We abstract the following from a paper on electric lighting by Prof.
J.A. Fleeming, read before the Iron and Steel Institute, Manchester.
The illustration is from _Engineering_.
[Illustration: ELECTRICAL GOVERNOR.]
One of the questions which most frequently occurs in reference to mill
and factory lighting is whether the factory engines can be used to run
the dynamo. As a broad, general rule, there can be no question that
the best results are obtained by using a separate dynamo engine,
controlled by a good governor, set apart for that purpose. With an
ordinary shunt dynamo, the speed ought not to vary more than 2 or 3
per cent. of its normal value on either side of that value. Hence, if
a dynamo has a normal speed of 1,000, it should certainly not vary
over a greater range than from 970 to 980 to 1,020 to 1,030. In many
cases there may be shafting from which the necessary power can be
taken, and of which the speed is variable only within these limits.
There are several devices by which it has been found possible to
enable a dynamo to maintain a constant electromotive force, even if
the speed of rotation varies over considerable limits. One of these is
that (see illustration) due to Messrs. Trotter & Ravenshaw, and
applicable to shunt or series machines.
In the circuit of the field magnet is placed a variable resistance.
This resistance is thrown in or out by means of a motor device
actuated by an electromotive force indicator. A plunger of soft iron
is suspended from a spring, and hangs within a solenoid of wire, which
solenoid is in connection with the terminals of the dynamo. Any
increase or diminution of the electromotive force causes this iron to
move in or out of the core, and its movement is made to connect or
disconnect the gearing which throws in the field magnet resistance
with a shaft driven by the engine itself. The principle of the
apparatus is therefore that small variations of electromotive force
are made to vary inversely the strength of the magnetic field through
the intervention of a relay mechanism in which the power required to
effect the movement is tapped from the engine.
With the aid of such a governor it is possible to drive a dynamo from
a mill shaft providing the requisite power, but of which the speed of
rotation is not sufficiently uniform to secure alone efficient
regulation of electromotive force. Another device, patented by Mr.
Crompton, is a modification of that method of field magnet winding
commonly known as compound winding. The field magnets are wound over
with two wires, one of which has a high resistance and is arranged as
a shunt, and the other of which has a low resistance and is arranged
in series. Instead, however, of the magnetizing powers of these coils
being united in the same direction as an ordinary compound winding,
they are opposed to one another. That is to say, the current in the
shunt wire tends to magnetize the iron of the field magnets in an
opposite direction to that of the series wire. It results from this
that any slight increase of speed diminishes the strength of the
magnetic field, and _vice versa_. Accordingly, within certain limits,
the electromotive force of the dynamo is independent of the speed of
rotation.
* * * * *
THE ELECTRIC CURRENT AS A MEANS OF INCREASING THE TRACTIVE ADHESION OF
RAILWAY MOTORS AND OTHER ROLLING CONTACTS.[1]
[Footnote 1: Read before the American Association for the
Advancement of Science. New York meeting, 1887.]
By ELIAS E. RIES.
The object of this paper is to lay before you the results of some
recent experiments in a comparatively new field of operation, but one
that, judging from the results already attained, is destined to become
of great importance and value in its practical application to various
branches of industry.
I say "comparatively new" because the underlying principles involved
in the experiments referred to have, to a certain extent, been
employed (in, however, a somewhat restricted sense) for purposes
analogous to those that form the basis of this communication.
As indicated by the title, the subject that will now occupy our
attention is the use of the electric current as a means of increasing
and varying the frictional adhesion of rolling contacts and other
rubbing surfaces, and it is proposed to show how this effect may be
produced, both by means of the direct action of the current itself and
by its indirect action through the agency of electro-magnetism.
Probably the first instance in which the electric current was directly
employed to vary the amount of friction between two rubbing surfaces
was exemplified in Edison's electro-motograph, in which the variations
in the strength of a telephonic current caused corresponding
variations in friction between a revolving cylinder of moistened chalk
and the free end of an adjustable contact arm whose opposite extremity
was attached to the diaphragm of the receiving telephone. This device
was extremely sensitive to the least changes in current strength, and
if it were not for the complication introduced by the revolving
cylinder, it is very likely that it would to-day be more generally
used.
