FREQUENCY CONVERTER 191
produces electric power in both circuits, primary and secondary,
thus can not be called a frequency converter, and the distinc-
tion between primary and secondary circuits ceases, but both
circuits are generator circuits. The machine then is a two-fre-
quency induction generator. As the electric power generated
at the two frequencies is proportional to the frequencies, this
gives a limitation to the usefulness of the machine, and it appears
suitable only in two cases:
(a) If s = — 1, both frequencies are the same, and stator
and rotor circuits can be connected together, in parallel or in
series, giving the " double synchronous-induction generator."
Such machines have been proposed for steam-turbine alternators
of small and moderate sizes, as they permit, with bipolar con-
struction, to operate at twice the maximum speed available for
the synchronous machine, which is 1500 revolutions for 25 cycles,
and 3600 revolutions for 60 cycles.
(&) If s is very small, so that the power produced in the low-
frequency circuit is very small and may be absorbed by a small
"low-frequency exciter."
Further discussion of both of these types is given in the
Chapter XIII on the " Synchronous Induction Generator."
111. The use of the general alternating-current transformer as
frequency converter is always accompanied by the production
of mechanical power when lowering, and by the consumption
of mechanical power when raising the frequency. Thus a second
machine, either induction or synchronous, would be placed on the
frequency converter shaft to supply the mechanical power as
motor when raising the frequency, or absorb the power as
generator, when lowering the frequency. This machine may be
of either of the two frequencies, but would naturally, for eco-
nomical reasons, be built for the supply frequency, when motor,
and for the generated or secondary frequency, when generator.
Such a couple of frequency converter and driving motor and
auxiliary generator has over a motor-generator set the advan-
tage, that it requires a total machine capacity only equal to the
output, while with a motor-generator set the total machine
capacity equals twice the output. It has, however, the dis-
advantage not to be as standard as the motor and the generator.
If a synchronous machine is used, the frequency is constant;
if an induction machine is used, there is a slip, increasing with
the load, that is, the ratio of the two frequencies slightly varies