Of all the wonderful operations accomplished by the aid of electricity
at the present time, none so completely mystifies the beholder
as the action of the trolley car. The electric light, although
incomprehensible to the average layman, does not excite his curiosity
to the same extent. The glowing filament of an incandescent lamp or
the dazzling carbon points of an arc light stimulate the inquisitive
proclivities to some extent, but as the popular notion with respect
to the nature of electricity is that it is some kind of fluid that
can flow through wires and other things like water through a pipe,
the conclusion arrived at is that the current, in its passage through
the filament or the carbon points, generates a sufficient amount of
heat to raise the temperature of the material to the luminous point.
The fact that energy is required to raise the temperature of the mass
to the incandescent point is not taken into consideration by those
not versed in technical matters, owing to the fact that, as nothing
moves, it is not supposed that power can be expended. When a trolley
car is seen coming down the street at a high rate of speed the effect
upon the mind is very different. Here we see a vast amount of weight
propelled at a high velocity, and yet the only source through which
the power to accomplish this result is supplied is a small wire. The
mystifying cause does not stop here, for if we look further into the
matter we see that the energy has to pass from the trolley wire to the
car through the very small contact between it and the trolley wheel.
After contemplating these facts, it appears remarkable that the energy
that can creep through this diminutive passage can by any means be made
to develop the force necessary to propel a car with a heavy load up a
steep grade. An electrical engineer, if asked to explain the action,
would say that the force of magnetic attraction was made use of to
accomplish the result, but this explanation would fail to throw any
light upon the subject. In what follows, it is proposed to explain the
matter in a simple manner, and then it will be seen that what appears
to be an incomprehensible mystery, when not understood, is, in fact, no
mystery at all.
NOTE.--The illustrations of railway motor, generator, and
switchboard (Figs. 15, 16, 17) were made from photographs kindly
furnished by the manufacturers, the Westinghouse Electric and
Manufacturing Company.
Electricity and magnetism are two forces that are intimately associated
with each other, and, although radically different, it is difficult,
if not impossible, to obtain one without the other, although it is a
simple matter to make one inactive under certain conditions. It is very
generally understood that a magnet possesses the power of attraction,
and that it will draw toward it pieces of iron, steel, and other
magnets. The laws governing the attractive properties of magnets,
however, are not so well understood, and many are not aware of the fact
that under certain conditions one magnet will repel another, but such
is nevertheless the case.
[Illustration: FIGS. 1, 2, 3.--DIAGRAMS ILLUSTRATING THE ATTRACTION AND
REPULSION OF MAGNETS.]
In Fig. 1 the lower outline, _M_, represents a magnet fixed in
position, and the upper bar represents another magnet arranged to
swing freely around the pivot _a_. A magnet, as is generally known,
will arrange itself in a north-to-south position if suspended from its
center, like a scale beam, and allowed to swing freely, and the same
end will always point toward the north. On this account the ends of a
magnet are called its poles, and the one that will point toward the
north is designated the north pole, while the other one is the south
pole. The terms north and south poles were applied to magnets centuries
ago, but at the present time the ends are more commonly designated as
positive and negative. In Fig. 1 it will be noticed that the stationary
magnet has its positive end upward, and this attracts the negative end
of the swinging magnet. If the order of the poles is reversed, so that
the positive of the swinging magnet will come opposite the positive of
the stationary one, then there will be a repulsive action instead of an
attraction, as is shown in Fig. 2. If the two negative ends were placed
opposite, the effect would be the same. From this we see that to obtain
an attraction we must place the magnets so that opposite poles come
together, and that by reversing the order we obtain a repulsive action.
If the swinging magnet is replaced by a bar of iron, as is shown in
Fig. 3, there will be an attraction, no matter what end of the magnet
may be uppermost, thus showing that either end of a magnet will attract
a bar of iron. The explanation of these different actions is that when
two magnets are brought into proximity to each other each one exerts
its force without any regard to the other, and if the two are set
to act together they will attract one another, but if set to act in
opposition they will repel. When one of the bars is not a magnet, but
simply a piece of iron or steel, this bar, having no attractive or
repulsive force of its own, can only obey the attractive action of the
other, which is the only one that exerts a force.