It has also been discovered more recently that in the operation of
electric railways in which the track rails form part of the circuit, a
considerable increase in the tractive adhesion of the driving wheels
is manifested, due to the passage of the return current from the
wheels into the track. In the Baltimore and Hampden electric railway,
using the Daft "third rail" system, this increased tractive adhesion
enables the motors to ascend without slipping a long grade of 350 feet
to the mile, drawing two heavily loaded cars, which result, it is
claimed, is not attainable by steam or other self-propelling motors of
similar weight. In the two instances just cited the conditions are
widely different, as regards the nature of the current employed, the
mechanical properties of the surfaces in contact, and the electrical
resistance and the working conditions of the respective circuits. In
both, however, as clearly demonstrated by the experiments hereinafter
referred to, the cause of the increased friction is substantially the
same.
In order to ascertain the practical value of the electric current as a
means of increasing mechanical friction, and, if possible, render it
commercially and practically useful wherever such additional friction
might be desirable, as for example in the transmission of power, etc.,
a series of experiments were entered into by the author, which, though
not yet fully completed, are sufficiently advanced to show that an
electric current, when properly applied, is capable of very materially
increasing the mechanical friction of rotating bodies, in some cases
as much as from 50 to 100 per cent., with a very economical
expenditure of current; this increase depending upon the nature of the
substances in contact and being capable of being raised by an
increased flow of current.
Before entering into a description of the means by which this result
is produced, and how it is proposed to apply this method practically
to railway and other purposes, it may be well to give a general
outline of what has so far been determined. These experiments have
shown that the coefficient of friction between two conducting surfaces
is very much increased by the passage therethrough of an electric
current of _low electromotive force and large volume_, and this is
especially noticeable between two rolling surfaces in peripheral
contact with each other, or between a rolling and a stationary
surface, as in the case of a driving wheel running upon a railway
rail. This effect increases with the number of amperes of current
flowing through the circuit, of which the two surfaces form part, and
is not materially affected by the electromotive force, so long as the
latter is sufficient to overcome the electrical resistance of the
circuit. This increase in frictional adhesion is principally
noticeable in iron, steel, and other metallic bodies, and is due to a
molecular change in the conducting substances at their point of
contact (which is also the point of greatest resistance in the
circuit), caused by the heat developed at that point. This heat is
ordinarily imperceptible, and becomes apparent only when the current
strength is largely augmented. It is therefore probable that a portion
of this increased tractive adhesion is due directly to the current
itself aside from its heating effect, although I have not as yet been
able to ascertain this definitely. The most economical and efficient
results have been obtained by the employment of a transformed current
of extremely low electromotive force (between ½ and 1 volt), but of
very large volume or quantity, this latter being variable at will, so
as to obtain different degrees of frictional resistance in the
substances under observation.
These experiments were originally directed mainly toward an endeavor
to increase the tractive adhesion of the driving wheels of locomotives
and other vehicles, and to utilize the electric current for this
purpose in such a manner as to render it entirely safe, practical, and
economical. It will be apparent at once that a method of increasing
the tractive power of the present steam locomotives by more than 50
per cent. without adding to their weight and without injury to the
roadbed and wheel tires, such as is caused by the sand now commonly
used, would prove of considerable value, and the same holds true with
respect to electrically propelled street cars, especially as it has
been found exceedingly difficult to secure sufficient tractive
adhesion on street railways during the winter season, as well as at
other times, on roads having grades of more than ordinary steepness.
As this, therefore, is probably the most important use for this
application of the electric current, it has been selected for
illustrating this paper.
