If the successful operation of a street-railway car by mechanical power
depended wholly upon the ability to produce a motor of sufficient
capacity to do the work, the problem would be an easy one to solve,
and would have been solved long before the advent of the electric
motor. Mere ability to furnish the necessary power, however, is not
enough to meet the requirements. As already shown, the mechanism
must be light, strong, compact, simple, and so well protected that
it can not be injured except under abnormal conditions. In addition,
speed-controlling devices must be provided whereby the velocity may
be changed at will and in the shortest possible time, and with as
nearly absolute precision as possible. This controlling mechanism must
also be so arranged that the direction of motion may be varied with
the greatest certainty and as rapidly as may at any time be required.
The way in which these results are accomplished in an electrically
operated car can be understood from Figs. 18 and 19, which are line
drawings, in a simplified form, of an ordinary trolley car. Fig. 18
is an elevation showing the outline of the car body and the wheels in
broken lines, while the motors and the wires through which the current
is conveyed thereto are drawn in solid lines. Fig. 19 is a plan in
which the outline of the car floor and the platforms is represented in
broken lines, the solid lines being the motors and connecting wires.
In almost every instance railway cars are provided with two motors, as
shown at _M M_ in these two figures. This arrangement is adopted not
because one motor can not furnish all the power required, but simply
for the purpose of making the equipment more reliable. Everything of
human make is liable to fail; hence if only one motor were used there
would be more or less liability of its giving out at a critical moment,
and then the car would be helpless. If two motors are provided, should
one give out the car would not be disabled, for the remaining machine
would be able to run it to its destination. In order that this result
may be successfully accomplished, each motor is made of sufficient
capacity to run the car without being overtaxed, unless the load is
abnormally large; but even under the latter conditions the machine will
in ninety-nine cases out of a hundred withstand the strain. Some roads,
in small towns, where the traffic is light and the expense must be
kept down to the lowest point, use single-motor cars, so as to effect
a saving in first cost. This course, however, is very seldom followed,
except in places where there are no heavy grades or where there is
very little probability of the loads becoming excessive, except at
rare intervals. If the cars are provided with a single motor, when one
becomes disabled from any cause it has to wait until overtaken by the
car behind it, so that it may be pushed by the latter to the end of the
road.
The electric current for operating the motors is generated in a power
house that is located at some convenient point along the route. The
current is conveyed to the moving cars by means of a trolley wire,
which is marked _T_ in the drawings. Unless the road is very small and
operates but a few cars, this wire will not be sufficient to carry all
the current, hence in most cases there are a number of supplementary
wires, which are called feeders. These wires are carried along on
poles, and at proper intervals are connected with the trolley wire _T_.
The electric current passes from the trolley wire through the motors
on the car, and thence to the rails _R_, and through these, and also
through the ground, back to the power house. The exact path of the
current is as follows:
[Illustration: FIGS. 18, 19.--OUTLINE ELEVATION AND PLAN OF ELECTRIC
RAILWAY CAR, SHOWING LOCATION OF MOTORS, CONTROLLING SWITCHES, AND
CONNECTING WIRES.]
From the trolley wire, through the trolley pole _t_, to the fixture
on top of the car which holds the latter. From this fixture, as shown
by the heavy full line, the current passes to _a_, which is a switch
located under the car hood overhanging the platform. From this switch
the current passes to a similar one, marked _b_, located in a like
position at the other end of the car. These two switches are called
emergency switches; they are provided simply as a safety device,
and are used only when the main switches get out of order and the
motorman can not turn the current off in the regular way. From the
last hood switch _b_ the current passes to the bottom of the car,
where it enters the wire _d d_ at the point _c_. This wire _d d_, as
will be seen, runs in both directions, and ends in the stands _C C_.
