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*NOTES ON A UNIPOLAR DYNAMO*
Written by Nikola Tesla and published in:
* The Electrical Engineer, N.Y., Sept. 2, 1891 *
It is characteristic of fundamental discoveries, of great
achievements of intellect, that they retain an undiminished
power upon the imagination of the thinker. The memorable
experiment of Faraday with a disc rotating bctwe'en the two
poles of a magnet, which has borne such magnificent fruit,
has long passed into every-day experience; yet there are certain
features about this embryo of the present dynamos and motors
which even to-day appear to us Striking, and are worthy of the
most careful study.
Consider, for instance, the case of a disc of iron or other
metal revolving between the two opposite poles of a magnet,
and the Polar surfaces completely covering both sides of the
disc, and assume the current to be taken off or conveyed to
the same by contacts uniformly from all points of the periphery
of the disc. Take first the case of a motor.
In all ordinary motors the operation is dependent upon some
shifting or change of the resultant of the magnetic attraction
exerted upon the armature, this process being effected either
by some mechanical contrivance on the motor or by the action
of currents of the proper character. We may explain the operation
of such a motor just as we can that of a water-wheel.
But in the above example of the disc surrounded completely by
the polar surfaces, there is no shifting of the ;magnetic action,
no change whatever, as far as we know, and yet rotation ensues.
Here, then, ordinary considerations do not apply; we cannot even
give a superficial explanation, as in ordinary motors, and the
operation will be clear to us only when we shall have recognized
the very nature of the forces concerned, and fathomed the mystery
of the invisible connecting mechanism.
Considered as a dynamo machine, the disc is an equally interesting
object of study. In addition to its peculiarity of giving currents
of one direction without the employment of commutating devices, such
a machine differs from ordinary dynamos in that there is no reaction
between armature and field. The armature current tends to set up a
magnetization at right angles to that of the field current, but since
the current is taken off uniformly from all points of the periphery,
and since, to be exact, the external circuit may also be arranged
perfectly symmetrical to the field magnet, no reaction can occur.
This, however, is true only as long as the magnets are weakly
energized, for when the magnets are more or less saturated, both
magnetizations at right angles seemingly interfere with each other.
For the above reason alone it would appear that the output of
such a machine should, for the same weight, be much greater than
that of any other machine in which the armature current tends to
demagnetize the field. The extraordinary output of the Forbes
unipolar dynamo and the experience of the writer confirm this view.
Again, the facility with which such a machine may be made to
excite itself is striking, but this may be due m besides to the
absence of armature reaction -- to the perfect smoothness of the
current and non-existence of self-induction.
If the poles do not cover the disc completely on both
sides, then, of course, unless the disc be properly subdivided,
the machine will be very inefficient. Again, in this case there
are points worthy of notice. If-the disc be rotated and the
field current interrupted, the current through the armature
will continue to flow and the field magnets will lose their
strength comparatively slowly. The reason for this will at once
appear when we consider the direction of the currents set up
in the disc.
Referring to the diagram Fig. 1, d represents the disc with
the sliding contacts B B' on the shaft and periphery. N and S
represent the two poles of a magnet. If the pole N be above, as
indicated in the diagram, the disc being supposed to be in the
plane of the paper, and rotating in the direction of the arrow D,
the current set up in the disc will flow from the centre to the
periphery, as indicated by the arrow A. Since the magnetic action
is more o.or less confined to the space between the poles N S,
the other portions of the disc may be considered inactive.
The current set up will therefore not wholly pass through the
external circuit F, but will close through the disc itself, and
generally, if the disposition be in any way similar to the one
illustrated, by far the greater portion of the current
generated will not appear externally, as the circuit F is
practically short-circuited by the inactive portions of the disc.
The direction of the resulting currents in the latter may be
assumed to be as indicated by the dotted lines and arrows
m and n; and the direction of the energizing field current
being indicated by the arrows a b c d, an inspection of the
figure shows that one of the two branches of the eddy current,
that is, A B' m B, will tend to demagnetize the field, while
the other branch, that is, A B' n B, will have the opposite
Therefore, the branch A B' m B, that is, the one which is
approaching the field, will repel the lines of the same, while
branch A B' n B, that is, the one leaving the field, will gather
the lines of force upon itself. In consequence of this there
will be a constant tendency to reduce the current flow in the
path A B' m B, while on the other hand no such opposition will
exist in path A B' n B, and the effect of the latter branch or
path will be more or less preponderating over that of the former.
