232 ELECTRICAL APPARATUS
to balance the polyphase system, that is, to shift half of the single
phase power by a quarter period, and thus produce a uniform
flow of power.
Such synchronous phase balancer constructively is a synchro-
nous machine, having two sets of field poles, A and By in quad-'
rature with each other. Then by varying or reversing the
excitation of the two sets of field poles, any phase relation of the
reversely rotating polyphase system of the balancer to that of the
converter can be produced, from zero to 360°.
137. Large single-phase powers, such as are required for single-
phase railroading, thus can be produced.
(a) By using single-phase generators and separate single-phase
supply circuits.
(f>) By using single-phase generators running in multiple with
the general three-phase system, and controlling voltage and me-
chanical power supply so as to absorb the single-phase load by the
single-phase generators. In this case, however, if the single-
phase load uses the same transmission line as the three-phase
load, phase balancing at the receiving circuit may be necessary.
(c) By taking the single-phase load from the three-phase
system. If the load is considerable, this may require special
construction of the generators, and phase balancers.
(<$) By taking the power all as balanced three-phase power
from the generating system, and converting the required amount
to single-phase, by phase converter and phase balancer. This
may be -done in the generating station, or at the receiving station
where the single-phase power is required.
Assuming that in addition to a balanced three-phase load of
power, Po, a single-phase load of power, P, is required. Estimating
roughly, that the single-phase capacity of a machine structure is
half the three-phase capacity of the structure—which probably
is not far wrong—then the use of single-phase generators gives
us Po-kw. three-phase, and P-kw. single-phase generators, and as
the latter is equal in size to 2 P-kw. three-phase capacity, the
total machine.capacity would be P0 + 2 P.
Three-phase generation and phase conversion would require
Po + P kw. in three-phase generators, and phase converters
transferring half the single-phase power from the phase which is
loaded by single-phase, to the quadrature phase. That is, the
phase converter must have a capacity of P/2 kw. in the motor
phase, and P/2 kw. capacity in the generator phase, or a total