[Illustration: FIGS 4, 5.--DIAGRAMS ILLUSTRATING THE METHOD OF
OBTAINING ROTARY MOTION WITH MAGNETS.]
In Fig. 4 _M_ is a magnet bent into the form of a U, commonly called
a horseshoe magnet. The short bar set between the upper ends is also
a magnet, and is arranged so as to revolve around the shaft _s_. From
what has just been explained in connection with Figs. 1 and 2 it
will be understood that, with the poles as indicated by the letters,
there will be an attractive force set up between the top end of the
straight bar and the _P_ end of the horseshoe, and thus rotation will
be produced in the direction of the arrow. The rotation, however, will
necessarily stop when the bar reaches the position shown in Fig. 5, for
then the attraction between the poles will resist further movement.
If the straight bar were not a magnet, but simply a piece of iron or
steel, it is evident that when in the position of Fig. 4 the attraction
would be just as much toward the right as toward the left, and if
the bar were placed accurately in the central position it would not
swing in either direction. It would be in the condition called, in
mechanics, unstable equilibrium. In practice this condition could not
be very well realized, as it would be difficult to set and retain the
bar in a position where the attraction from both sides would be the
same, therefore the rotation would be in one direction or the other;
but whichever way the bar might move, it would only swing through one
quarter of a revolution, into the horizontal position of Fig. 5.
If we reflect upon these actions we can see that if we could destroy
the magnetism of both parts before the straight bar reaches the
position of Fig. 5 it would be possible to obtain rotation through
a greater distance than one quarter of a turn, for then the headway
acquired by the rotating part would cause it to continue its motion.
If, after the completion of one half of a revolution, we could
remagnetize both parts, we would then set up an attraction between the
lower end of the straight bar and the left side of the horseshoe, for
then the polarity of the former would be the reverse of that shown in
Fig. 4--that is, the lower end would be negative. By means of this
second attraction we would cause the bar to rotate through the third
quarter of the revolution, and if, just before completing this last
quarter, we were to remove all the magnetism again, the headway would
keep up the motion through the final quarter of the revolution, thus
completing one full turn. From this it will be realized that if we
could magnetize and demagnetize the two parts twice in each revolution
a continuous rotation could be obtained.
If the magnetizing and demagnetizing action were only applied to the
rotating part we would fail to keep up a continuous rotation, for,
as was shown in connection with Fig. 3, the action when the straight
bar reached the position of Fig. 5 would be the same as if it were
magnetized, owing to the fact that a magnet always exerts an attraction
upon a mass of iron. Suppose, however, that we were to reverse the
polarity of the rotating part just as it reaches the position of Fig.
5, then there would be two poles of the same polarity opposite each
other, and, as shown in Fig. 2, the force acting between them would be
repulsive, and would push the bar around in the direction of rotation.
Not only would the right-side pole of the horseshoe force the end
of the bar away from it, but the negative pole, on the left side,
would attract this same end, and thus a force would be exerted by the
two poles of _M_ to keep up the rotation through the next half of a
circle. On reaching this last position the rotation would stop if the
polarity of the revolving bar were left unchanged, for then the poles
facing each other would be of opposite polarity. If, however, we again
reversed the polarity, a repulsion would be set up between the poles
facing each other, and thus a force would be exerted to continue the
rotation. Thus we see that if the polarity of the horseshoe magnet is
not disturbed it is necessary to reverse that of the rotating part to
obtain a continuous motion, but if we change the magnetic conditions of
both parts, then it is only necessary to magnetize and demagnetize them
alternately.
From the foregoing it is seen that there are two ways in which the
force of magnetism could be utilized to keep up a continuous rotation,
and the question now is, Can either of them be made available in
practice? To this we answer that, by the aid of the relations existing
between electricity and magnetism, both can be and are made available,
as will be shown in the following paragraphs:
[Illustration: FIGS. 6, 7, 8.--DIAGRAMS ILLUSTRATING THE PRINCIPLES OF
ELECTRO-MAGNETS.]
