214:

ELECTRICAL APPARATUS

com ation and inconvenience incident to the use of two trolley
wires. In this case, as large amounts of polyphase power are
required, and economy in weight is important, momentum is
enerally used for energy storage, that is an induction machine
is employed as phase converter, and then is used either in series
or in shunt to the motor.

For the small amounts of power required by use (a), generally
inductance or capacity are employed; and even then usually the
conversion is made not to polyphase, but to monocyclic, as the
latter is far more economical in apparatus.

Conversion from polyphase to single-phase obviously means
the problem of deriving single-phase power from a balanced
polyphase system. A single-phase load can be taken from any
phase of a polyphase system, but such a load, when consider-
able, unbalances the polyphase system, that is, makes the vol-
tages of the phases unequal and lowers the generator capacity.
The problem thus is, to balance the voltages and the reaction of
the load on the generating system.

This problem has become of considerable importance in the
last years, for the purpose of taking large single-phase loads, for
electric railway, furnace work, etc., from a three-phase supply
system as a central station or transmission line. For this pur-
pose, usually synchronous phase converters with synchronous
phase balancers are used.

As illustration may thus be considered in the following the
monocyclic device, the induction phase converter, and the
synchronous phase converter and balancer.

Monocyclic Devices
127. The name "monocyclic" is applied to a polyphase sys-
tem of voltages (whether symmetrical or unsymmetrical), in
which the flow of energy is essentially single-phase.
For instance^ if, as shown diagrammatically in Fig. 67, we
connect, between single-phase mains, AB, two pairs of non-in-
ductive resistances, r, and inductive reactances, x (or in general,
two pairs of impedances of different inductance factors), such
that r = x, consuming the voltages EI and E2 respectively, then
the voltage eQ = CD is in quadrature with, and equal to, the
voltage e = AB, and the two voltages, e and eo, constitute a
monocyclic-system of quarter-phase voltages: e gives the energy