THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.
The best description that can be given of what he attempted, and
succeeded in doing, with the rotating magnetic field, is to be found in
Mr. Tesla's brief paper explanatory of his rotary current, polyphase
system, read before the American Institute of Electrical Engineers, in
New York, in May, 1888, under the title "A New System of Alternate
Current Motors and Transformers." As a matter of fact, which a perusal
of the paper will establish, Mr. Tesla made no attempt in that paper to
describe all his work. It dealt in reality with the few topics
enumerated in the caption of this chapter. Mr. Tesla's reticence was no
doubt due largely to the fact that his action was governed by the wishes
of others with whom he was associated, but it may be worth mention that
the compiler of this volume--who had seen the motors running, and who
was then chairman of the Institute Committee on Papers and Meetings--had
great difficulty in inducing Mr. Tesla to give the Institute any paper
at all. Mr. Tesla was overworked and ill, and manifested the greatest
reluctance to an exhibition of his motors, but his objections were at
last overcome. The paper was written the night previous to the meeting,
in pencil, very hastily, and under the pressure just mentioned.
In this paper casual reference was made to two special forms of motors
not within the group to be considered. These two forms were: 1. A motor
with one of its circuits in series with a transformer, and the other in
the secondary of the transformer. 2. A motor having its armature circuit
connected to the generator, and the field coils closed upon themselves.
The paper in its essence is as follows, dealing with a few leading
features of the Tesla system, namely, the rotating magnetic field,
motors with closed conductors, synchronizing motors, and rotating field
transformers:--
The subject which I now have the pleasure of bringing to your notice is
a novel system of electric distribution and transmission of power by
means of alternate currents, affording peculiar advantages, particularly
in the way of motors, which I am confident will at once establish the
superior adaptability of these currents to the transmission of power and
will show that many results heretofore unattainable can be reached by
their use; results which are very much desired in the practical
operation of such systems, and which cannot be accomplished by means of
continuous currents.
Before going into a detailed description of this system, I think it
necessary to make a few remarks with reference to certain conditions
existing in continuous current generators and motors, which, although
generally known, are frequently disregarded.
In our dynamo machines, it is well known, we generate alternate currents
which we direct by means of a commutator, a complicated device and, it
may be justly said, the source of most of the troubles experienced in
the operation of the machines. Now, the currents so directed cannot be
utilized in the motor, but they must--again by means of a similar
unreliable device--be reconverted into their original state of alternate
currents. The function of the commutator is entirely external, and in no
way does it affect the internal working of the machines. In reality,
therefore, all machines are alternate current machines, the currents
appearing as continuous only in the external circuit during their
transit from generator to motor. In view simply of this fact, alternate
currents would commend themselves as a more direct application of
electrical energy, and the employment of continuous currents would only
be justified if we had dynamos which would primarily generate, and
motors which would be directly actuated by, such currents.
But the operation of the commutator on a motor is twofold; first, it
reverses the currents through the motor, and secondly, it effects
automatically, a progressive shifting of the poles of one of its
magnetic constituents. Assuming, therefore, that both of the useless
operations in the systems, that is to say, the directing of the
alternate currents on the generator and reversing the direct currents on
the motor, be eliminated, it would still be necessary, in order to cause
a rotation of the motor, to produce a progressive shifting of the poles
of one of its elements, and the question presented itself--How to
perform this operation by the direct action of alternate currents? I
will now proceed to show how this result was accomplished.
[Illustration: FIG. 1.]
[Illustration: FIG. 1a.]
[Illustration: FIG. 2.]
[Illustration: FIG. 2a.]
In the first experiment a drum-armature was provided with two coils at
right angles to each other, and the ends of these coils were connected
to two pairs of insulated contact-rings as usual. A ring was then made
of thin insulated plates of sheet-iron and wound with four coils, each
two opposite coils being connected together so as to produce free poles
on diametrically opposite sides of the ring. The remaining free ends of
the coils were then connected to the contact-rings of the generator
armature so as to form two independent circuits, as indicated in Fig. 9.