In Fig. 6 _W_ represents a coil of wire provided with a cotton
covering, so that there may be no actual contact between the adjoining
convolutions. If the ends _p n_ of this coil are connected with a
source of electric energy, an electric current will flow through it,
and if a bar, as indicated by _N P_, of iron or steel is placed within
the coil it will become magnetized. If the bar is made of steel and
is hardened it will retain the magnetism, and become what is called a
permanent magnet; such a magnet, in fact, as we have considered in all
the previous figures. If the bar is made of iron it will not retain the
magnetism, but will only be a magnet as long as the electric current
flows through the coil _W_. A magnet of the latter type is called
an electro-magnet. If the iron is of poor quality--that is, from an
electrical standpoint--it will require an appreciable time to lose its
magnetism, but if it is soft and high grade, electrically considered,
it will lose its magnetism instantly, or nearly so. If we take two bars
of soft iron and arrange them side by side, as in Fig. 7, and wind
coils around them as indicated each one will become magnetized when the
ends _p n_ of the coils are connected with an electric circuit. If the
lower ends of the two bars are joined by a piece, as shown at _M_, we
will have a horseshoe electro-magnet. If we take a round disk of iron,
as in Fig. 8, and wind a coil around it, it will also become a magnet
when an electric current traverses the coil. Thus it will be seen that
it makes little difference what the shape of the iron may be, providing
it is surrounded by a coil of wire and an electric current is passed
through the latter. This being the case, it is evident that either of
the processes explained in connection with Figs. 4 and 5 can be made
available for the production of a continuous rotation by the aid of
electro-magnets. Suppose we make a drum, as shown in Fig. 9, and wind
a wire coil around it in the direction indicated, then when a current
passes through the wire the drum will be magnetized, with poles at top
and bottom. If the electric current passes through the wire from end
_p_ to end _n_ the drum will be magnetized positively at the top and
negatively at the bottom, and if the direction of the current through
the wire is reversed the polarity of the drum will be reversed. If we
construct a horseshoe magnet of the shape shown in Fig. 10, and place
within the circular opening between its ends the drum of Fig. 9, we
will have a device that is capable of developing a continuous rotation,
providing we have suitable means for reversing the direction of the
electric current through the wire coil; and this machine constitutes an
electric motor in its simplest form.
[Illustration: FIGS. 9, 10.--DIAGRAMS ILLUSTRATING THE PRINCIPLES OF
THE ELECTRIC MOTOR.]
In an electric motor the horseshoe magnet is called the field magnet,
and the rotating part is called the armature, while the device by
means of which the direction of the current through the armature coil
is reversed is called the commutator. In this last figure it will be
noticed that the coils wound upon the field magnet are represented
as of wire much finer than that wound upon the armature. In actual
practice machines are sometimes wound in this way, and sometimes the
field wire is twice as large as that on the armature. When the field
wire is very much finer than that of the armature the machine is what
is known as shunt wound, which means that only a small portion of the
current that passed through the armature passes through the field
coils. Although with this type of winding the current that passes
through the field coils is very weak, the magnetism developed thereby
can be made greater than that of the armature if desired. This result
is accomplished by increasing the number of turns of wire in the field
coils. Thus if the current through the armature is one hundred times
as strong as that through the field coils, the latter can be made to
equal the effect of the former by increasing the number of turns in the
proportion of one hundred to one, and if the increase is still greater
the field coils will develop the strongest magnetism. The reason why a
small current passing around a magnet a great many times will develop
as strong a magnetization as a large current, can be readily understood
when we say that the magnetism is in proportion to the total strength
of the electric current that circulates around the magnet. Suppose we
have two currents, one of which is one thousand times as strong as the
other, then if the weak one is passed through a coil consisting of
one thousand turns it will develop just as strong a magnetization as
the large current passing through a coil of only one turn. This last
explanation enables us to see how it is that the comparatively small
current that can pass through the contact between the trolley wire and
the trolley wheel can develop in the motor force sufficient to propel
a heavy car up a steep grade. When that small current reaches the car
motors it passes through a thousand or more turns of wire, and thus its
effect is increased a corresponding number of times.
