METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.
With the object of obtaining the desired speed in motors operated by
means of alternating currents of differing phase, Mr. Tesla has devised
various plans intended to meet the practical requirements of the case,
in adapting his system to types of multipolar alternating current
machines yielding a large number of current reversals for each
revolution.
For example, Mr. Tesla has pointed out that to adapt a given type of
alternating current generator, you may couple rigidly two complete
machines, securing them together in such a way that the requisite
difference in phase will be produced; or you may fasten two armatures to
the same shaft within the influence of the same field and with the
requisite angular displacement to yield the proper difference in phase
between the two currents; or two armatures may be attached to the same
shaft with their coils symmetrically disposed, but subject to the
influence of two sets of field magnets duly displaced; or the two sets
of coils may be wound on the same armature alternately or in such manner
that they will develop currents the phases of which differ in time
sufficiently to produce the rotation of the motor.
Another method included in the scope of the same idea, whereby a single
generator may run a number of motors either at its own rate of speed or
all at different speeds, is to construct the motors with fewer poles
than the generator, in which case their speed will be greater than that
of the generator, the rate of speed being higher as the number of their
poles is relatively less. This may be understood from an example, taking
a generator that has two independent generating coils which revolve
between two pole pieces oppositely magnetized; and a motor with
energizing coils that produce at any given time two magnetic poles in
one element that tend to set up a rotation of the motor. A generator
thus constructed yields four reversals, or impulses, in each
revolution, two in each of its independent circuits; and the effect upon
the motor is to shift the magnetic poles through three hundred and sixty
degrees. It is obvious that if the four reversals in the same order
could be produced by each half-revolution of the generator the motor
would make two revolutions to the generator's one. This would be readily
accomplished by adding two intermediate poles to the generator or
altering it in any of the other equivalent ways above indicated. The
same rule applies to generators and motors with multiple poles. For
instance, if a generator be constructed with two circuits, each of which
produces twelve reversals of current to a revolution, and these currents
be directed through the independent energizing-coils of a motor, the
coils of which are so applied as to produce twelve magnetic poles at all
times, the rotation of the two will be synchronous; but if the
motor-coils produce but six poles, the movable element will be rotated
twice while the generator rotates once; or if the motor have four poles,
its rotation will be three times as fast as that of the generator.
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
These features, so far as necessary to an understanding of the
principle, are here illustrated. Fig. 28 is a diagrammatic illustration
of a generator constructed in accordance with the invention. Fig. 29 is
a similar view of a correspondingly constructed motor. Fig. 30 is a
diagram of a generator of modified construction. Fig. 31 is a diagram of
a motor of corresponding character. Fig. 32 is a diagram of a system
containing a generator and several motors adapted to run at various
speeds.
In Fig. 28, let C represent a cylindrical armature core wound
longitudinally with insulated coils A A, which are connected up in
series, the terminals of the series being connected to collecting-rings
_a a_ on the shaft G. By means of this shaft the armature is mounted to
rotate between the poles of an annular field-magnet D, formed with polar
projections wound with coils E, that magnetize the said projections. The
coils E are included in the circuit of a generator F, by means of which
the field-magnet is energized. If thus constructed, the machine is a
well-known form of alternating-current generator. To adapt it to his
system, however, Mr. Tesla winds on armature C a second set of coils B B
intermediate to the first, or, in other words, in such positions that
while the coils of one set are in the relative positions to the poles of
the field-magnet to produce the maximum current, those of the other set
will be in the position in which they produce the minimum current. The
coils B are connected, also, in series and to two connecting-rings,
secured generally to the shaft at the opposite end of the armature.
[Illustration: FIG. 30.]
[Illustration: FIG. 31.]
The motor shown in Fig. 29 has an annular field-magnet H, with four
pole-pieces wound with coils I. The armature is constructed similarly to
the generator, but with two sets of two coils in closed circuits to
correspond with the reduced number of magnetic poles in the field. From
the foregoing it is evident that one revolution of the armature of the
generator producing eight current impulses in each circuit will produce
two revolutions of the motor-armature.
