456 ELECTRICAL APPARATUS
sections, 1 and 1', 2 and 2', 3 and 3', 4 and 4', are connected in
multiple (to equalize the flux distribution) between four pairs of
collector rings, shown in Fig. 225 as 1 and li, 2 and 2i, 3 and 3i,
4 and 4i. The latter are connected in series. This machine,
Figs. 224 and 225, thus could also be used as a three-wire or
five-wire machine, or as a direct-current converter, by bringing
out intermediary connections, from the collector rings 2, 3, 4.
250. As each conductor of the unipolar machine requires a
separate pair of collector rings, with a reasonably moderate
number of collector rings, unipolar machines of medium capacity
are suited for low voltages only, such as for electrolytic machines,
and have been built for this purpose to a limited extent, but in
general it has been found more economical by series connection
of the electrolytic cells to permit the use of higher voltages, and
then employ standard machines.
For commercial voltages, 250 or 600, to keep the number of
collector rings reasonably moderate, unipolar machines require
very large magnetic fluxes—that is, large units of capacity—and
very high peripheral speeds. The latter requirement made this
machine type unsuitable during the days of the slow-speed direct-
connected steam engine, but when the high-speed steam turbine
arrived, the study-of the design of high-powered steam-turbine-
driven unipolars was undertaken, and a number of such machines
built and installed.
In the huge turbo-alternators of today, the largest loss is the
core loss: hysteresis and eddies in the iron, which often is more
than all the other losses together. Theoretically, the unipolar
machine has-no core loss, as the magnetic flux does not change
anywhere, and solid steel thus is used throughout—and has to
be used, due to the shape of the magnetic circuit. However,
with the enormous magnetic fluxes of these machines, in solid
iron, the least variation of the magnetic circuit, such as caused
by small ^inequalities of the air gap, by the reaction of the arma-
ture currents, etc., causes enormous core losses, mostly eddies,
and while theoretically the unipolar has no core loss, designing
experience has shown, that it is a very difficult problem to keep
the core loss in such machines down to reasonable values. Fur-
thermore, in and at the collector rings, the magnetic reaction of
the armature currents is alternating or pulsating. Thus- in Figs.
224 and 225, the point of entrance of the current from the arma-
ture conductors into the collector rings revolves with the rotation