COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.
In the preceding descriptions relative to synchronizing motors and
methods of operating them, reference has been made to the plan adopted
by Mr. Tesla, which consists broadly in winding or arranging the motor
in such manner that by means of suitable switches it could be started as
a multiple-circuit motor, or one operating by a progression of its
magnetic poles, and then, when up to speed, or nearly so, converted into
an ordinary synchronizing motor, or one in which the magnetic poles were
simply alternated. In some cases, as when a large motor is used and when
the number of alternations is very high, there is more or less
difficulty in bringing the motor to speed as a double or
multiple-circuit motor, for the plan of construction which renders the
motor best adapted to run as a synchronizing motor impairs its
efficiency as a torque or double-circuit motor under the assumed
conditions on the start. This will be readily understood, for in a large
synchronizing motor the length of the magnetic circuit of the polar
projections, and their mass, are so great that apparently considerable
time is required for magnetization and demagnetization. Hence with a
current of a very high number of alternations the motor may not respond
properly. To avoid this objection and to start up a synchronizing motor
in which these conditions obtain, Mr. Tesla has combined two motors, one
a synchronizing motor, the other a multiple-circuit or torque motor, and
by the latter he brings the first-named up to speed, and then either
throws the whole current into the synchronizing motor or operates
jointly both of the motors.
This invention involves several novel and useful features. It will be
observed, in the first place, that both motors are run, without
commutators of any kind, and, secondly, that the speed of the torque
motor may be higher than that of the synchronizing motor, as will be the
case when it contains a fewer number of poles or sets of poles, so that
the motor will be more readily and easily brought up to speed. Thirdly,
the synchronizing motor may be constructed so as to have a much more
pronounced tendency to synchronism without lessening the facility with
which it is started.
Fig. 77 is a part sectional view of the two motors; Fig. 78 an end view
of the synchronizing motor; Fig. 79 an end view and part section of the
torque or double-circuit motor; Fig. 80 a diagram of the circuit
connections employed; and Figs. 81, 82, 83, 84 and 85 are diagrams of
modified dispositions of the two motors.
[Illustration: FIG. 77.]
Inasmuch as neither motor is doing any work while the current is acting
upon the other, the two armatures are rigidly connected, both being
mounted upon the same shaft A, the field-magnets B of the synchronizing
and C of the torque motor being secured to the same base D. The
preferably larger synchronizing motor has polar projections on its
armature, which rotate in very close proximity to the poles of the
field, and in other respects it conforms to the conditions that are
necessary to secure synchronous action. The pole-pieces of the armature
are, however, wound with closed coils E, as this obviates the employment
of sliding contacts. The smaller or torque motor, on the other hand,
has, preferably, a cylindrical armature F, without polar projections and
wound with closed coils G. The field-coils of the torque motor are
connected up in two series H and I, and the alternating current from the
generator is directed through or divided between these two circuits in
any manner to produce a progression of the poles or points of maximum
magnetic effect. This result is secured by connecting the two
motor-circuits in derivation with the circuit from the generator,
inserting in one motor circuit a dead resistance and in the other a
self-induction coil, by which means a difference in phase between the
two divisions of the current is secured. If both motors have the same
number of field poles, the torque motor for a given number of
alternations will tend to run at double the speed of the other, for,
assuming the connections to be such as to give the best results, its
poles are divided into two series and the number of poles is virtually
reduced one-half, which being acted upon by the same number of
alternations tend to rotate the armature at twice the speed. By this
means the main armature is more easily brought to or above the required
speed. When the speed necessary for synchronism is imparted to the main
motor, the current is shifted from the torque motor into the other.
[Illustration: FIG. 78.]
[Illustration: FIG. 79.]
A convenient arrangement for carrying out this invention is shown in
Fig. 80, in which J J are the field coils of the synchronizing, and H I
the field coils of the torque motor. L L' are the conductors of the main
line. One end of, say, coils H is connected to wire L through a
self-induction coil M. One end of the other set of coils I is connected
to the same wire through a dead resistance N. The opposite ends of these
two circuits are connected to the contact _m_ of a switch, the handle or
lever of which is in connection with the line-wire L'. One end of the
field circuit of the synchronizing motor is connected to the wire L. The
other terminates in the switch-contact _n_. From the diagram it will be
readily seen that if the lever P be turned upon contact _m_, the torque
motor will start by reason of the difference of phase between the
currents in its two energizing circuits. Then when the desired speed is
attained, if the lever P be shifted upon contact _n_ the entire current
will pass through the field coils of the synchronizing motor and the
other will be doing no work.
The torque motor may be constructed and operated in various ways, many
of which have already been touched upon. It is not necessary that one
motor be cut out of circuit while the other is in, for both may be acted
upon by current at the same time, and Mr. Tesla has devised various
dispositions or arrangements of the two motors for accomplishing this.
Some of these arrangements are illustrated in Figs. 81 to 85.
[Illustration: FIG. 80.]
Referring to Fig. 81, let T designate the torque or multiple circuit
motor and S the synchronizing motor, L L' being the line-wires from a
source of alternating current. The two circuits of the torque motor of
different degrees of self-induction, and designated by N M, are
connected in derivation to the wire L. They are then joined and
connected to the energizing circuit of the synchronizing motor, the
opposite terminal of which is connected to wire L'. The two motors are
thus in series. To start them Mr. Tesla short-circuits the synchronizing
motor by a switch P', throwing the whole current through the torque
motor. Then when the desired speed is reached the switch P' is opened,
so that the current passes through both motors. In such an arrangement
as this it is obviously desirable for economical and other reasons that
a proper relation between the speeds of the two motors should be
observed.
In Fig. 82 another disposition is illustrated. S is the synchronizing
motor and T the torque motor, the circuits of both being in parallel. W
is a circuit also in derivation to the motor circuits and containing a
switch P''. S' is a switch in the synchronizing motor circuit. On the
start the switch S' is opened, cutting out the motor S. Then P'' is
opened, throwing the entire current through the motor T, giving it a
very strong torque. When the desired speed is reached, switch S' is
closed and the current divides between both motors. By means of switch
P'' both motors may be cut out.
[Illustration: FIGS. 81, 82, 83, 84 and 85.]
In Fig. 83 the arrangement is substantially the same, except that a
switch T' is placed in the circuit which includes the two circuits of
the torque motor. Fig. 84 shows the two motors in series, with a shunt
around both containing a switch S T. There is also a shunt around the
synchronizing motor S, with a switch P'. In Fig. 85 the same disposition
is shown; but each motor is provided with a shunt, in which are switches
P' and T'', as shown.