ELECTRICITY IN LOCOMOTION
The electric tramcar has become such a necessary feature of our
everyday life that it is very difficult to realize how short a time
it has been with us. To most of us a horse-drawn tramcar looks like a
relic of prehistoric times, and yet it is not so many years since the
horse tram was in full possession of our streets. Strikes of tramway
employees are fortunately rare events, but a few have occurred during
the past two or three years in Leeds and in other towns, and they have
brought home to us our great dependence upon the electric tram. During
the Leeds strike the streets presented a most curious appearance, and
the city seemed to have made a jump backward to fifty years ago. Every
available article on wheels was pressed into service to bring business
men into the city from the outlying districts, and many worthy citizens
were seen trying to look dignified and unconcerned as they jogged
along in conveyances which might have come out of the Ark. On such an
occasion as this, if we imagine the electric light supply stopped also,
we can form some little idea of our indebtedness to those who have
harnessed electricity and made it the greatest power of the twentieth
century.
There are three distinct electric tramway systems; the trolley or
overhead system, the surface contact system, and the conduit system.
The trolley system has almost driven the other two from the field, and
it is used almost exclusively throughout Great Britain and Ireland.
On the Continent and in the United States the conduit system still
survives, but probably it will not be long before the trolley system is
universally employed.
The superiority of the trolley system lies in the fact that it is
cheaper to construct and to maintain than the other two, and also in
its much greater reliability under all working conditions. The overhead
wire is not one continuous cable, but is divided into sections of about
half a mile in length, each section being supplied with current from a
separate main. At each point where the current is fed to the trolley
wire a sort of metal box may be seen at the side of the street. These
boxes are called “feeder pillars,” and each contains a switch by means
of which the current can be cut off from that particular section, for
repairing or other purposes. Above the car is fixed an arm provided
with a trolley wheel which runs along the wire, and this wheel takes
the current from the wire. From the wheel the current passes down
the trolley arm to the controller, which is operated by the driver,
and from there to the motors beneath the car. Leaving the motors it
passes to the wheels and then to the rails, from which it is led off
at intervals by cables and so returned to the generating station. The
current carried by the rails is at a pressure of only a few volts, so
that there is not the slightest danger of shock from them. There are
generally two electric motors beneath the car, and the horse-power of
each varies from about fifteen to twenty-five.
The controller consists mainly of a number of graduated resistances. To
start the car the driver moves a handle forward notch by notch, thus
gradually cutting out the resistance, and so the motors receive more
and more current until they are running at full speed. The movement of
the controller handle also alters the connexion of the motors. When
the car is started the motors are connected in series, so that the full
current passes through each, while the pressure is divided between
them; but when the car is well on the move the controller connects the
motors in parallel, so that each receives the full pressure of the
current.
The conduit and surface contact systems are much the same as the
trolley system except in the method of supplying the current to the
cars. In the conduit system two conductors conveying the current are
placed in an underground channel or conduit of concrete strengthened
by iron yokes. The top of the conduit is almost closed in so as to
leave only a narrow slot, through which passes the current collector of
the car. This current collector, or “plough” as it is called, carries
two slippers which make contact with the conductors, and thus take
current from them. In this system the current returns along one of
the conductors, so that no current passes along the track rails. This
is the most expensive of the three systems, both in construction and
maintenance.
The surface contact or stud system is like the conduit system in having
conductors placed in a sort of underground trough, but in this case
contact with the conductors is made by means of metal studs fixed at
intervals in the middle of the track. The studs are really the tops of
underground boxes each containing a switch, which, when drawn up to a
certain position, connects the stud to the conductors. These switches
are arranged to be moved by magnets fixed beneath the car, and thus
when the car passes over a stud the magnets work the switch and connect
the stud to the conductors, so that the stud is then “alive.” The
current is taken from the studs by means of sliding brushes or skates
which are carried by the car. The studs are thus alive only when the
car is passing over them, and at all other times they are dead, and
not in any way dangerous.