These latter are the controlling switches, and are provided with a
handle _h_, by means of which the current is turned on or off from
the motors, and is directed through them in such a way as to make the
car run in whichever direction may be desired. From the controllers
_C C_ several wires are run under the car, as shown at _e e e_. These
wires are generally bunched into one or two cables, but they are kept
separate from each other by means of strong insulating coverings. Four
wires lead into each motor, and three or four into each of the boxes
marked _G G_. If the motors were required to run in one direction
only, then two wires would be sufficient to convey the current to
them; but as they have to run in either direction, at least three
wires are necessary, but in almost every case four are used, as the
results obtained thereby are more satisfactory. The boxes _G G_ are
called rheostats, and are simply devices through which the current is
run so as to reduce the speed of the car, and also for the purpose of
graduating the strength of the current that passes through the motors
in the act of starting. These rheostats are very seldom in use when the
car is in motion, because it is a waste of power to pass the current
through them. After the current has passed through the motors it enters
the ironwork, and thus gets into the car wheels and finally to the
track.
The lines drawn in Fig. 18 to indicate the position of the wires in the
car do not show their actual position, but only the general direction
they follow. From the trolley base to the first hood switch the wire,
as a rule, is run along the car roof on one side of the ventilator,
and the wire leading from the first to the second hood switch occupies
a corresponding position on the opposite side of the roof. From the
last hood switch, _b_, the wire is run down one corner of the car body,
being either within the car body, or, if not, so covered by moldings as
to not be reached by the hands of passengers. The wires _d_ and _e_ are
generally run under the car, and are firmly secured to it by means of
suitable fastenings.
The controlling switches _C C_ are provided with one and sometimes two
handles, one of which is used to regulate the speed of the car and
stop and start it, while the other is for the purpose of reversing
the direction in which it runs. The handle _h_ is for the purpose of
regulating the speed, and by means of _k_ the direction of motion
is changed. Before _h_ is moved from the inactive position _k_ is
turned so that the car may run either forward or backward, as may be
desired; then, when _h_ is moved, the car will start, and by varying
the position of _h_ the speed can be changed. If it is desired to
reverse the car, _h_ is brought back to the stop position, _k_ is
shifted to the reverse motion, and then _h_ is again turned to the
running position. When the controlling switch is provided with only one
handle this is turned in one direction to run the car ahead, and in
the opposite direction to run it backward, the graduations in velocity
being obtained by placing the handle in positions intermediate between
the stop position and the highest speed position.
As will be noticed, the wire _d d_ branches at _c_ and runs in both
directions. Now, when the controller handles are both turned to the
stop position the current from the trolley can get no farther than
the ends of _d_ in either switch, but if one of them is turned to
the running position, the current at once passes to the wires in the
cable _e e e_, and thus to the two motors. If the switches _C C_ are
in proper working order and there is no disarrangement of the wires
leading to the motors or those within the latter, the current will obey
the movements of the handle _h_, but under other conditions it may not.
If such an emergency arises, the motorman reaches up to the hood and
turns the safety switch _a_ or _b_, and thus cuts the current off.
The force with which the motors turn the car wheels around depends
upon the strength of the current; this is owing to the fact that the
magnetic force is increased or decreased by variations in the current
strength. If the current is doubled the magnetic force of the armature
is nearly doubled, and so is that of the field magnet, therefore the
pull between the poles is nearly four times as great. From this it
will be seen that the force with which the car is pushed ahead can be
increased enormously by a comparatively small increase in the strength
of the current. If the current strength is doubled, the propelling
force is practically quadrupled; and if the current is increased four
times, the propelling force is made nearly sixteen times as great.
The speed at which the car runs depends upon the force that impels the
current through the wire, and which is called electro-motive force.
The greater the electro-motive force, the higher the velocity. If the
current passes from the wires in the cable _e e e_ through each motor
separately, and thence to the rails _R_, each machine will receive
the effect of the whole electro-motive force of the current; but if
after the current has passed through one motor it is directed through
the other, then each machine will be acted upon by only one half the
electro-motive force, and, as can be seen at once, the velocity in the
first instance will be twice as great as in the last. This fact is
taken advantage of in regulating the speed of the car, and controlling
switches arranged so as to direct the current through the motors in
this way are designated as belonging to the series parallel type,
the name being given from the fact that when the car is running slow,
the current passes through the two motors in series--that is, through
one after the other; but when the motors are running fast, a separate
current passes through each machine.
[Illustration: FIG. 20.--VIEW OF ELECTRIC RAILWAY TRUCK WITH ONE MOTOR
ON EACH AXLE AND PROVIDED WITH MAGNETIC BRAKES.]