It may now be seen what results were secured in this combination, and
with this view I would refer to the diagrams, Figs. 1 to 8_a_. The field
of the generator being independently excited, the rotation of the
armature sets up currents in the coils C C_{1}, varying in strength and
direction in the well-known manner. In the position shown in Fig. 1, the
current in coil C is nil, while coil C_{1} is traversed by its maximum
current, and the connections may be such that the ring is magnetized by
the coils c_{1} c_{1}, as indicated by the letters N S in Fig. 1_a_,
the magnetizing effect of the coils c c being nil, since these coils
are included in the circuit of coil C.
[Illustration: FIG. 3.]
[Illustration: FIG. 3a.]
In Fig. 2, the armature coils are shown in a more advanced position,
one-eighth of one revolution being completed. Fig. 2_a_ illustrates the
corresponding magnetic condition of the ring. At this moment the coil
C_{1} generates a current of the same direction as previously, but
weaker, producing the poles n_{1} s_{1} upon the ring; the coil C also
generates a current of the same direction, and the connections may be
such that the coils c c produce the poles n s, as shown in Fig. 2_a_.
The resulting polarity is indicated by the letters N S, and it will be
observed that the poles of the ring have been shifted one-eighth of the
periphery of the same.
[Illustration: FIG. 4.]
[Illustration: FIG. 4a.]
In Fig. 3 the armature has completed one quarter of one revolution. In
this phase the current in coil C is a maximum, and of such direction as
to produce the poles N S in Fig. 3_a_, whereas the current in coil C_{1}
is nil, this coil being at its neutral position. The poles N S in Fig.
3_a_ are thus shifted one quarter of the circumference of the ring.
Fig. 4 shows the coils C C in a still more advanced position, the
armature having completed three-eighths of one revolution. At that
moment the coil C still generates a current of the same direction as
before, but of less strength, producing the comparatively weaker poles
n s in Fig. 4_a_. The current in the coil C_{1} is of the same strength,
but opposite direction. Its effect is, therefore, to produce upon the
ring the poles n_{1} s_{1}, as indicated, and a polarity, N S, results,
the poles now being shifted three-eighths of the periphery of the ring.
[Illustration: FIG. 5.]
[Illustration: FIG. 5a.]
In Fig. 5 one half of one revolution of the armature is completed, and
the resulting magnetic condition of the ring is indicated in Fig. 5_a_.
Now the current in coil C is nil, while the coil C_{1} yields its
maximum current, which is of the same direction as previously; the
magnetizing effect is, therefore, due to the coils, c_{1} c_{1} alone,
and, referring to Fig. 5_a_, it will be observed that the poles N S are
shifted one half of the circumference of the ring. During the next half
revolution the operations are repeated, as represented in the Figs. 6 to
8_a_.
[Illustration: FIG. 6.]
[Illustration: FIG. 6a.]
A reference to the diagrams will make it clear that during one
revolution of the armature the poles of the ring are shifted once around
its periphery, and, each revolution producing like effects, a rapid
whirling of the poles in harmony with the rotation of the armature is
the result. If the connections of either one of the circuits in the ring
are reversed, the shifting of the poles is made to progress in the
opposite direction, but the operation is identically the same. Instead
of using four wires, with like result, three wires may be used, one
forming a common return for both circuits.
[Illustration: FIG. 7.]
[Illustration: FIG. 7_a_.]
This rotation or whirling of the poles manifests itself in a series of
curious phenomena. If a delicately pivoted disc of steel or other
magnetic metal is approached to the ring it is set in rapid rotation,
the direction of rotation varying with the position of the disc. For
instance, noting the direction outside of the ring it will be found that
inside the ring it turns in an opposite direction, while it is
unaffected if placed in a position symmetrical to the ring. This is
easily explained. Each time that a pole approaches, it induces an
opposite pole in the nearest point on the disc, and an attraction is
produced upon that point; owing to this, as the pole is shifted further
away from the disc a tangential pull is exerted upon the same, and the
action being constantly repeated, a more or less rapid rotation of the
disc is the result. As the pull is exerted mainly upon that part which
is nearest to the ring, the rotation outside and inside, or right and
left, respectively, is in opposite directions, Fig. 9. When placed
symmetrically to the ring, the pull on the opposite sides of the disc
being equal, no rotation results. The action is based on the magnetic
inertia of iron; for this reason a disc of hard steel is much more
affected than a disc of soft iron, the latter being capable of very
rapid variations of magnetism. Such a disc has proved to be a very
useful instrument in all these investigations, as it has enabled me to
detect any irregularity in the action. A curious effect is also produced
upon iron filings. By placing some upon a paper and holding them
externally quite close to the ring, they are set in a vibrating motion,
remaining in the same place, although the paper may be moved back and
forth; but in lifting the paper to a certain height which seems to be
dependent on the intensity of the poles and the speed of rotation, they
are thrown away in a direction always opposite to the supposed movement
of the poles. If a paper with filings is put flat upon the ring and the
current turned on suddenly, the existence of a magnetic whirl may easily
be observed.
