MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.
We now come to a new class of motors in which resort is had to
condensers for the purpose of developing the required difference of
phase and neutralizing the effects of self-induction. Mr. Tesla early
began to apply the condenser to alternating apparatus, in just how many
ways can only be learned from a perusal of other portions of this
volume, especially those dealing with his high frequency work.
Certain laws govern the action or effects produced by a condenser when
connected to an electric circuit through which an alternating or in
general an undulating current is made to pass. Some of the most
important of such effects are as follows: First, if the terminals or
plates of a condenser be connected with two points of a circuit, the
potentials of which are made to rise and fall in rapid succession, the
condenser allows the passage, or more strictly speaking, the
transference of a current, although its plates or armatures may be so
carefully insulated as to prevent almost completely the passage of a
current of unvarying strength or direction and of moderate electromotive
force. Second, if a circuit, the terminals of which are connected with
the plates of the condenser, possess a certain self-induction, the
condenser will overcome or counteract to a greater or less degree,
dependent upon well-understood conditions, the effects of such
self-induction. Third, if two points of a closed or complete circuit
through which a rapidly rising and falling current flows be shunted or
bridged by a condenser, a variation in the strength of the currents in
the branches and also a difference of phase of the currents therein is
produced. These effects Mr. Tesla has utilized and applied in a variety
of ways in the construction and operation of his motors, such as by
producing a difference in phase in the two energizing circuits of an
alternating current motor by connecting the two circuits in derivation
and connecting up a condenser in series in one of the circuits. A
further development, however, possesses certain novel features of
practical value and involves a knowledge of facts less generally
understood. It comprises the use of a condenser or condensers in
connection with the induced or armature circuit of a motor and certain
details of the construction of such motors. In an alternating current
motor of the type particularly referred to above, or in any other which
has an armature coil or circuit closed upon itself, the latter
represents not only an inductive resistance, but one which is
periodically varying in value, both of which facts complicate and
render difficult the attainment of the conditions best suited to the
most efficient working conditions; in other words, they require, first,
that for a given inductive effect upon the armature there should be the
greatest possible current through the armature or induced coils, and,
second, that there should always exist between the currents in the
energizing and the induced circuits a given relation of phase. Hence
whatever tends to decrease the self-induction and increase the current
in the induced circuits will, other things being equal, increase the
output and efficiency of the motor, and the same will be true of causes
that operate to maintain the mutual attractive effect between the field
magnets and armature at its maximum. Mr. Tesla secures these results by
connecting with the induced circuit or circuits a condenser, in the
manner described below, and he also, with this purpose in view,
constructs the motor in a special manner.
[Illustration: FIG. 86.]
[Illustration: FIG. 88.]
[Illustration: FIG. 89.]
[Illustration: FIG. 87.]
[Illustration: FIG. 90.]
Referring to the drawings, Fig. 86, is a view, mainly diagrammatic, of
an alternating current motor, in which the present principle is applied.
Fig. 87 is a central section, in line with the shaft, of a special form
of armature core. Fig. 88 is a similar section of a modification of the
same. Fig. 89 is one of the sections of the core detached. Fig. 90 is a
diagram showing a modified disposition of the armature or induced
circuits.
The general plan of the invention is illustrated in Fig. 86. A A in this
figure represent the the frame and field magnets of an alternating
current motor, the poles or projections of which are wound with coils B
and C, forming independent energizing circuits connected either to the
same or to independent sources of alternating currents, so that the
currents flowing through the circuits, respectively, will have a
difference of phase. Within the influence of this field is an armature
core D, wound with coils E. In motors of this description heretofore
these coils have been closed upon themselves, or connected in a closed
series; but in the present case each coil or the connected series of
coils terminates in the opposite plates of a condenser F. For this
purpose the ends of the series of coils are brought out through the
shaft to collecting rings G, which are connected to the condenser by
contact brushes H and suitable conductors, the condenser being
independent of the machine. The armature coils are wound or connected in
such manner that adjacent coils produce opposite poles.
The action of this motor and the effect of the plan followed in its
construction are as follows: The motor being started in operation and
the coils of the field magnets being traversed by alternating currents,
currents are induced in the armature coils by one set of field coils, as
B, and the poles thus established are acted upon by the other set, as C.
The armature coils, however, have necessarily a high self-induction,
which opposes the flow of the currents thus set up. The condenser F not
only permits the passage or transference of these currents, but also
counteracts the effects of self-induction, and by a proper adjustment of
the capacity of the condenser, the self-induction of the coils, and the
periods of the currents, the condenser may be made to overcome entirely
the effect of self-induction.
It is preferable on account of the undesirability of using sliding
contacts of any kind, to associate the condenser with the armature
directly, or make it a part of the armature. In some cases Mr. Tesla
builds up the armature of annular plates K K, held by bolts L between
heads M, which are secured to the driving shaft, and in the hollow space
thus formed he places a condenser F, generally by winding the two
insulated plates spirally around the shaft. In other cases he utilizes
the plates of the core itself as the plates of the condenser. For
example, in Figs. 88 and 89, N is the driving shaft, M M are the heads
of the armature-core, and K K' the iron plates of which the core is
built up. These plates are insulated from the shaft and from one
another, and are held together by rods or bolts L. The bolts pass
through a large hole in one plate and a small hole in the one next
adjacent, and so on, connecting electrically all of plates K, as one
armature of a condenser, and all of plates K' as the other.
To either of the condensers above described the armature coils may be
connected, as explained by reference to Fig. 86.
In motors in which the armature coils are closed upon themselves--as,
for example, in any form of alternating current motor in which one
armature coil or set of coils is in the position of maximum induction
with respect to the field coils or poles, while the other is in the
position of minimum induction--the coils are best connected in one
series, and two points of the circuit thus formed are bridged by a
condenser. This is illustrated in Fig. 90, in which E represents one set
of armature coils and E' the other. Their points of union are joined
through a condenser F. It will be observed that in this disposition the
self-induction of the two branches E and E' varies with their position
relatively to the field magnet, and that each branch is alternately the
predominating source of the induced current. Hence the effect of the
condenser F is twofold. First, it increases the current in each of the
branches alternately, and, secondly, it alters the phase of the currents
in the branches, this being the well-known effect which results from
such a disposition of a condenser with a circuit, as above described.
This effect is favorable to the proper working of the motor, because it
increases the flow of current in the armature circuits due to a given
inductive effect, and also because it brings more nearly into
coincidence the maximum magnetic effects of the coacting field and
armature poles.
It will be understood, of course, that the causes that contribute to the
efficiency of condensers when applied to such uses as the above must be
given due consideration in determining the practicability and efficiency
of the motors. Chief among these is, as is well known, the periodicity
of the current, and hence the improvements described are more
particularly adapted to systems in which a very high rate of alternation
or change is maintained.
Although this invention has been illustrated in connection with a
special form of motor, it will be understood that it is equally
applicable to any other alternating current motor in which there is a
closed armature coil wherein the currents are induced by the action of
the field, and the feature of utilizing the plates or sections of a
magnetic core for forming the condenser is applicable, generally, to
other kinds of alternating current apparatus.