A motor having a single coil of wire upon the armature, as in Fig. 10,
would not give very satisfactory results, owing to the fact that the
rotative force developed by it would not be uniform. Such motors are
made in very small sizes, but never when a machine of any capacity is
required. For large machines it is necessary to wind the armature with
a number of coils, so that the rotating force may be uniform, and also
so that the current may be reversed by the commutator without producing
sparks so large as to destroy the device. When an armature is wound
with a number of coils the direction of the current is reversed, by the
commutator, in each coil as it reaches the point where its usefulness
ends, and where, if it continued to flow in the same direction, it
would act to hold the armature back. The effect of this reversal of
the current in one coil after another is to maintain the polarity of
the armature practically at the same point, so that the strongest pull
is exerted between it and the field magnet poles at all times. To
explain clearly the way in which the commutator reverses the current
in one coil at a time it will be necessary to make use of a diagram
illustrating what is called a ring armature. Such a diagram is shown
in Fig. 11. The ring _A_ is the armature core, and is made of iron;
the wire coils are represented as consisting of one turn to each coil,
and are marked _w w w_. The current enters the wire through the spring
_B_, and passes out through _C_. As can be seen, the current from _B_
can flow through the coils _w w_ in both directions, thus dividing
into two currents, each one of which will traverse one half of the
wire wound upon the armature. The two half currents will meet at _C_.
If the armature is rotated the springs _B_ and _C_ (which are called
commutator brushes) will pass from one turn of the wire coil to another
just back of it as the rotation progresses, and each time that contact
is made with a new turn the direction of the current in the turn just
ahead will be reversed. The current in the wire as a whole, however,
will always be in the same direction--that is, in all the turns to the
right of the two brushes; the current will flow toward the center of
the shaft on the front side of the armature, and away from the shaft
in all the turns on the left side. As the direction of the current on
opposite sides of the brushes is always the same, the poles of the
armature will remain under _B_ and _C_, therefore the relation between
the position of the poles of the armature and the field magnet will
be the same substantially as that illustrated in Fig. 10, and, as a
result, the force tending to produce rotation will at all times be the
greatest possible for the strength of the current used and the size of
the magnets.
Armatures are wound with a number of turns of wire in each coil, unless
the machine is very large, and present an appearance more like Fig.
12. In this figure the brushes are arranged to make contact with the
outer surface of the ring _C_, which is the commutator. The segments
_s s_ are connected with the ends of the armature coils _c c c_, but
are separated from each other by some kind of material that will not
conduct electricity--that is, they are electrically insulated. As will
be noticed from this, the armature in Fig. 11 acts as a commutator as
well as an armature, its outer surface performing the former office.
In the winding the difference between Figs. 11 and 12 is simply in
the number of turns in each coil, there being one turn in Fig. 11 and
several in Fig. 12.
[Illustration: FIGS. 11, 12.--DIAGRAMS ILLUSTRATING THE METHOD OF
WINDING ARMATURES OF ELECTRIC MOTORS AND GENERATORS.]
The armature shown in Fig. 10 is of the type called drum armature,
but it can be wound so as to produce the same result as the ring,
although it is not so easy to explain this style of winding. It will
be sufficient for the present explanation to say that whatever type of
armature may be used, the winding is always such that the direction of
the current through the wire coils is reversed progressively, so that
the magnetic polarity is maintained practically at the same point;
therefore there is a continuous pull between this point of the armature
core and the poles of the field magnet. The commutator is secured to
the armature shaft, and the brushes through which the current enters
and leaves are held stationary; keeping this fact in mind, it can be
seen at once that in Fig. 12 the current will flow from the brush _a_
through the two sides of the armature wire to brush _b_, hence all
the coils on the right of the vertical line will be traversed by the
current in the same direction--that is, either to or from the center of
the shaft--and in the coils on the left the direction will be opposite,
which is just the same order as was explained in connection with Fig.
11.
[Illustration: FIGS. 13, 14.--DIAGRAMS ILLUSTRATING THE DIFFERENCE
BETWEEN AN ELECTRIC MOTOR AND A GENERATOR.]