I have here a model car and track arranged to show the equipment and
operation of the system as applied to railway motors. The current in
the present instance is one of alternating polarity which is converted
by this transformer into one having the required volume. The
electromotive force of this secondary current is somewhat higher than
is necessary. In practice it would be about half a volt. You will
notice upon a closer inspection that one of the forward driving wheels
is insulated from its axle, and the transformed current, after passing
to a regulating switch under the control of the engineer or driver,
goes to this insulated wheel, from which it enters the track rail,
then through the rear pair of driving wheels and axles to the opposite
rail, and then flows up through the forward uninsulated wheel, from
the axle of which it returns by way of a contact brush to the opposite
terminal of the secondary coil of the transformer. Thus the current is
made to flow _seriatim_ through all four of the driving wheels,
completing its circuit through that portion of the rails lying between
the two axles, and generating a sufficient amount of heat at each
point of contact to produce the molecular change before referred to.
By means of the regulating switch the engineer can control the amount
of current flowing at any time, and can even increase its strength to
such an extent, in wet or slippery weather, as to _evaporate any
moisture_ that may adhere to the surface of the rails at the point of
contact with the wheels while the locomotive or motor car is under
full speed.
It will be apparent that inasmuch as the "traction circuit" moves
along with the locomotive, and is complete through its driving wheel
base, the track rails in front and rear of the same are at all times
entirely free from current, _and no danger whatever can occur by
coming in contact with the rails between successive motors_. Moreover,
the potential used in the present arrangement, while sufficient to
overcome the extremely low resistance of the moving circuit, is too
small to cause an appreciable loss of current from that portion of the
rails in circuit, even under the most unfavorable conditions of the
weather. In practice the primary current necessary is preferably
generated by a small high speed alternating dynamo on the locomotive,
the current being converted by means of an inductional transformer. To
avoid the necessity for electrically bridging the rail joints, a
modified arrangement may be employed, in which the electrical
connection is made directly with a fixed collar on the forward and
rear driving axles, the current dividing itself in parallel between
the two rails in such a manner that, if a defective joint exists in
the rail at one side, the circuit is still complete through the rail
on the other; and as the rails usually break joints on opposite sides,
this arrangement is found very effective. The insulation of the
driving wheels is very easily effected in either case.
As the amount of additional tractive adhesion produced depends upon
the _quantity_ of current flowing rather than upon its pressure, the
reason for transforming the current as described will be apparent, and
its advantages over a direct current of higher tension and less
quantity, both from an economical and practical standpoint, will for
this reason be clear. The amount of heat produced at the point of
contact between the wheels and rails is never large enough to injure
or otherwise affect them, although it may be quite possible to
increase the current sufficiently to produce a very considerable
heating effect. The amount of current sent through the traction
circuit will of course vary with the requirements, and as the extent
to which the resistance to slipping may be increased is very great,
this method is likely to prove of considerable value. While in some
cases the use of such a method of increasing the tractive power of
locomotives would be confined to ascending gradients and the movement
of exceptionally heavy loads, in others it would prove useful as a
_constant_ factor in the work of transportation. In cases like that of
the New York elevated railway system, where the traffic during certain
hours is much beyond the capacity of the trains, and the structure
unable to support the weight of heavier engines, a system like that
just described would prove of very great benefit, as it would easily
enable the present engines to draw two or three additional cars with
far less slipping and lost motion than is the case with mechanical
friction alone, at a cost for tractive current that is insignificant
compared to the advantages gained. Other cases may be cited in which
this method of increasing friction will probably be found useful,
aside from its application to railway purposes, but these will
naturally suggest themselves and need not be further dwelt upon.
In the course of the experiments above described, another and somewhat
different method of increasing the traction of railway motors has been
devised, which is more particularly adapted to electric motors for
street railways, and is intended to be used in connection with a
system of electric street railways now being developed by the author.
In this system _electro-magnetism_ provides the means whereby the
increase in tractive adhesion is produced, and this result is attained
in an entirely novel manner. Several attempts have heretofore been
made to utilize magnetism for this purpose, but apparently without
success, chiefly because of the crude and imperfect manner in which
most of these attempts have been carried out.
