SYNCHRONOUS INDUCTION G$N$RATOR 199
synchronism of its generated frequency. Such'"a-"double syn-
chronous alternator" so consists of an induction machine, in
which the stator and the rotor are connected with each other in
parallel or in series, supplied with the reactive exciting current
by a synchronous machine—for instance, by using synchronous
converters with overexcited field as load—and driven at a speed
equal to twice the frequency required. This type of machine
may be useful for prime movers of very high speeds, such as
stearn turbines, as it permits a speed equal to twice that of the
bipolar synchronous machine (3000 revolutions at 25, and 7200
revolutions at 60 cycles).
2. If of the two frequencies, one is chosen so low that the
amount of power generated at this frequency is very small, and
can be taken up by a synchronous machine or other low-fre-
quency machine, the latter then may also be called an exciter.
For instance, connecting the rotor of an induction machine to a
synchronous motor of /2 = 4 cycles, and driving it at a speed
of /o = 64 cycles, generates in the stator an e.m.f. at fi = 60
cycles, and the amount of power generated at 60 cycles is 6% =
15 times the power generated by 4 cycles. The machine then
is an induction generator driven at 15 times its synchronous
speed. Where the power at frequency, /2, is very small, it would
be no serious objection if this power were not generated, but con-
sumed. That is, by impressing /2 = 4 cycles upon the rotor,
and driving it at /o = 56 cycles, in opposite direction to the rotat-
ing field produced in it by the impressed frequency of 4 cycles,
the stator also generates an e.m.f. at fi = 60 cycles. In this
case, electric power has to be put into the machine by a generator
at /2 = 4 cycles, and mechanical power at a speed of /0 = 56
cycles, and electric power is produced as output at/i = 60 cycles.
The machine thus operated is an ordinary frequency converter,
which transforms from a very, low frequency, /2 = 4 cycles, to
frequency /i = 60 cycles or 15 times the impressed frequency,
and the electric power input so is only one-fifteenth of the electric
power output, the other fourteen-fifteenths are given by the
mechanical power input, and the generator supplying the im-
pressed frequency, /2 = 4 cycles, accordingly is so small that it
can be considered as an exciter.
118. 3. If the rotor of frequency, /2, driven at speed, /0, is
connected to the external circuit through a commutator, the
effective frequency supplied by the commutator "brushes to the