The joint effect of both the assumed branch currents might be
represented by that of one single current of the same direction
as that energizing the field. In other words, the eddy currents
circulating in the disc will energize the field magnet. This is
a result quite contrary to what we might be led to
suppose at first, for we would naturally expect that the
resulting effect of the armature currents would be such
as to oppose the field current, as generally occurs when
a primary and secondary conductor are placed in inductive
relations to each other.
But it must be remembered that this result from the
peculiar disposition in this case, namely, two paths being
afforded to the current, and the latter selecting that path
which offers the least opposition to its flow. From this we
see that the eddy currents flowing in the disc partly
energize the field, and for this reason when the field
current is interrupted the currents in the disc will continue
to flow, and the field magnet will lose its strength with
comparative slowness and may even retain a certain strength
as long as the rotation of the disc is continued.
Figure 2 & 3 together
The result will, of course, largely depend on the
resistance and geometrical dimensions of the path of
the resulting eddy current and on the speed of rotation;
these elements, namely; determine the retardation of
this current and its position relative to the field.
For a certain speed there would be a maximum energizing
action; then at higher speeds, it would gradually fall off
to zero and finally reverse, that is, the resultant eddy
current effect would be to weaken the field. The reaction
would be best demonstrated experimentally by arranging the
fields N S, N' S', freely movable o.on an axis concentric
with the shaft of the disc.
If the latter were rotated as before in the direction of
the arrow D, the field would be dragged in the same direction
with a torque, which, up to a certain point, would go on
increasing with the speed of rotation, then fall off, and
passing through zero, finally become negative; that is: the
field would begin to rotate in opposite direction to the disc.
In experiments with alternate current motors in which the
field was shifted by currents of differing phase, this
interesting result was observed. For very low speeds of
rotation of the field the motor would show a torque of
900 lbs or more, measured on a pulley 12 inches in
When the speed of rotation of the poles was increased, the
torque would diminish, would finally go down to zero, become
negative, and then the armature would begin to rotate in
opposite direction to the field.
To return to the principal subject; assume the conditions
to be such that the eddy currents generated by the rotation
of the disc strengthen the field, and suppose the latter gradually
removed while the disc is kept rotating at an increased rate.
The current, once started, may then be sufficient to maintain
itself and even increase in strength, and then we have the case
of Sir William Thomson's "current accumulator."
But from the above considerations it would seem that for the
success of the experiment the employment of a disc not subdivided
would be essential, for if there should be a radial subdivision,
the eddy currents could not form and the self-exciting action would
cease. If such a radially subdivided disc .were used it would be
necessary to connect the spokes by a conducting rim or in any
proper manner so as to form a symmetrical system of dosed circuits.
The action of the eddy currents may be utilized to excite a machine
of any construction. For instance, in Figs. 2 and 3 an arrangement
is shown by which a machine with a disc armature might be excited.
Here a number of magnets, N S, N S, are placed radially on each
side of a metal disc D carrying on its rim a set of insulated coils,
C C. The magnets form two separate fields, an internaY and an
external one, the solid disc rotating in the field nearest the axis,
and the coils in the field further from it.
Assume the magnets slightly energized at the start; they could be
strengthened by the action of the eddy currents in the solid disc
so as to afford a stronger field for the peripheral coils. Although
there is no doubt that under proper conditions a machine might be
excited in this or a similar manner, there being sufficient
experimental evidence to warrant such an assertion, such a mode of
excitation would be wasteful.
But a unipolar dynamo or motor, such as shown in Fig. 1 may be
excited in an efficient manner by simply properly subdividing the
disc or cylinder in which the currents are set up, and it is
practicable to do away with the field coils which are usually employed.
Such a plan is illustrated in Fig. 4.