The weight and speed of electric cars make it important to have a
thoroughly reliable system of brakes. First of all there are ordinary
mechanical brakes, which press against the wheels. Then there are
electro-magnetic slipper brakes which press on the rails instead of on
the wheels of the car. These brakes are operated by electro-magnets
of great power, the current necessary to excite the magnets being
taken from the motors. Finally there is a most interesting and
ingenious method of regenerative control. Before a car can be stopped
after it has attained considerable speed a certain amount of energy
has to be got rid of in some way. With the ordinary mechanical or
electro-magnetic brakes this energy is wasted, but in the regenerative
method it is turned into electric current, which is sent back into the
circuit. If an electric motor is supplied with mechanical power instead
of electric current it becomes a dynamo, and generates current instead
of using it. In the regenerative system, when a car is “coasting” down
a hill it drives the wheels, and the wheels drive the motors, so that
the latter become dynamos and generate current which is sent back to
the power station. In this way some of the abnormal amount of current
taken by a car in climbing a hill is returned when the car descends the
hill. The regenerative system limits the speed of the car, so that it
cannot possibly get beyond control.
[Illustration: PLATE VII.
_By permission of_ _Siemens Brothers Dynamo Works Ltd._
ELECTRIC COLLIERY RAILWAY.]
A large tramway system spreads outwards from the centre of a city to
the suburbs, and usually terminates at various points on the outskirts
of these suburbs. It often happens that there are villages lying some
distance beyond these terminal points, and it is very desirable that
there should be some means of transport between these villages and
the city. An extension of the existing tramway is not practicable
in many cases, because the traffic would not be sufficient to pay for
the heavy outlay, and also because the road may not be of sufficient
width to admit of cars running on a fixed track. The difficulty may
be overcome satisfactorily by the use of trackless trolley cars. With
these cars the costly business of laying a rail track is altogether
avoided, only a system of overhead wires being necessary. As there is
no rail to take the return current, a second overhead wire is required.
The car is fitted with two trolley arms, and the current is taken from
one wire by the first arm, sent through the controller and the motors,
and returned by the second arm to the other wire, and so back to the
generating station. The trolley poles are so arranged that they allow
the car to be steered round obstructions or slow traffic, and the car
wheels are usually fitted with solid rubber tyres. Trackless cars are
not capable of dealing with a large traffic, but they are specially
suitable where an infrequent service, say a half-hourly one, is enough
to meet requirements.
We come now to electric railways. These may be divided into two
classes, those with separate locomotives and those without. The
separate locomotive method is largely used for haulage purposes in
collieries and large works of various kinds. In Plate VII. is seen an
electric locomotive hauling a train of coal waggons in a colliery near
the Tyne, and it will be seen that the overhead system is used, the
trolley arm and wheel being replaced by sliding bows. In a colliery
railway it is generally impossible to select the most favourable track
from the railway constructor’s point of view, as the line must be
arranged to serve certain points. This often means taking the line
sometimes through low tunnels or bridges where the overhead wire must
be low, and sometimes over public roads where the wire must be high;
and the sliding bow is better able than the trolley arm and wheel to
adapt itself to these variations. In the colliery where this locomotive
is used the height of the overhead wire ranges from 10 feet 6 inches
through tunnels or bridges, to 21 feet where the public road is
crossed. The locomotive weighs 33½ tons, and has four electric motors
each developing 50 horse-power with the current employed. It will be
noticed that the locomotive has two sets of buffers. This is because
it has to deal with both main line waggons and the smaller colliery
waggons, the upper set of buffers being for the former, and the lower
and narrower set for the latter. Plate VIII. shows a 50-ton locomotive
on the British Columbia Electric Railway, and a powerful locomotive in
use in South America. In each case it will be seen that the trolley
wheel is used.
In this country electric railways for passenger traffic are mostly
worked on what is known as the multiple-unit system, in which no
separate locomotives are used, the motors and driving mechanism being
placed on the cars themselves. There are also other cars without this
equipment, so that a train consists of a single motor-car with or
without trailer, or of two motor-cars with trailer between, or in fact
of any other combination. When a train contains two or more motor-cars
all the controllers, which are very similar to those on electric
tramcars, are electrically connected so as to be worked together from
one master controller. This system allows the length of the train to
be adjusted to the number of passengers, so that no power is wasted
in running empty cars during periods of small traffic. In suburban
railways, where the stopping-places are many and close together, the
efficiency of the service depends to a large extent upon the time
occupied in bringing the trains from rest to full speed. In this
respect the electric train has a great advantage over the ordinary
train hauled by a steam locomotive, for it can pick up speed at three
or more times the rate of the latter, thus enabling greater average
speeds and a more frequent service to be maintained.
Electric trains are supplied with current from a central generating
station, just as in the case of electric tramcars, but on passenger
lines the overhead wire is in most cases replaced by a third rail. This
live rail is placed upon insulators just outside the track rail, and
the current is collected from it by sliding metal slippers which are
carried by the cars. The return current may pass along the track rails
as in the case of trolley tramcars, or be conveyed by another insulated
conducting rail running along the middle of the track.