The present system owes its efficiency to the formation of _a complete
and constantly closed magnetic circuit_, moving with the vehicle and
completed through the two driving axles, wheels, and that portion of
the track rails lying between the two pairs of wheels, in a manner
similar to that employed in the electrical method before shown. We
have here a model of a second motor car equipped with the apparatus,
mounted on a section of track and provided with means for measuring
the amount of tractive force exerted both with and without the passage
of the current.
You will notice that each axle of the motor car is wound with a helix
of insulated wire, the helices in the present instance being divided
to permit the attachment to the axles of the motor connections. The
helices on both axles are so connected that, when energized, they
induce magnetic lines of force that flow in the same direction through
the magnetic circuit. There are, therefore, four points at which the
circuit is maintained closed by the rolling wheels, and as the
resistance to the flow of the lines of force is greatest at these
points, the magnetic saturation there is more intense, and produces
the most effective result just where it is most required. Now, when
the battery circuit is closed through the helices, it will be observed
that the torque, or pull, exerted by the motor car is fully twice that
exerted by the motor with the traction circuit open, and, by
increasing the battery current until the saturation point of the iron
is reached, the tractive force is _increased nearly 200 per cent._, as
shown by the dynamometer. A large portion of this resistance to the
slipping or skidding of the driving wheels is undoubtedly due to
direct magnetic attraction between the wheels and track, this
attraction depending upon the degree of magnetic saturation and the
relative mass of metal involved.
But by far the greatest proportion of the increased friction is purely
the result of the change in position of the iron molecules due to the
well known action of magnetism, which causes a direct and close
_interlocking action_, so to speak, between the molecules of the two
surfaces in contact. This may be illustrated by drawing a very thin
knife blade over the poles of an ordinary electro-magnet, first with
the current on and then off.
In the model before you, the helices are fixed firmly to, and revolve
with, the axles, the connections being maintained by brushes bearing
upon contact rings at each end of the helices. If desired, however,
the axles may revolve loosely within the helices, and instead of the
latter being connected for cumulative effects, they may be arranged in
other ways so as to produce either subsequent or opposing magnetic
forces, leaving certain portions of the circuit neutral and
concentrating the lines of force wherever they maybe most desirable.
Such a disposition will prove of advantage in some cases.
The amount of current required to obtain this increased adhesion in
practice is extremely small, and may be entirely neglected when
compared to the great benefits derived. The system is very simple and
inexpensive, and the amount of traction secured is entirely within the
control of the motor man, as in the electric system. It will be seen
that the car here will not, with the traction circuit open, propel
itself up hill when one end of the track is raised more than 5 inches
above the table; but with the circuit energized it will readily ascend
the track as you now see it, with one end about 13½, inches above the
other in a length of three feet, _or the equivalent of a 40 per cent.
grade_; and this could be increased still further if the motor had
power enough to propel itself against the force of gravity on a
steeper incline. As you will notice, the motor adheres very firmly to
the track and requires a considerable push to force it down this 40
per cent. grade, whereas with the traction circuit open it slips down
in very short order, notwithstanding the efforts of the driving
mechanism to propel it up.
The resistance of the helices on this model is less than two ohms, and
this will scarcely be exceeded when applied to a full sized car, the
current from two or three cells of secondary batteries being probably
sufficient to energize them.
The revolution of the driving axles and wheels is not interfered with
in the slightest, because in the former the axle boxes are outside the
path of the lines of force, and in the case of the latter because each
wheel practically forms a single pole piece, and in revolving presents
continuously a new point of contact, of the same polarity, to the
rail; the flow of the lines of force being most intense through the
lower half of the wheels, and on a perpendicular line connecting the
center of the axle with the rail. In winter all that is necessary is
to provide each motor car with a suitable brush for cleaning the track
rails sufficiently to enable the wheels to make good contact
therewith, and any tendency to slipping or skidding may be effectually
checked. By this means it is easily possible to increase the tractive
adhesion of an ordinary railway motor from 50 to 100 per cent.,
without any increase in the load or weight upon the track; for it must
be remembered that even that portion of the increased friction due to
direct attraction does not increase the weight upon the roadbed, as
this attraction is mutual between the wheels and track rails; and if
this car and track were placed upon a scale and the circuit closed, it
would not weigh a single ounce more than with the circuit open.
