CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTOR.
A description is given elsewhere of a method of operating alternating
current motors by first rotating their magnetic poles until they have
attained synchronous speed, and then alternating the poles. The motor is
thus transformed, by a simple change of circuit connections from one
operated by the action of two or more independent energizing currents to
one operated either by a single current or by several currents acting as
one. Another way of doing this will now be described.
At the start the magnetic poles of one element or field of the motor are
progressively shifted by alternating currents differing in phase and
passed through independent energizing circuits, and short circuit the
coils of the other element. When the motor thus started reaches or
passes the limit of speed synchronous with the generator, Mr. Tesla
connects up the coils previously short-circuited with a source of direct
current and by a change of the circuit connections produces a simple
alternation of the poles. The motor then continues to run in synchronism
with the generator. The motor here shown in Fig. 41 is one of the
ordinary forms, with field-cores either laminated or solid and with a
cylindrical laminated armature wound, for example, with the coils A B at
right angles. The shaft of the armature carries three collecting or
contact rings C D E. (Shown, for better illustration, as of different
diameters.)
One end of coil A connects to one ring, as C, and one end of coil B
connects with ring D. The remaining ends are connected to ring E.
Collecting springs or brushes F G H bear upon the rings and lead to the
contacts of a switch, to be presently described. The field-coils have
their terminals in binding-posts K K, and may be either closed upon
themselves or connected with a source of direct current L, by means of a
switch M. The main or controlling switch has five contacts _a b c d e_
and two levers _f g_, pivoted and connected by an insulating cross-bar
_h_, so as to move in parallelism. These levers are connected to the
line wires from a source of alternating currents N. Contact _a_ is
connected to brush G and coil B through a dead resistance R and wire P.
Contact _b_ is connected with brush F and coil A through a
self-induction coil S and wire O. Contacts _c_ and _e_ are connected to
brushes G F, respectively, through the wires P O, and contact _d_ is
directly connected with brush H. The lever _f_ has a widened end, which
may span the contacts _a b_. When in such position and with lever _g_ on
contact _d_, the alternating currents divide between the two
motor-coils, and by reason of their different self-induction a
difference of current-phase is obtained that starts the motor in
rotation. In starting, the field-coils are short circuited.
[Illustration: FIG. 41.]
When the motor has attained the desired speed, the switch is shifted to
the position shown in dotted lines--that is to say, with the levers _f
g_ resting on points _c e_. This connects up the two armature coils in
series, and the motor will then run as a synchronous motor. The
field-coils are thrown into circuit with the direct current source when
the main switch is shifted.