The electric railways already described are run on continuous current,
but there are also railways run on alternating current. A section of
the London, Brighton, and South Coast Railway is electrically operated
by alternating current, the kind of current used being that known as
single-phase. The overhead system is used, and the current is led to
the wire at a pressure of about 6000 volts. This current is collected
by sliding bows and conveyed to transformers carried on the trains,
from which it emerges at a pressure of about 300 volts, and is then
sent through the motors. The overhead wires are not fixed directly to
the supports as in the case of overhead tramway wires, but instead two
steel cables are carried by the supports, and the live wires are hung
from these. The effect of this arrangement is to make the sliding bows
run steadily and evenly along the wires without jumping or jolting. If
ever electricity takes the place of steam for long distance railway
traffic, this system, or some modification of it, probably will be
employed.
Mention must be made also of the Kearney high speed electric
mono-railway. In this system the cars, which are electrically driven,
are fitted above and below with grooved wheels. The lower wheels run on
a single central rail fixed to sleepers resting on the ground, and the
upper wheels run on an overhead guide rail. It is claimed that speeds
of 150 miles an hour are attainable with safety and economy in working.
This system is yet only just out of the experimental stage, but its
working appears to be exceedingly satisfactory.
A self-contained electric locomotive has been constructed by the North
British Locomotive Company. It is fitted with a steam turbine which
drives a dynamo generating continuous current, and the current is used
to drive four electric motors. This locomotive has undergone extensive
trials, but its practical value as compared with the ordinary type of
electric locomotive supplied with current from an outside source is not
yet definitely established.
At first sight it appears as though the electric storage cell or
accumulator ought to provide an almost perfect means of supplying
power for self-propelled electric vehicles of all kinds. In practice,
however, it has been found that against the advantages of the
accumulator there are to be set certain great drawbacks, which have not
yet been overcome. Many attempts have been made to apply accumulator
traction to electric tramway systems, but they have all failed, and
the idea has been abandoned. There are many reasons for the failure
of these attempts. The weight of a battery of accumulators large
enough to run a car with a load of passengers is tremendous, and this
is of course so much dead weight to be hauled along, and it becomes
a very serious matter when steep hills have to be negotiated. When a
car is started on a steep up-gradient a sudden and heavy demand for
current is made, and this puts upon the accumulators a strain which
they are not able to bear without injury. Another great drawback is
the comparatively short time for which accumulators can give a heavy
current, for this necessitates the frequent return of the cars to the
central station in order to have the batteries re-charged. Finally,
accumulators are sensitive things, and the continuous heavy vibration
of a tramcar is ruinous to them.
The application of accumulators to automobiles is much more feasible,
and within certain limits the electric motor-car may be considered
a practical success. The electric automobile is superior to the
petrol-driven car in its delightfully easy and silent running, and its
freedom from all objectionable smells. On the other hand high speeds
cannot be attained, and there is the trouble of having the accumulators
re-charged, but for city work this is not a serious matter. Two sets
of accumulators are used, so that one can be left at the garage to be
charged while the other is in use, the replacing of the exhausted set
by the freshly charged one being a matter of only a few minutes. The
petrol-driven car is undoubtedly superior in every way for touring
purposes. Petrol can now be obtained practically anywhere, whereas
accumulator charging stations are comparatively few and far between,
especially in country districts; and there is no comparison as regards
convenience between the filling of a petrol tank and the charging of a
set of accumulators, for one process takes a few minutes and the other
a few hours.
Accumulator-driven locomotives are not in general use, but for certain
special purposes they have proved very satisfactory. A large locomotive
of this kind was used for removing excavated material and for taking
in the iron segments, sleepers, rails, and other materials in the
construction of the Great Northern, Piccadilly, and Brompton Tube
Railway. This locomotive is 50 feet 6 inches long, and it carries
a battery of eighty large “chloride” cells, the total weight of
locomotive and battery being about 64 tons. It is capable of hauling a
load of 60 tons at a rate of from 7 to 9 miles an hour on the level.
Amongst the latest developments of accumulator traction is a complete
train to take the place of a steam locomotive hauling a single
coach on the United Railways of Cuba. According to the _Scientific
American_ the train consists of three cars, each having a battery of
216 cells, supplying current at 200 volts to the motors. Each car has
accommodation for forty-two passengers, and the three are arranged
to work on the multiple-unit system from one master controller. The
batteries will run from 60 to 100 miles for each charging of seven
hours.