It is obvious that this increase in friction between two moving
surfaces can also be applied to _check_, as well as augment, the
tractive power of a car or train of cars, and I have shown in
connection with this model a system of braking that is intended to be
used in conjunction with the electro-magnetic traction system just
described. You will have noticed that in the experiments with the
traction circuit the brake shoes here have remained idle; that is to
say, they have not been attracted to the magnetized wheels. This is
because a portion of the traction current has been circulating around
this coil on the iron brake beam, inducing in the brake shoes
magnetism of like polarity to that in the wheels to which they apply.
They have therefore been _repelled_ from the wheel tires instead of
being attracted to them. Suppose now that it is desired to stop the
motor car; instead of opening the traction circuit, the current
flowing through the helices is simply reversed by means of this pole
changing switch, whereupon the axles are magnetized in the opposite
direction and the brake shoes are instantly drawn to the wheels with a
very great pressure, as the current in the helices and brake coil now
assist each other in setting up a very strong magnetic flow,
sufficient to bring the motor car almost to an instant stop, if
desired.
The same tractive force that has previously been applied to increase
the tractive adhesion now exercises its influence upon the brake shoes
and wheels, with the result of not only causing a very powerful
pressure between the two surfaces due to the magnetic attraction, but
offering an extremely large frictional resistance in virtue of the
molecular interlocking action before referred to. As shown in the
present instance, a portion of the current still flows through the
traction circuit and prevents the skidding of the wheels.
The method thus described is equally applicable to increase the
coefficient of friction in apparatus for the transmission of power,
its chief advantage for this purpose being the ease and facility with
which the amount of friction between the wheels can be varied to suit
different requirements, or increased and diminished (either
automatically or manually) according to the nature of the work being
done. With soft iron contact surfaces the variation in friction is
very rapid and sensitive to slight changes in current strength, and
this fact may prove of value in connection with its application to
regulating and measuring apparatus. In all cases the point to be
observed is to maintain a closed magnetic circuit of low resistance
through the two or more surfaces the friction of which it is desired
to increase, and the same rule holds good with respect to the electric
system, except that in the latter case the best effects are obtained
when the area of surface in contact is smallest.
For large contact areas the magnetic system is found to be most
economical, and this system might possibly be used to advantage to
prevent slipping of short wire ropes and belts upon their driving
pulleys, in cases where longer belts are inapplicable as in the
driving of dynamos and other machinery. Experiments have also been,
and are still being, made with the object of increasing friction by
means of permanent magnetism, and also with a view to _diminishing_
the friction of revolving and other moving surfaces, the results of
which will probably form the subject matter of a subsequent paper.
Enough has been said to indicate that the development of these two
methods of increasing mechanical friction opens up a new and extensive
field of operation, and enables electricity to score another important
point in the present age of progress. The great range and flexibility
of this method peculiarly adapt it to the purposes we have considered
and to numerous others that will doubtless suggest themselves to you.
Its application to the increase of the tractive adhesion of railway
motors is probably its most prominent and valuable feature at present,
and is calculated to act as an important stimulus to the practical
introduction of electric railways on our city streets, inasmuch as the
claims heretofore made for cable traction in this respect are now no
longer exclusively its own. On trunk line railways the use of sand and
other objectionable traction-increasing appliances will be entirely
dispensed with, and locomotives will be enabled to run at greater
speed with less slipping of the wheels and less danger of derailment.
Their tractive power can be nearly doubled without any increase in
weight, enabling them to draw heavier trains and surmount steeper
grades without imposing additional weight or strain upon bridges and
other parts of the roadbed. Inertia of heavy trains can be more
readily overcome, loss of time due to slippery tracks obviated, and
the momentum of the train at full speed almost instantly checked by
_one and the same means_.
* * * * *