If, when a car is running, the controlling switch is turned to cut the
current off, the effect will be that the speed will gradually reduce;
but if it is desired to effect a sudden stop, it becomes necessary to
check the headway by means of a brake. For this purpose the hand brake
ordinarily used on all types of cars is employed, but magnetic brakes
are also used in some cases. Fig. 20 shows a car truck equipped with
two motors and magnetic brakes, one on each axle. Looking at the front
end of the truck, the brake is seen on the left side of the axle,
between the motor bearing and the car wheel. The larger drum, on the
right side, is the casing within which the gear wheel and pinion are
inclosed. These magnetic brakes are operated by a current generated
by the motors, and not by that of the main line. As was explained in
the first article, an electric motor can be made to act as a generator
of electric current by simply reversing the direction in which the
armature revolves. If we do not desire to reverse the direction of
rotation, the result can still be attained by reversing the direction
in which the current passes through the armature coils. It is evident
that the direction of a car motor can not be reversed at the instant
that it is desired to have it act as a generator--that is, when it
is desired to put the brakes on; hence the direction of the current
through the armature is reversed.
When a car is provided with magnetic brakes, the controlling switches
are so made that when the handle _h_ is moved back to the stop position
it disconnects the motors from the trolley wire and at the same time
connects them with the magnetic brakes in such a manner that they
will act as generators and thus send current through the coils of the
latter. In order that the force with which the brakes are applied may
be graduated, the controlling switches are arranged so as to be moved
several steps back of the point which in the ordinary type of switch
would be the final stop position. When the handle _h_ is placed on
the first brake position the current generated by the motors is not
very strong, and as a consequence the force of the brake is light, but
sufficient to bring the car to a stop in a reasonable distance. If a
quicker stop is desired the handle is moved to the second, third, or
fourth brake position, thus increasing the retarding force as much as
may be desired. Magnetic brakes are very desirable, as they save the
car wheels, and furthermore afford an additional safety in cases where
it is necessary to arrest the speed instantly.
The position of the motors with reference to the truck and car wheels
is very well shown in Fig. 20, and also the manner in which they are
held in place. The covers of the openings through which access to the
commutator brushes is obtained are removed from both motors, and in
the forward one the top of the commutator and one of the brushes can
be readily seen. The manner in which the motors are suspended from the
truck is not the same in this figure as in those previously shown, but
this is simply because the machines are not made by the same concern,
and each manufacturer has his own design.
Fig. 21 shows the appearance of the interior of the controlling
switches _C C_, Figs. 18 and 19. It will be noticed that there are two
upright shafts, the ends of which project above the top of the box. The
handle _h_ is placed upon the shaft to the left, and _k_ on that to the
right. The first is the main controller, and the other is the reversing
switch. It will be noticed that the main controller shaft carries a
number of circular segments of different lengths; these are so disposed
that they come in contact with suitable stationary pieces as the handle
_h_ is turned around, and thus vary the path of the current through the
motors and the rheostats in the manner required to effect the desired
changes in the velocity of the car. The reversing shaft is also
provided with a number of segments, but these are not so easily seen,
although they can be discerned on close examination. The wires from the
cable _e e e_ and also wire _d d_ are attached to the stationary pieces
with which the segments carried by the two shafts make contact when
the latter are moved around by the motorman. These wires can be seen
back of the main switch shaft, and also above the board located at the
lower left-hand corner. All these wires enter the controller through an
opening in the bottom.
[Illustration: FIG. 21.--VIEW OF INTERIOR OF CAR CONTROLLER.]