The disc or cylinder D is supposed to be arranged to rotate between
the two poles N and S of a magnet, which completely cover it on both
sides, the contours of the disc and poles being represented by the
Figure 3 & 4 together
But from the above considerations it would seem that for
the success of the experiment the employment of a disc not
subdivided would be essential, for if there should be a
radial subdivision, the eddy currents could not form and
the self-exciting action would cease. If such a radially
subdivided disc were used it would be necessary to connect
the spokes by a conducting rim or in any proper manner so
as to form a symmetrical system of dosed circuits.
The action of the eddy currents may be utilized to excite a
machine of any construction. For instance, in Figs. 2 and 3
an arrangement is shown by which a machine with a disc armature
might be excited. Here a number of magnets, N S, N S, are
placed radially on each side of a metal disc D carrying on its
rim a set of insulated coils, C C. The magnets form two
separate fields, an internal and an external one, the solid disc
rotating in the field nearest the axis, and the coils in the
field further from it.
Assume the magnets slightly energized at the start; they could
be strengthened by the action of the eddy currents in the solid
disc so as to afford a stronger field for the peripheral coils.
Although there is no doubt that under proper conditions a machine
might be excited in this or a similar manner, there being
sufficient experimental evidence to warrant such an assertion,
such a mode of excitation would be wasteful.
But a unipolar dynamo or motor, such as shown in Fig. 1 may be
excited in an efficient manner by simply properly subdividing
the disc or cylinder in which the currents are set up, and it
is practicable to do away with the field coils which are usually
Such a plan is illustrated in Fig. 4. The disc or cylinder D
is supposed to be arranged to rotate between the two poles N and S
of a magnet, which completely cover it on both sides, the contours
of the disc and poles being represented by the circles d and d'
respectively, the upper pole being omitted for the sake of clearness.
The cores of the magnet are supposed to be hollow, the shaft C of
the disc passing through them. If the unmarked pole be below, and
the disc be rotated screw fashion, the current will be, as before,
from the centre to the periphery, and may be taken off by suitable
sliding contacts, B B', on the shaft and periphery respectively.
In this arrangement the current flowing through the disc and
external circuit will have no appreciable effect on the field
magnet. But let us now suppose the disc to be subdivided, spirally
as indicated by the full or dotted lines, Fig. 4. The difference
of potential between a point on the shaft and a point on the
periphery will remain unchanged, in sign as well as in amount.
The only difference will be that the resistance o,of the disc will
be augmented and' that there will be a greater fall of potential
from a point on the shaft to a point on the periphery when the same
current is traversing the external circuit. But since the current
is forced to follow the lines of subdivision, we see that it will
tend either to energize or de-energize
the field, and this will depend, other things being equal,
upon the direction of the lines of subdivision. If the
subdivision be as indicated by the full lines in Fig. 4,
it is evident that if the current is of the same direction
as before, that is, from centre to periphery, its effect
will be to strengthen the field magnet; whereas, if the
subdivision be as indicated by the dotted lines, the
current generated will tend to weaken the magnet.
In the former case the machine will be capable of exciting
itself when the disc is rotated in the direction of arrow D;
in the latter case the direction of rotation must be reversed.
Two such discs may be combined, however, as indicated, the
two discs rotating in opposite fields, and in the same or
Similar disposition may, of course, be made in a type of
machine in which, instead of a disc, a cylinder is rotated.
In such unipolar machines, in the manner indicated, the usual
field coils and poles may be omitted and the machine may be
made to consist only of a cylinder or of two discs enveloped
by a metal casting.
Instead of subdividing the disc or cylinder spirally, as
indicated in Fig. 4, it is more convenient to interpose one
or more turns between the disc and the contact ring on the
periphery, as illustrated in Fig. 5.
A Forbes dynamo may, for instance, be excited in such a manner.
In the experience of the writer it has been found that instead
of taking the current from two such discs by sliding contacts,
as usual, a flexible conducting: belt may be employed to advantage.
The discs are in such case provided with large flanges, affording
a very great contact surface. The belt should be made to bear
on the flanges with spring pressure to take up the expansion.
Several machines with belt contact were constructed by the
writer two years ago, and worked satisfactorily; but for want
of time the work in that direction has been temporarily suspended.
A number of features pointed out above. have also been
used by, the writer in connection with some types of
alternating current motors.