To demonstrate the complete analogy between the ring and a revolving
magnet, a strongly energized electro-magnet was rotated by mechanical
power, and phenomena identical in every particular to those mentioned
above were observed.
Obviously, the rotation of the poles produces corresponding inductive
effects and may be utilized to generate currents in a closed conductor
placed within the influence of the poles. For this purpose it is
convenient to wind a ring with two sets of superimposed coils forming
respectively the primary and secondary circuits, as shown in Fig. 10. In
order to secure the most economical results the magnetic circuit should
be completely closed, and with this object in view the construction may
be modified at will.
[Illustration: FIG. 8.]
[Illustration: FIG. 8_a_.]
The inductive effect exerted upon the secondary coils will be mainly due
to the shifting or movement of the magnetic action; but there may also
be currents set up in the circuits in consequence of the variations in
the intensity of the poles. However, by properly designing the generator
and determining the magnetizing effect of the primary coils, the latter
element may be made to disappear. The intensity of the poles being
maintained constant, the action of the apparatus will be perfect, and
the same result will be secured as though the shifting were effected by
means of a commutator with an infinite number of bars. In such case the
theoretical relation between the energizing effect of each set of
primary coils and their resultant magnetizing effect may be expressed by
the equation of a circle having its centre coinciding with that of an
orthogonal system of axes, and in which the radius represents the
resultant and the co-ordinates both of its components. These are then
respectively the sine and cosine of the angle _a_ between the radius and
one of the axes (_OX_). Referring to Fig. 11, we have r^2 = x^2 + y^2;
where x = r cos _a_, and y = r sin _a_.
Assuming the magnetizing effect of each set of coils in the transformer
to be proportional to the current--which may be admitted for weak
degrees of magnetization--then x = Kc and y = Kc^1, where K is a
constant and c and c^1 the current in both sets of coils respectively.
Supposing, further, the field of the generator to be uniform, we have
for constant speed
c^1 = K^1 sin _a_ and
c = K^1 sin (90 deg. + _a_) = K^1 cos _a_,
where K^1 is a constant. See Fig. 12.
Therefore,
x = Kc = K K^1 cos _a_;
y = Kc^1 = K K^1 sin _a_; and
K K^1 = r.
[Illustration: FIG. 9.]
That is, for a uniform field the disposition of the two coils at right
angles will secure the theoretical result, and the intensity of the
shifting poles will be constant. But from r^2 = x^2 + y^2 it follows
that for y = 0, r = x; it follows that the joint magnetizing effect
of both sets of coils should be equal to the effect of one set when at
its maximum action. In transformers and in a certain class of motors the
fluctuation of the poles is not of great importance, but in another
class of these motors it is desirable to obtain the theoretical result.
In applying this principle to the construction of motors, two typical
forms of motor have been developed. First, a form having a comparatively
small rotary effort at the start but maintaining a perfectly uniform
speed at all loads, which motor has been termed synchronous. Second, a
form possessing a great rotary effort at the start, the speed being
dependent on the load.
These motors may be operated in three different ways: 1. By the
alternate currents of the source only. 2. By a combined action of these
and of induced currents. 3. By the joint action of alternate and
continuous currents.
[Illustration: FIG. 10.]