In addition to the apparatus shown in Figs. 18 to 21, electric cars
are provided with a safety fuse and a lightning arrester, the object
of the latter being to protect the motors from the destructive effects
of lightning strokes. The object of the safety fuse is to protect the
motor from injury when the current becomes too strong. An electric
current in passing through a wire generates heat, and the stronger the
current the greater the heat. If the wire is large and the current
weak, the heat developed may be insufficient to raise the temperature
to a noticeable degree; but, on the other hand, if the wire is small
or the current very strong, the heat generated may be capable of
raising the temperature of the metal to the fusing point. In fact, the
incandescent lamp operates upon this principle; the carbon filament
is traversed by a current of a strength sufficient to heat it to a
point where it becomes intensely luminous, and sometimes, through
accident or otherwise, the current becomes strong enough to melt the
filament, and then the light goes out. In an electric motor it is not
necessary to raise the temperature of the wire to the melting point to
do serious injury; in fact, if the heat is sufficient to char paper or
cloth, the machine will be rendered useless until suitable repairs are
made. The insulation of the wire coils is made principally of cotton,
which is a very good electrical insulator in its natural state, but
when carbonized by excessive heat it becomes a conductor. As soon as
it becomes a conductor the current is no longer confined to the proper
channel, but cuts through the insulation to find the shortest path
through the machine. If safety fuses were not provided the danger of
destroying the insulation of the motors and thus disabling the car
would be decidedly great, for, as already said, the motors can not be
stalled with an overload, the only effect produced being a reduction in
the speed and an increase in current strength. Now, if there were no
way of limiting the increase in current strength the motors, if greatly
overloaded, would continue to operate until the insulation gave out.
The safety fuse is simply a piece of wire of such size that it will be
melted by a current that the motors can carry without being injured;
hence when the current strength reaches a point where the safety of the
apparatus is endangered the fuse melts and thus breaks the circuit and
stops the further flow of current. Fuses are generally made of an alloy
that melts at a low temperature, so that the molten metal may not set
fire to anything upon which it may fall. These easily fused alloys are
inferior to copper as electrical conductors, and on this account the
fuse wire is as a rule much larger than that wound upon the motors,
which fact makes its action somewhat mysterious to the uninitiated;
but whatever its size may be, it is so proportioned that it will melt
before the current rises to a strength that would injure the motor
coils.
The manner in which the electric current generated in the power house
reaches the motors is illustrated in Fig. 22. In this figure four
tracks are shown, which may be taken to represent roads running in
as many different directions. The three squares at the left side
represent generators located in the power house. The circles _a a a_
represent switches, by means of which the generators are connected
or disconnected from the trolley lines. _A_ and _B_ represent heavy
metallic rods, generally made of copper, with which the generators are
connected by means of the switches _a a a_. These rods are called bus
bars. The circles _b b b b_ represent switches by means of which the
current is turned on or off from the several tracks.
[Illustration: FIG. 22.--DIAGRAM ILLUSTRATING THE MANNER IN WHICH THE
ELECTRIC CURRENT FLOWS FROM THE GENERATORS TO THE CARS UPON THE TRACKS.]
Electric currents must always circulate in closed paths--that is, the
current that starts out from a generator must return to it, and the
amount coming back is the same as that which leaves. The action of
an electric generator can be understood by comparing it with that of
a water pump pumping into a pipe which runs around from the delivery
end to the suction. With such an arrangement it can be seen that the
action of the pump would be to keep the water in circulation, but
the same water would be pumped through the pump and the pipe all the
time. With an electric generator the action is the same, and in Fig.
22 the current flowing along any one of the tracks follows the course
indicated by the arrows. The currents pass out to the several tracks
through the trolley wires _T T T T_, and return through the tracks
_R R R R_. The bus bar _A_ is connected with a plate _D_, which is
imbedded in the ground, and is also connected with the ends of the
rails _R R R R_. Suppose for a moment that the two lower generators are
out of service, their switches _a a_ being turned so as to disconnect
them from the bus _A_, and, further, suppose that the three lower _b_
switches are open, so that the current can only pass to the upper
track; then the top generator will feed into the top road only. Tracing
the path of the current under these conditions, we find that it will
start from the upper side of the generator through the _a_ switch to
the _B_ bus, and thence to the trolley wire at the top of the figure.
On reaching the first car a portion of the current passes to the track
_R_, the amount being dependent upon the speed of the car and the load.
Why the whole current does not follow this path generally puzzles the
layman, but the explanation is that the motors hold the current back,
and only allow as much to pass through them as is necessary to perform
the required work--that is to say, the current flowing through each
car is not controlled by the generator or by the force of the current,
but by the requirements of the motors. The amount of current delivered
by the generator is governed by the demands of the motors. The current
that does not pass through the first car goes on to the second one,
and if there were more cars there would be current left in the trolley
wire to supply them. After passing through the motors of the two cars
the current returns through the rails _R_ to the plate _D_, and thus
to bus _A_, from which it enters the lower side of the top generator.