An electric motor can be turned into an electric generator by simply
reversing the direction in which the armature rotates--that is, any
electric machine is either a generator or a motor. This fact can
be illustrated by means of Figs. 13 and 14, both of which show the
armature and the poles of the field magnet. The first figure represents
an electric motor, and, as can be seen, the pull between the _N_ pole
of the armature and the _P_ pole of the field is in the direction of
arrow _b_, hence the armature will rotate in the same direction, as
indicated by arrow _a_. To obtain the polarity of the armature and
field it is necessary to pass an electric current through both--that
is to say, we must expend electrical energy to obtain power from the
machine. As soon as the current ceases to flow, the polarity of the
armature and field dies out, and the rotation of the former comes to an
end. The magnetism, however, does not die out entirely; a small residue
is always left, although it is never sufficient to produce rotation,
and even if it were it could only cause the armature to revolve through
one quarter of a turn. If, after the current has been shut off, the
armature shaft is rotated in the reverse direction, as indicated by
arrow _a_ in Fig. 14, the motion will be against the pull of the
magnetism; therefore, although the poles may be very weak, an amount of
power sufficient to overcome their attraction must be applied to the
pulley, otherwise rotation can not be accomplished. In consequence of
the backward rotation a current is generated in the armature coils, and
this current, as it traverses the field coils as well as those of the
armature, causes the polarity of both parts to increase. As a result
of the increased polarity the resistance to rotation is increased,
and more power has to be applied to the pulley. The increase in the
strength of the poles results in increasing the current generated, and
this in turn further increases the pole strength, so that one effect
helps the other, the result being that the current, which starts with
an infinitesimal strength, soon rises to the maximum capacity of the
machine.
The motor shown in Fig. 10 does not in any way resemble an electric
railway motor, nevertheless the principle of action is precisely the
same in both. The design of a machine of any kind has to conform to
the practical requirements, and this is true of railway motors, just
as it is true of printing presses, sawmills, or any other mechanism.
A railway motor must be designed to run at a comparatively slow speed
and to develop a strong rotative force, or torque, as it is technically
called. It must also be so constructed that it will not be injured if
covered with mud and water. It must be compact, strong, and light, and
capable of withstanding a severe strain without giving out. To render
the machine water- and mud-proof it is formed with an outer iron shell,
which entirely incases the internal parts. The first railway motors
were not inclosed, and the result was that they frequently came to
grief from the effects of a shower of mud. When the modern inclosed
type of motor, which is called the iron-clad type, first made its
appearance it was frequently spoken of as the clam-shell type, and
the name is not altogether inappropriate, for while the outside may
be covered with mud to such an extent as to entirely obliterate the
design, the interior will remain perfectly clean and dry, and therefore
its effectiveness will not be impaired.
[Illustration: FIG. 15.--EXTERNAL VIEW OF ELECTRIC RAILWAY MOTOR
MOUNTED UPON CAR-WHEEL AXLE.]
To enable the motor to give a strong torque and run at a slow speed
the number of poles in the field and armature is increased. The design
of Fig. 10 has two poles in the field and two in the armature, and
is what is known as the bipolar type. Machines having more than two
poles in each part are called multipolar machines. The number of poles
can be increased by pairs, but not by a single pole--that is, we can
have four, six, eight, or any other even number of poles, but not
five, seven, or any odd number. This is owing to the fact that there
must always be as many positive as negative poles, no more and no
less. Railway motors at the present time are made with four poles. The
external appearance can be understood from Fig. 15, while Fig. 16 and
Fig. 17 will serve to elucidate the internal construction. In Fig.
