SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.
In the first chapters of this section we have, bearing in mind the broad
underlying principle, considered a distinct class of motors, namely,
such as require for their operation a special generator capable of
yielding currents of differing phase. As a matter of course, Mr. Tesla
recognizing the desirability of utilizing his motors in connection with
ordinary systems of distribution, addressed himself to the task of
inventing various methods and ways of achieving this object. In the
succeeding chapters, therefore, we witness the evolution of a number of
ideas bearing upon this important branch of work. It must be obvious to
a careful reader, from a number of hints encountered here and there,
that even the inventions described in these chapters to follow do not
represent the full scope of the work done in these lines. They might,
indeed, be regarded as exemplifications.
We will present these various inventions in the order which to us
appears the most helpful to an understanding of the subject by the
majority of readers. It will be naturally perceived that in offering a
series of ideas of this nature, wherein some of the steps or links are
missing, the descriptions are not altogether sequential; but any one who
follows carefully the main drift of the thoughts now brought together
will find that a satisfactory comprehension of the principles can be
gained.
As is well known, certain forms of alternating-current machines have the
property, when connected in circuit with an alternating current
generator, of running as a motor in synchronism therewith; but, while
the alternating current will run the motor after it has attained a rate
of speed synchronous with that of the generator, it will not start it.
Hence, in all instances heretofore where these "synchronizing motors,"
as they are termed, have been run, some means have been adopted to bring
the motors up to synchronism with the generator, or approximately so,
before the alternating current of the generator is applied to drive
them. In some instances mechanical appliances have been utilized for
this purpose. In others special and complicated forms of motor have been
constructed. Mr. Tesla has discovered a much more simple method or plan
of operating synchronizing motors, which requires practically no other
apparatus than the motor itself. In other words, by a certain change in
the circuit connections of the motor he converts it at will from a
double circuit motor, or such as have been already described, and which
will start under the action of an alternating current, into a
synchronizing motor, or one which will be run by the generator only when
it has reached a certain speed of rotation synchronous with that of the
generator. In this manner he is enabled to extend very greatly the
applications of his system and to secure all the advantages of both
forms of alternating current motor.
The expression "synchronous with that of the generator," is used here in
its ordinary acceptation--that is to say, a motor is said to synchronize
with the generator when it preserves a certain relative speed determined
by its number of poles and the number of alternations produced per
revolution of the generator. Its actual speed, therefore, may be faster
or slower than that of the generator; but it is said to be synchronous
so long as it preserves the same relative speed.
In carrying out this invention Mr. Tesla constructs a motor which has a
strong tendency to synchronism with the generator. The construction
preferred is that in which the armature is provided with polar
projections. The field-magnets are wound with two sets of coils, the
terminals of which are connected to a switch mechanism, by means of
which the line-current may be carried directly through these coils or
indirectly through paths by which its phases are modified. To start such
a motor, the switch is turned on to a set of contacts which includes in
one motor circuit a dead resistance, in the other an inductive
resistance, and, the two circuits being in derivation, it is obvious
that the difference in phase of the current in such circuits will set up
a rotation of the motor. When the speed of the motor has thus been
brought to the desired rate the switch is shifted to throw the main
current directly through the motor-circuits, and although the currents
in both circuits will now be of the same phase the motor will continue
to revolve, becoming a true synchronous motor. To secure greater
efficiency, the armature or its polar projections are wound with coils
closed on themselves.
In the accompanying diagrams, Fig. 38 illustrates the details of the
plan above set forth, and Figs. 39 and 40 modifications of the same.
[Illustration: FIGS. 38, 39 and 40.]
Referring to Fig. 38, let A designate the field-magnets of a motor, the
polar projections of which are wound with coils B C included in
independent circuits, and D the armature with polar projections wound
with coils E closed upon themselves, the motor in these respects being
similar in construction to those described already, but having on
account of the polar projections on the armature core, or other similar
and well-known features, the properties of a synchronizing-motor. L L'
represents the conductors of a line from an alternating current
generator G.
Near the motor is placed a switch the action of which is that of the one
shown in the diagrams, which is constructed as follows: F F' are two
conducting plates or arms, pivoted at their ends and connected by an
insulating cross-bar, H, so as to be shifted in parallelism. In the path
of the bars F F' is the contact 2, which forms one terminal of the
circuit through coils C, and the contact 4, which is one terminal of the
circuit through coils B. The opposite end of the wire of coils C is
connected to the wire L or bar F', and the corresponding end of coils B
is connected to wire L' and bar F; hence if the bars be shifted so as to
bear on contacts 2 and 4 both sets of coils B C will be included in the
circuit L L' in multiple arc or derivation. In the path of the levers F
F' are two other contact terminals, 1 and 3. The contact 1 is connected
to contact 2 through an artificial resistance, I, and contact 3 with
contact 4 through a self-induction coil, J, so that when the switch
levers are shifted upon the points 1 and 3 the circuits of coils B and C
will be connected in multiple arc or derivation to the circuit L L', and
will include the resistance and self-induction coil respectively. A
third position of the switch is that in which the levers F and F' are
shifted out of contact with both sets of points. In this case the motor
is entirely out of circuit.