It will from this explanation be seen that the action of the generator
is simply to keep the current circulating. If two of the generators
are connected with the bus bars _A_ and _B_, the current required by
the motors will be delivered by the two machines, and if the three
generators are placed in service the current will be divided among them.
When two or more generators are used, it is necessary to provide
means to prevent the current from dividing unequally between them;
if this were not done, one machine might do nearly all the work,
while the other one would be practically idle. The means employed to
accomplish the result is simply an additional bus bar, which is called
an equalizing bus. We will not undertake to explain the principle upon
which this arrangement acts; it is sufficient to say that by such means
the work can be distributed in amounts directly proportional to the
capacity of the generators, so that if one machine is very much larger
than the others it will take a portion of the load corresponding to
its size. In order that these results may be attained it is necessary
to properly adjust the several generators, and as no machine can be
made to work with the accuracy of perfection, the work will not be
distributed in true proportion for all conditions of load; thus if the
generators are adjusted so as to each take its proper share when all
the cars are in operation, one machine may do too much or not enough
when only one half the number are running, but the excess or deficiency
will not be more than a few per cent unless the adjustment is very
defective.
Electric generators for railway work are made in all sizes, from those
only large enough to operate four or five cars to others capable of
furnishing sufficient current for thirty or forty or even more. Small
generators are made so as to be driven by a belt running over a pulley
mounted on the end of the armature shaft, or they may be arranged
to be connected to the end of a steam-engine shaft, and thus become
what is called direct connected machines. Large generators are almost
invariably of the latter type. A machine of this class is illustrated
in Fig. 23. The driving engine is shown at _E_, the cylinder being
in the background and the crank toward the front, the shaft being
clearly seen at _S_, while _F_ is the fly wheel. The generator is
mounted directly upon the engine shaft, between the bearing at the
crank end and the fly wheel. The large ring marked _G_ is the field
magnet ring, and at _D D D_ the field coils are shown. These coils
are equally spaced all the way around the circle. The commutator is
marked _C_, and the commutator brushes are located at _B B_. The
armature can not be seen very well, as it is covered by the brush
holders and their supporting frames, but it is located within the
ring _G_ in the position designated by _A_. This machine is one of
a number used to operate the roads of Troy, N. Y., and is of about
one-thousand-horsepower capacity, which is enough to furnish all the
current required to run sixty or seventy cars.
[Illustration: FIG. 23.--LARGE-SIZE DIRECT CONNECTED ELECTRIC RAILWAY
GENERATOR.]
The switches _a a_ and _b b_, shown in Fig. 22, and the bus bars _A
B_, are mounted upon a large panel, made of marble or slate, called a
switch board. These switches are sufficient for the purpose of turning
the current on or off any track or for connecting and disconnecting
the generators, but for the successful operation of the plant it is
necessary to have other devices by means of which the strength of
the current may be ascertained, and also the electro-motive force.
It is necessary to provide each generator with means for varying the
electro-motive force of the current it generates, otherwise the load
could not be properly equalized between the several machines. All these
different devices are located upon the switch board, so as to have
them in an accessible position. A railway switch board, arranged for
four generators and a large number of distributing circuits, is shown
in Fig. 24. The four generator switches are shown at _a a a a_, and
the circles marked _R_, directly under them, are the devices by means
of which the electro-motive force of the current is regulated. These
devices are called field regulators, from the fact that their office
is to regulate the strength of the field magnets of the generators,
making them stronger to increase the electro-motive force and weaker
to reduce it. The part seen upon the front of the switch board is not
the regulator proper, but only the handle and the contact points over
which this swings. The instruments marked _A A A A_ are for measuring
the strength of the current of each individual generator, and are
called ammeters. The instruments marked _V V V V_ are for the purpose
of indicating the electro-motive force of the currents of the several
generators, and are called voltmeters. _Ag_ is an ammeter used to
measure the strength of the total current, and _Vg_ is a voltmeter that
indicates the electro-motive force of the current passing out to the
cars on the various lines. The ammeter _Ag_ is not an actual necessity,
for the strength of the total current can be ascertained by adding the
readings of the four instruments connected with the generators, but it
is a convenience, as it saves the trouble of performing the addition.
