264

ELECTRICAL APPARATUS

Instead of cutting out segments of the rotor, in Pig. 126, the
same effect can be produced, with a cylindrical rotor, by a short-
circuited turn, S, as shown in Fig. 128, This gives a periodic
variation of the effective reluctance, from a minimum, shown in
Fig. 128, to a maximum in the position shown in dotted lines in
Fig. 128.

This latter structure is the so-called " synchronous-induction
motor," Chapter VIII, which here appears as a special form of
the reaction machine.

If a direct current is sent through the winding of the machine,

FIG. 126.—Bipolar reac- FIG. 127.—Four-polar FIG. 128.—Synchronous-
tiorx machine. reaction machine, induction motor as reac-
tion machine.
Fig. 126 or 127, a pulsating voltage and current is produced in
this winding. By having two separate windings, and energizing
the one by a direct current, we get a converter, from direct cur-
rent'in the first, to alternating current in the second winding.
The maximum voltage in the second winding can not exceed the
voltage, per turn, in the exciting winding, thus is very limited,
and so is the current. Higher values are secured by inserting a
high "inductance in series in the direct-current winding. In this
case, a single winding may be used and the alternating-circuit
shunted across the machine terminals, inside of the inductance.
151. Obviously, if the reactance or reluctance is variable, it
will perform a complete cycle during the time the armature coil
moves from one field pole to the next field pole, that is, during
one-half wave of the main current. That is, in other words,
the reluctance and reactance vary with twice the frequency of
the alternating main current. Such a. case is shown in Figs.
129 and 130. The impressed e.m.f., and thus at negligible
resistance, the counter e.m.f., is represented by the sine wave,