246

ELECTRICAL APPARATUS

magnitude, that is, larger than the exciting current of the trans-
former, it saturates the transformer iron. Running at or beyond
magnetic saturation, the primary exciting current of the trans-
former then becomes excessive, the hysteresis heating due to the
unsymmetrical magnetic cycle is greatly increased, and the
transformer endangered or destroyed.

PIG. 97.—Contact-making rectifier
with direct-current rotor.

FIG. 98.—Contact-making rectifier
with alternating-current rotor.

Half-wave rectifiers thus are impracticable except for extremely
small power.

The full-wave contact-making rectifier, Fig. 97 or 98, does not
have this objection. In this type of rectifier, the connection be-
tween rectified receiver circuit and
alternating supply circuit are not
synchronously reversed, as in Fig. 95
or 96, but in Fig. 97 one side of the
rectified circuit, B, is permanently
connected to the middle m of the
alternating supply circuit, T, while the
other side of the rectified circuit is
synchronously connected and discon-
nected with the two sides, a and
6, of the alternating supply circuit.
Or we may say: the rectified circuit takes one-half wave from
the one transformer half coil, ma, the other half wave from
the other transformer half coil, mb. Thus, while each of the two
transformer half coils carries unidirectional current, the uni-
directional currents in the two half coils flow in opposite direc-
tion, thus give magnetically the same effect as one alternating

FIG. 99.—Half-wave rectifier,
contact making.