The simplest form of a synchronous motor is obtained by winding a
laminated ring provided with pole projections with four coils, and
connecting the same in the manner before indicated. An iron disc having
a segment cut away on each side may be used as an armature. Such a motor
is shown in Fig. 9. The disc being arranged to rotate freely within the
ring in close proximity to the projections, it is evident that as the
poles are shifted it will, owing to its tendency to place itself in such
a position as to embrace the greatest number of the lines of force,
closely follow the movement of the poles, and its motion will be
synchronous with that of the armature of the generator; that is, in the
peculiar disposition shown in Fig. 9, in which the armature produces by
one revolution two current impulses in each of the circuits. It is
evident that if, by one revolution of the armature, a greater number of
impulses is produced, the speed of the motor will be correspondingly
increased. Considering that the attraction exerted upon the disc is
greatest when the same is in close proximity to the poles, it follows
that such a motor will maintain exactly the same speed at all loads
within the limits of its capacity.
To facilitate the starting, the disc may be provided with a coil closed
upon itself. The advantage secured by such a coil is evident. On the
start the currents set up in the coil strongly energize the disc and
increase the attraction exerted upon the same by the ring, and currents
being generated in the coil as long as the speed of the armature is
inferior to that of the poles, considerable work may be performed by
such a motor even if the speed be below normal. The intensity of the
poles being constant, no currents will be generated in the coil when the
motor is turning at its normal speed.
Instead of closing the coil upon itself, its ends may be connected to
two insulated sliding rings, and a continuous current supplied to these
from a suitable generator. The proper way to start such a motor is to
close the coil upon itself until the normal speed is reached, or nearly
so, and then turn on the continuous current. If the disc be very
strongly energized by a continuous current the motor may not be able to
start, but if it be weakly energized, or generally so that the
magnetizing effect of the ring is preponderating, it will start and
reach the normal speed. Such a motor will maintain absolutely the same
speed at all loads. It has also been found that if the motive power of
the generator is not excessive, by checking the motor the speed of the
generator is diminished in synchronism with that of the motor. It is
characteristic of this form of motor that it cannot be reversed by
reversing the continuous current through the coil.
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
The synchronism of these motors may be demonstrated experimentally in a
variety of ways. For this purpose it is best to employ a motor
consisting of a stationary field magnet and an armature arranged to
rotate within the same, as indicated in Fig. 13. In this case the
shifting of the poles of the armature produces a rotation of the latter
in the opposite direction. It results therefrom that when the normal
speed is reached, the poles of the armature assume fixed positions
relatively to the field magnet, and the same is magnetized by
induction, exhibiting a distinct pole on each of the pole-pieces. If a
piece of soft iron is approached to the field magnet, it will at the
start be attracted with a rapid vibrating motion produced by the
reversals of polarity of the magnet, but as the speed of the armature
increases, the vibrations become less and less frequent and finally
entirely cease. Then the iron is weakly but permanently attracted,
showing that synchronism is reached and the field magnet energized by
induction.
The disc may also be used for the experiment. If held quite close to the
armature it will turn as long as the speed of rotation of the poles
exceeds that of the armature; but when the normal speed is reached, or
very nearly so, it ceases to rotate and is permanently attracted.
[Illustration: FIG. 13.]
A crude but illustrative experiment is made with an incandescent lamp.
Placing the lamp in circuit with the continuous current generator and in
series with the magnet coil, rapid fluctuations are observed in the
light in consequence of the induced currents set up in the coil at the
start; the speed increasing, the fluctuations occur at longer intervals,
until they entirely disappear, showing that the motor has attained its
normal speed. A telephone receiver affords a most sensitive instrument;
when connected to any circuit in the motor the synchronism may be easily
detected on the disappearance of the induced currents.
In motors of the synchronous type it is desirable to maintain the
quantity of the shifting magnetism constant, especially if the magnets
are not properly subdivided.
To obtain a rotary effort in these motors was the subject of long
thought. In order to secure this result it was necessary to make such a
disposition that while the poles of one element of the motor are shifted
by the alternate currents of the source, the poles produced upon the
other elements should always be maintained in the proper relation to the
former, irrespective of the speed of the motor. Such a condition exists
in a continuous current motor; but in a synchronous motor, such as
described, this condition is fulfilled only when the speed is normal.
[Illustration: FIG. 14.]