The voltmeter _Vg_, however, can not be regarded in this light; in
fact, its presence is decidedly serviceable, for it indicates the
average electro-motive force of all the generators; therefore if any
one of the instruments _V V V V_ is higher or lower it shows at once
that the generator to which it is attached is out of adjustment and not
doing its proper share of the work. The switches _b b b_, by means of
which the current is turned on to the several external circuits, are
shown at the extreme end of the switch board.
The instrument marked _W_, located between the _a_ switches, is
called a wattmeter, and its office is to indicate the amount of power
furnished by the generators. This instrument is not always used, as
it is a convenience but not a necessity. It can be seen at once that
whether it is used or not, the amount of power required to operate
the roads will be the same, but it is thought by most railroad
managers that it is desirable, for then the relation between the coal
consumed and the power developed can be seen; and if the showing is
not as good as it should be, the engineer can remind the firemen that
they are not exercising as much care in feeding the boilers as they
should. Considered in this light, the watt-meter acts as a check to
wastefulness on the part of the employees.
[Illustration: FIG. 24.--ELECTRIC RAILWAY SWITCHBOARD.]
The instruments marked _C C C C_ serve the same purpose in connection
with the generators as the safety fuses do with respect to the car
motors; they are electro-magnetic devices used to open the generator
circuits whenever the current reaches a strength that is sufficient
to injure the machine. These devices are called circuit breakers.
As will be noticed, there are four located directly above the four
_a_ switches, and, at the farther end of the board, a large number
located directly above the _b_ switches. The latter act to open the
individual circuits when the currents flowing in them become too
strong, and the former are controlled entirely by the current of the
generator circuits. A circuit breaker is more reliable than a safety
fuse, because it acts quicker. With the fuse the current must act for
some time before it can melt the metal, as a sufficient amount of heat
can not be generated instantly. With the circuit breaker, however,
the action is instantaneous, for as soon as the current reaches the
predetermined strength the magnetism of the operative parts of the
device becomes sufficiently strong to cause it to act. A circuit
breaker is simply a switch that is arranged to be opened automatically
by the action of a magnet, instead of by the hand of the operator. The
switch part of the apparatus is held in place by a catch that is set
much after the fashion of the catch in a mouse trap--that is, so that
the least pressure will disengage it. A strong spring acts to throw
the switch open, and as soon as the catch is tripped by the actuating
magnet the force of the spring comes into action and the circuit is
opened.
The circuit breaker is a very valuable apparatus, for it frequently
happens that, through delays of one kind or another, a large number of
cars concentrate at one point on the road, and, as all the motormen
are anxious to make headway, they all start up at once at the first
opportunity. If there were no circuit breakers at the power house the
result would be that some of the generators would be greatly overloaded
and perhaps disabled; but, owing to the presence of the circuit
breakers, the actual result is that the circuit is broken, and then
the motormen have to wait until the current is turned on again. If too
many of them try to start their cars at the second trial the current
will again stop. After two or three ineffectual efforts have been made
to start all the cars together the motormen will conclude to go easy,
and set a few in motion at a time. In this way the cars will become
more evenly distributed along the line, and the demand for current at
the point of blockade will reduce to the normal amount, or nearly so,
and the running of the cars will continue without further interruption,
for the current drawn by the motors having been reduced to the average
amount, the circuit breaker will cease to act.
The bus bars and all the connections between them and the generators
and external circuits, as well as with all the instruments, are located
behind the switch board. All these connections are so secured that
they can not come in contact with each other except where contacts
are required; care is also taken to prevent any connection being made
with the iron framing that supports the marble slabs. The front of a
switch board is generally very attractive, the surface being of highly
polished marble, while all the switches and instruments are finely
finished and, as a rule, of decidedly ornamental design.
The switch board might be looked upon as the fountain head from which
the entire operation of an electric railway system is controlled. By
the movement of one set of switches upon it the generators are thrown
in or out of service, and by the movement of another set of switches
the several branches of the road are rendered active or inactive.