The application of the principle of this invention is not, however,
confined to any particular form of machine. In Figs. 30 and 31 a
generator and motor of another well-known type are shown. In Fig. 30, J
J are magnets disposed in a circle and wound with coils K, which are in
circuit with a generator which supplies the current that maintains the
field of force. In the usual construction of these machines the
armature-conductor L is carried by a suitable frame, so as to be rotated
in face of the magnets J J, or between these magnets and another similar
set in front of them. The magnets are energized so as to be of
alternately opposite polarity throughout the series, so that as the
conductor C is rotated the current impulses combine or are added to one
another, those produced by the conductor in any given position being all
in the same direction. To adapt such a machine to his system, Mr. Tesla
adds a second set of induced conductors M, in all respects similar to
the first, but so placed in reference to it that the currents produced
in each will differ by a quarter-phase. With such relations it is
evident that as the current decreases in conductor L it increases in
conductor M, and conversely, and that any of the forms of Tesla motor
invented for use in this system may be operated by such a generator.
Fig. 31 is intended to show a motor corresponding to the machine in Fig.
30. The construction of the motor is identical with that of the
generator, and if coupled thereto it will run synchronously therewith.
J' J' are the field-magnets, and K' the coils thereon. L' is one of the
armature-conductors and M' the other.
Fig. 32 shows in diagram other forms of machine. The generator N in this
case is shown as consisting of a stationary ring O, wound with
twenty-four coils P P', alternate coils being connected in series in two
circuits. Within this ring is a disc or drum Q, with projections Q'
wound with energizing-coils included in circuit with a generator R. By
driving this disc or cylinder alternating currents are produced in the
coils P and P', which are carried off to run the several motors.
The motors are composed of a ring or annular field-magnet S, wound with
two sets of energizing-coils T T', and armatures U, having projections
U' wound with coils V, all connected in series in a closed circuit or
each closed independently on itself.
Suppose the twelve generator-coils P are wound alternately in opposite
directions, so that any two adjacent coils of the same set tend to
produce a free pole in the ring O between them and the twelve coils P'
to be similarly wound. A single revolution of the disc or cylinder Q,
the twelve polar projections of which are of opposite polarity, will
therefore produce twelve current impulses in each of the circuits W W'.
Hence the motor X, which has sixteen coils or eight free poles, will
make one and a half turns to the generator's one. The motor Y, with
twelve coils or six poles, will rotate with twice the speed of the
generator, and the motor Z, with eight coils or four poles, will revolve
three times as fast as the generator. These multipolar motors have a
peculiarity which may be often utilized to great advantage. For example,
in the motor X, Fig. 32, the eight poles may be either alternately
opposite or there may be at any given time alternately two like and two
opposite poles. This is readily attained by making the proper electrical
connections. The effect of such a change, however, would be the same as
reducing the number of poles one-half, and thereby doubling the speed
of any given motor.
[Illustration: FIG. 32.]
It is obvious that the Tesla electrical transformers which have
independent primary currents may be used with the generators described.
It may also be stated with respect to the devices we now describe that
the most perfect and harmonious action of the generators and motors is
obtained when the numbers of the poles of each are even and not odd. If
this is not the case, there will be a certain unevenness of action which
is the less appreciable as the number of poles is greater; although this
may be in a measure corrected by special provisions which it is not here
necessary to explain. It also follows, as a matter of course, that if
the number of the poles of the motor be greater than that of the
generator the motor will revolve at a slower speed than the generator.
In this chapter, we may include a method devised by Mr. Tesla for
avoiding the very high speeds which would be necessary with large
generators. In lieu of revolving the generator armature at a high rate
of speed, he secures the desired result by a rotation of the magnetic
poles of one element of the generator, while driving the other at a
different speed. The effect is the same as that yielded by a very high
rate of rotation.
In this instance, the generator which supplies the current for operating
the motors or transformers consists of a subdivided ring or annular core
wound with four diametrically-opposite coils, E E', Fig. 33. Within the
ring is mounted a cylindrical armature-core wound longitudinally with
two independent coils, F F', the ends of which lead, respectively, to
two pairs of insulated contact or collecting rings, D D' G G', on the
armature shaft. Collecting brushes _d d' g g'_ bear upon these rings,
respectively, and convey the currents through the two independent
line-circuits M M'. In the main line there may be included one or more
motors or transformers, or both. If motors be used, they are of the
usual form of Tesla construction with independent coils or sets of coils
J J', included, respectively, in the circuits M M'. These
energizing-coils are wound on a ring or annular field or on pole pieces
thereon, and produce by the action of the alternating currents passing
through them a progressive shifting of the magnetism from pole to pole.