The object has been attained by placing within the ring a properly
subdivided cylindrical iron core wound with several independent coils
closed upon themselves. Two coils at right angles as in Fig. 14, are
sufficient, but a greater number may be advantageously employed. It
results from this disposition that when the poles of the ring are
shifted, currents are generated in the closed armature coils. These
currents are the most intense at or near the points of the greatest
density of the lines of force, and their effect is to produce poles upon
the armature at right angles to those of the ring, at least
theoretically so; and since this action is entirely independent of the
speed--that is, as far as the location of the poles is concerned--a
continuous pull is exerted upon the periphery of the armature. In many
respects these motors are similar to the continuous current motors. If
load is put on, the speed, and also the resistance of the motor, is
diminished and more current is made to pass through the energizing
coils, thus increasing the effort. Upon the load being taken off, the
counter-electromotive force increases and less current passes through
the primary or energizing coils. Without any load the speed is very
nearly equal to that of the shifting poles of the field magnet.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
[Illustration: FIG. 17.]
It will be found that the rotary effort in these motors fully equals
that of the continuous current motors. The effort seems to be greatest
when both armature and field magnet are without any projections; but as
in such dispositions the field cannot be concentrated, probably the best
results will be obtained by leaving pole projections on one of the
elements only. Generally, it may be stated the projections diminish the
torque and produce a tendency to synchronism.
A characteristic feature of motors of this kind is their property of
being very rapidly reversed. This follows from the peculiar action of
the motor. Suppose the armature to be rotating and the direction of
rotation of the poles to be reversed. The apparatus then represents a
dynamo machine, the power to drive this machine being the momentum
stored up in the armature and its speed being the sum of the speeds of
the armature and the poles.
[Illustration: FIG. 18.]
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
[Illustration: FIG. 21.]
If we now consider that the power to drive such a dynamo would be very
nearly proportional to the third power of the speed, for that reason
alone the armature should be quickly reversed. But simultaneously with
the reversal another element is brought into action, namely, as the
movement of the poles with respect to the armature is reversed, the
motor acts like a transformer in which the resistance of the secondary
circuit would be abnormally diminished by producing in this circuit an
additional electromotive force. Owing to these causes the reversal is
instantaneous.
If it is desirable to secure a constant speed, and at the same time a
certain effort at the start, this result may be easily attained in a
variety of ways. For instance, two armatures, one for torque and the
other for synchronism, may be fastened on the same shaft and any desired
preponderance may be given to either one, or an armature may be wound
for rotary effort, but a more or less pronounced tendency to synchronism
may be given to it by properly constructing the iron core; and in many
other ways.
As a means of obtaining the required phase of the currents in both the
circuits, the disposition of the two coils at right angles is the
simplest, securing the most uniform action; but the phase may be
obtained in many other ways, varying with the machine employed. Any of
the dynamos at present in use may be easily adapted for this purpose by
making connections to proper points of the generating coils. In closed
circuit armatures, such as used in the continuous current systems, it is
best to make four derivations from equi-distant points or bars of the
commutator, and to connect the same to four insulated sliding rings on
the shaft. In this case each of the motor circuits is connected to two
diametrically opposite bars of the commutator. In such a disposition the
motor may also be operated at half the potential and on the three-wire
plan, by connecting the motor circuits in the proper order to three of
the contact rings.
In multipolar dynamo machines, such as used in the converter systems,
the phase is conveniently obtained by winding upon the armature two
series of coils in such a manner that while the coils of one set or
series are at their maximum production of current, the coils of the
other will be at their neutral position, or nearly so, whereby both sets
of coils may be subjected simultaneously or successively to the inducing
action of the field magnets.
Generally the circuits in the motor will be similarly disposed, and
various arrangements may be made to fulfill the requirements; but the
simplest and most practicable is to arrange primary circuits on
stationary parts of the motor, thereby obviating, at least in certain
forms, the employment of sliding contacts. In such a case the magnet
coils are connected alternately in both the circuits; that is, 1, 3,
5 ... in one, and 2, 4, 6 ... in the other, and the coils of each set
of series may be connected all in the same manner, or alternately in
opposition; in the latter case a motor with half the number of poles
will result, and its action will be correspondingly modified. The Figs.
15, 16, and 17, show three different phases, the magnet coils in each
circuit being connected alternately in opposition. In this case there
will be always four poles, as in Figs. 15 and 17; four pole projections
will be neutral; and in Fig. 16 two adjacent pole projections will have
the same polarity. If the coils are connected in the same manner there
will be eight alternating poles, as indicated by the letters n' s'
in Fig. 15.
