transformer," it is, approximately:
Electric power input -f- electric power output -J- mechanical
power output = fi -5- /2 -f- /0.
If /2 > /i, that is, the frequency converter increases the fre-
queijcy, the rotation must be in backward direction, against the
rotating field, so as to give a slip, /2, greater than the impressed