The purpose and manner of operating the motor by these devices are as
follows: The normal position of the switch, the motor being out of
circuit, is off the contact points. Assuming the generator to be
running, and that it is desired to start the motor, the switch is
shifted until its levers rest upon points 1 and 3. The two
motor-circuits are thus connected with the generator circuit; but by
reason of the presence of the resistance I in one and the self-induction
coil J in the other the coincidence of the phases of the current is
disturbed sufficiently to produce a progression of the poles, which
starts the motor in rotation. When the speed of the motor has run up to
synchronism with the generator, or approximately so, the switch is
shifted over upon the points 2 and 4, thus cutting out the coils I and
J, so that the currents in both circuits have the same phase; but the
motor now runs as a synchronous motor.
It will be understood that when brought up to speed the motor will run
with only one of the circuits B or C connected with the main or
generator circuit, or the two circuits may be connected in series. This
latter plan is preferable when a current having a high number of
alternations per unit of time is employed to drive the motor. In such
case the starting of the motor is more difficult, and the dead and
inductive resistances must take up a considerable proportion of the
electromotive force of the circuits. Generally the conditions are so
adjusted that the electromotive force used in each of the motor circuits
is that which is required to operate the motor when its circuits are in
series. The plan followed in this case is illustrated in Fig. 39. In
this instance the motor has twelve poles and the armature has polar
projections D wound with closed coils E. The switch used is of
substantially the same construction as that shown in the previous
figure. There are, however, five contacts, designated as 5, 6, 7, 8, and
9. The motor-circuits B C, which include alternate field-coils, are
connected to the terminals in the following order: One end of circuit C
is connected to contact 9 and to contact 5 through a dead resistance, I.
One terminal of circuit B is connected to contact 7 and to contact 6
through a self-induction coil, J. The opposite terminals of both
circuits are connected to contact 8.
One of the levers, as F, of the switch is made with an extension, _f_,
or otherwise, so as to cover both contacts 5 and 6 when shifted into the
position to start the motor. It will be observed that when in this
position and with lever F' on contact 8 the current divides between the
two circuits B C, which from their difference in electrical character
produce a progression of the poles that starts the motor in rotation.
When the motor has attained the proper speed, the switch is shifted so
that the levers cover the contacts 7 and 9, thereby connecting circuits
B and C in series. It is found that by this disposition the motor is
maintained in rotation in synchronism with the generator. This principle
of operation, which consists in converting by a change of connections or
otherwise a double-circuit motor, or one operating by a progressive
shifting of the poles, into an ordinary synchronizing motor may be
carried out in many other ways. For instance, instead of using the
switch shown in the previous figures, we may use a temporary ground
circuit between the generator and motor, in order to start the motor, in
substantially the manner indicated in Fig. 40. Let G in this figure
represent an ordinary alternating-current generator with, say, two
poles, M M', and an armature wound with two coils, N N', at right angles
and connected in series. The motor has, for example, four poles wound
with coils B C, which are connected in series, and an armature with
polar projections D wound with closed coils E E. From the common joint
or union between the two circuits of both the generator and the motor an
earth connection is established, while the terminals or ends of these
circuits are connected to the line. Assuming that the motor is a
synchronizing motor or one that has the capability of running in
synchronism with the generator, but not of starting, it may be started
by the above-described apparatus by closing the ground connection from
both generator and motor. The system thus becomes one with a two-circuit
generator and motor, the ground forming a common return for the currents
in the two circuits L and L'. When by this arrangement of circuits the
motor is brought to speed, the ground connection is broken between the
motor or generator, or both, ground-switches P P' being employed for
this purpose. The motor then runs as a synchronizing motor.
In describing the main features which constitute this invention
illustrations have necessarily been omitted of the appliances used in
conjunction with the electrical devices of similar systems--such, for
instance, as driving-belts, fixed and loose pulleys for the motor, and
the like; but these are matters well understood.
Mr. Tesla believes he is the first to operate electro-magnetic motors by
alternating currents in any of the ways herein described--that is to
say, by producing a progressive movement or rotation of their poles or
points of greatest magnetic attraction by the alternating currents until
they have reached a given speed, and then by the same currents producing
a simple alternation of their poles, or, in other words, by a change in
the order or character of the circuit connections to convert a motor
operating on one principle to one operating on another.