15 the motor casing is marked _M_, and, as will be seen, it forms a
complete shell. The motion of the armature shaft is transmitted to the
car-wheel axle _F_ through a pinion, which engages with a spur gear
secured to the latter. In Fig. 16 the pinion and gear are marked _N_
and _L_ respectively. As it is necessary that the armature shaft and
the axle be kept in perfect alignment, the motor casing _M_ is provided
with suitable bearings for both, those for the armature shaft being
marked _P P_ in Fig. 16, and one of those for the axle being marked _T_
in Fig. 15. It will be understood from the foregoing that the motor
is mounted so as to swing around the car-wheel axle as a center, but,
as it is not desirable to have all this dead weight resting upon the
wheels without any elasticity, the motor is carried by the crossbars
_B B_, Fig. 15, which rest upon springs _s s_ at each end. The beam
_A_ and a similar one at the farther end of the _B B_ bars extend out
to the sides of the car truck and are suitably secured to the latter.
The coils _w w_ are the ends of the field coils and the armature
connections, and to these the wires conveying the current from the
trolley are connected. The cover _C_ on top of the motor at one end
closes an opening through which access to the commutator brushes is
obtained. The armature is shown at _H_ in Fig. 16 and the commutator at
_K_ in the same figure. Directly under the armature may be seen one of
the field magnet coils, it being marked _R_.
[Illustration: FIG. 16.--RAILWAY MOTOR WITH CASING OPEN, SHOWING
ARMATURE IN LOWER HALF.]
As will be noticed in Fig. 16, the motor casing is made so as to open
along the central line, and the lower half is secured to the top by
means of hinges, _g g_, Fig. 15, and also by a number of bolts, which
are not so clearly shown. The gear wheels are also located within a
casing, which (Fig. 16) is made so as to be readily opened whenever
it becomes necessary. All the vital parts of the machine are entirely
covered, and are not easily injured by mud or water.
The construction of the armature and commutator is well illustrated in
Fig. 17, which shows this part of the machine by itself. The armature
is marked _A_, the shaft _B_, and the commutator _C_. In the diagrams,
Figs. 9, 10, 11, and 12, the wire coils are represented as wound upon
the surface of the armature core, but, from Fig. 17, it will be noticed
that they are located in grooves. A railway motor armature core, when
seen without the wire coils, looks very much like a wide-faced cog
wheel with extra long teeth, not very well shaped for gear teeth. In
Fig. 17 the ends of the teeth are marked _D_, and the grooves within
which the wire is wound are marked _E_. The coils are not wound so that
their sides are on diametrically opposite sides of the armature core,
but so that they may be one quarter of the circumference apart, and, as
will be noticed, the wires are arranged so as to fit neatly into each
other at the ends of the armature core. The bands marked _F F F F_ are
provided for the purpose of holding the wire coils within the grooves.
The flanges _H_ and _I_ are simply shields to prevent oil, grease, or
even water, if it should pass through the bearings, from being thrown
upon the commutator or armature. The pinion through which the armature
imparts motion to the car-wheel axle is not shown in Fig. 17, but it is
mounted upon the taper end of the shaft.
[Illustration: FIG. 17.--ARMATURE OF ELECTRIC RAILWAY MOTOR.]
An electric railway motor is a machine that is characterized by extreme
simplicity (there being only one moving part), compactness, and great
strength. In addition, as none of the working parts is exposed it
can not be injured, no matter how much mud may accumulate upon it.
One of the reasons why the electric railway motor has met with such
unparalleled success is that it is a machine that can withstand the
roughest kind of usage without being damaged thereby. Another reason
is that an electric motor can, if called upon, develop an amount
of power two or three times greater than its full-rated capacity
without injury, providing the strain is not maintained too long.
A steam engine or any other type of motor that has ever been used
for railway propulsion if loaded beyond its capacity will come to a
standstill--that is, it will be stalled--but an electric motor can not
be stalled with any strain that is likely to be placed upon it. If the
load is increased the motor will run slower and the current will become
greater, thus increasing the pull, but the armature will continue
to rotate until the current becomes so great as to burn out the
insulation. A railway motor calculated to work up to twenty-five-horse
power will have to develop on an average about six-or seven-horse
power, but if the car runs off the track on a steep grade, and has such
a heavy load that the motor is called upon to develop one-hundred-horse
power for a few seconds, the machine will be equal to the occasion.
This result a steam, gas, or any other type of engine can not
accomplish, and it is this fact as much as anything else that has given
the electric motor the control of the street-railway field.
[_To be continued_.]