The employment of multipolar motors secures in this system an advantage
much desired and unattainable in the continuous current system, and that
is, that a motor may be made to run exactly at a predetermined speed
irrespective of imperfections in construction, of the load, and, within
certain limits, of electromotive force and current strength.
In a general distribution system of this kind the following plan should
be adopted. At the central station of supply a generator should be
provided having a considerable number of poles. The motors operated from
this generator should be of the synchronous type, but possessing
sufficient rotary effort to insure their starting. With the observance
of proper rules of construction it may be admitted that the speed of
each motor will be in some inverse proportion to its size, and the
number of poles should be chosen accordingly. Still, exceptional demands
may modify this rule. In view of this, it will be advantageous to
provide each motor with a greater number of pole projections or coils,
the number being preferably a multiple of two and three. By this means,
by simply changing the connections of the coils, the motor may be
adapted to any probable demands.
If the number of the poles in the motor is even, the action will be
harmonious and the proper result will be obtained; if this is not the
case, the best plan to be followed is to make a motor with a double
number of poles and connect the same in the manner before indicated, so
that half the number of poles result. Suppose, for instance, that the
generator has twelve poles, and it would be desired to obtain a speed
equal to 12/7 of the speed of the generator. This would require a motor
with seven pole projections or magnets, and such a motor could not be
properly connected in the circuits unless fourteen armature coils would
be provided, which would necessitate the employment of sliding
contacts. To avoid this, the motor should be provided with fourteen
magnets and seven connected in each circuit, the magnets in each circuit
alternating among themselves. The armature should have fourteen closed
coils. The action of the motor will not be quite as perfect as in the
case of an even number of poles, but the drawback will not be of a
serious nature.
However, the disadvantages resulting from this unsymmetrical form will
be reduced in the same proportion as the number of the poles is
augmented.
If the generator has, say, n, and the motor n_{1} poles, the speed of
the motor will be equal to that of the generator multiplied by n/n_{1}.
The speed of the motor will generally be dependent on the number of the
poles, but there may be exceptions to this rule. The speed may be
modified by the phase of the currents in the circuit or by the character
of the current impulses or by intervals between each or between groups
of impulses. Some of the possible cases are indicated in the diagrams,
Figs. 18, 19, 20 and 21, which are self-explanatory. Fig. 18 represents
the condition generally existing, and which secures the best result. In
such a case, if the typical form of motor illustrated in Fig. 9 is
employed, one complete wave in each circuit will produce one revolution
of the motor. In Fig. 19 the same result will be effected by one wave in
each circuit, the impulses being successive; in Fig. 20 by four, and in
Fig. 21 by eight waves.
By such means any desired speed may be attained, that is, at least
within the limits of practical demands. This system possesses this
advantage, besides others, resulting from simplicity. At full loads the
motors show an efficiency fully equal to that of the continuous current
motors. The transformers present an additional advantage in their
capability of operating motors. They are capable of similar
modifications in construction, and will facilitate the introduction of
motors and their adaptation to practical demands. Their efficiency
should be higher than that of the present transformers, and I base my
assertion on the following:
In a transformer, as constructed at present, we produce the currents in
the secondary circuit by varying the strength of the primary or exciting
currents. If we admit proportionality with respect to the iron core the
inductive effect exerted upon the secondary coil will be proportional
to the numerical sum of the variations in the strength of the exciting
current per unit of time; whence it follows that for a given variation
any prolongation of the primary current will result in a proportional
loss. In order to obtain rapid variations in the strength of the
current, essential to efficient induction, a great number of undulations
are employed; from this practice various disadvantages result. These
are: Increased cost and diminished efficiency of the generator; more
waste of energy in heating the cores, and also diminished output of the
transformer, since the core is not properly utilized, the reversals
being too rapid. The inductive effect is also very small in certain
phases, as will be apparent from a graphic representation, and there may
be periods of inaction, if there are intervals between the succeeding
current impulses or waves. In producing a shifting of the poles in a
transformer, and thereby inducing currents, the induction is of the
ideal character, being always maintained at its maximum action. It is
also reasonable to assume that by a shifting of the poles less energy
will be wasted than by reversals.