192 ELECTRICAL APPARATUS
with the load, so that the latter arrangement is less suitable when
tying together two systems of constant frequencies.
112. Frequency converters may be used:
(a) For producing a moderate amount of power of a higher or
a lower frequency, from a large alternating-current system.
(6) For tying together two alternating-current systems of
different frequencies, and interchange power between them, so
that either acts as reserve to the other. In this case, electrical
power transfer may be either way.
(c) For local frequency reduction for commutating machines,
by having the general alternating-current transformer lower the
frequency, for instance from 60 to 30 cycles, and take up the
lower frequency, as well as the mechanical power in a commu-
tating machine on the frequency converter shaft. Such a
combination has been called a "Motor Converter"
Thus, instead of a 60-cycle synchronous converter, such a
60/30-cycle motor converter would offer the advantage of the
lower frequency of 30 cycles in the commutating machine. The
commutating machine then would receive half its input electric-
ally, as synchronous converter, half mechanically, as direct-
current generator, and thus would be half converter and half
generator; the induction machine on the same shaft would change
half of its 60-cycle power input into mechanical power, half into
30-cycle electric power.
Such motor converter is smaller and more efficient than a
motor-generator set, but larger and less efficient than a syn-
chronous converter.
Where phase control of the direct-current voltage is desired,
the motor converter as a rule does not require reactors, as the
induction machine has sufficient internal reactance.
(d) For supplying low frequency to a second machine on the
same shaft, for speed control, as "concatenated motor couple/7
That is, two induction motors on the same shaft, operating
in parallel, give full speed, and half speed is produced,
at full efficiency, by concatenating the two induction ma-
chines, that is, using the one as frequency converter for feeding
the other.
By using two machines of different number of poles, pi and
p2, on the same shaft, four different speeds can be secured, corre-
sponding respectively to the number of poles: p\ + p2j p*, pi,
Pi — z>2. That is, concatenation of both machines, operation