The cylindrical armature H of the motor is wound with two coils at right
angles, which form independent closed circuits.
If transformers be employed, one set of the primary coils, as N N, wound
on a ring or annular core is connected to one circuit, as M', and the
other primary coils, N N', to the circuit M. The secondary coils K K'
may then be utilized for running groups of incandescent lamps P P'.
[Illustration: FIG. 33.]
With this generator an exciter is employed. This consists of two poles,
A A, of steel permanently magnetized, or of iron excited by a battery or
other generator of continuous currents, and a cylindrical armature core
mounted on a shaft, B, and wound with two longitudinal coils, C C'. One
end of each of these coils is connected to the collecting-rings _b c_,
respectively, while the other ends are both connected to a ring, _a_.
Collecting-brushes _b' c'_ bear on the rings _b c_, respectively, and
conductors L L convey the currents therefrom through the coils E and E
of the generator. L' is a common return-wire to brush _a'_. Two
independent circuits are thus formed, one including coils C of the
exciter and E E of the generator, the other coils C' of the exciter and
E' E' of the generator. It results from this that the operation of the
exciter produces a progressive movement of the magnetic poles of the
annular field-core of the generator, the shifting or rotary movement of
the poles being synchronous with the rotation of the exciter armature.
Considering the operative conditions of a system thus established, it
will be found that when the exciter is driven so as to energize the
field of the generator, the armature of the latter, if left free to
turn, would rotate at a speed practically the same as that of the
exciter. If under such conditions the coils F F' of the generator
armature be closed upon themselves or short-circuited, no currents, at
least theoretically, will be generated in these armature coils. In
practice the presence of slight currents is observed, the existence of
which is attributable to more or less pronounced fluctuations in the
intensity of the magnetic poles of the generator ring. So, if the
armature-coils F F' be closed through the motor, the latter will not be
turned as long as the movement of the generator armature is synchronous
with that of the exciter or of the magnetic poles of its field. If, on
the contrary, the speed of the generator armature be in any way checked,
so that the shifting or rotation of the poles of the field becomes
relatively more rapid, currents will be induced in the armature coils.
This obviously follows from the passing of the lines of force across the
armature conductors. The greater the speed of rotation of the magnetic
poles relatively to that of the armature the more rapidly the currents
developed in the coils of the latter will follow one another, and the
more rapidly the motor will revolve in response thereto, and this
continues until the armature generator is stopped entirely, as by a
brake, when the motor, if properly constructed, runs at the speed with
which the magnetic poles of the generator rotate.
The effective strength of the currents developed in the armature coils
of the generator is dependent upon the strength of the currents
energizing the generator and upon the number of rotations per unit of
time of the magnetic poles of the generator; hence the speed of the
motor armature will depend in all cases upon the relative speeds of the
armature of the generator and of its magnetic poles. For example, if the
poles are turned two thousand times per unit of time and the armature is
turned eight hundred, the motor will turn twelve hundred times, or
nearly so. Very slight differences of speed may be indicated by a
delicately balanced motor.
Let it now be assumed that power is applied to the generator armature to
turn it in a direction opposite to that in which its magnetic poles
rotate. In such case the result would be similar to that produced by a
generator the armature and field magnets of which are rotated in
opposite directions, and by reason of these conditions the motor
armature will turn at a rate of speed equal to the sum of the speeds of
the armature and magnetic poles of the generator, so that a
comparatively low speed of the generator armature will produce a high
speed in the motor.
It will be observed in connection with this system that on diminishing
the resistance of the external circuit of the generator armature by
checking the speed of the motor or by adding translating devices in
multiple arc in the secondary circuit or circuits of the transformer the
strength of the current in the armature circuit is greatly increased.
This is due to two causes: first, to the great differences in the speeds
of the motor and generator, and, secondly, to the fact that the
apparatus follows the analogy of a transformer, for, in proportion as
the resistance of the armature or secondary circuits is reduced, the
strength of the currents in the field or primary circuits of the
generator is increased and the currents in the armature are augmented
correspondingly. For similar reasons the currents in the armature-coils
of the generator increase very rapidly when the speed of the armature is
reduced when running in the same direction as the magnetic poles or
conversely.
It will be understood from the above description that the
generator-armature may be run in the direction of the shifting of the
magnetic poles, but more rapidly, and that in such case the speed of the
motor will be equal to the difference between the two rates.