SYNCHRONOUS RECTIFIER 237

by the increase of rectified current; if it is high, it has little effect.
Furthermore, this resistance should vary with the current.

The belt-driven alternators of former days frequently had a
compounding series field excited by such a rectifying commutator
on the machine shaft, and by shunting 40 to 50 per cent, of the
power through the two resistance shunts, with careful setting of
brushes as much as 2000 watts have been rectified from single-
phase 125-cycle supply.

Single-phase synchronous motors were started by such recti-
fying commutators through which the field current passed, in
series with the armature, and the first long-distance power trans-

o

iji

FIG. 79.—Open-circuit rectifier. FIG. 80.—Short-circuit rectifier.
mission in America (Telluride) was originally operated with
single-phase machines started by rectifying commutator—the
commutator, however, requiring frequent renewal.
139. The reversal of connection between the rectified circuit
and the supply circuit may occur either over open-circuit, or
over short-circuit. That is, either the rectified circuit is first
disconnected from the supply circuit—which open-circuits both
—and then connected in reverse direction, or the rectified circuit
is connected to the supply circuit in reverse direction, before
being disconnected in the previous direction—which short-circuits
both circuits. The former, open-circuit rectification, results if
the width of the gap between the commutator segments is greater
than the width of the brushes, Fig. 79, the latter, short-circuit
rectification, results if the width of the gap is less than the width
of the brushes, Fig. 80.
In open-circuit rectification, the alternating and the rectified
voltage are shown as e and eg in Fig. 81. If the circuit is non-
inductive, the rectified current, io, has the same shape as the vol-