THE ESSENTIAL ADVANTAGES OF ELECTRIC TRACTION ON TRAMWAYS
A railway journal once committed itself to the statement that horse
traction was superior to electric traction on roads because the horse
possessed the 'vital principle' of energy in its constitution.
It is distinctly curious to find an authority on locomotion describing
the essential drawback of horse traction as its distinguishing
advantage. The 'vital principle,' unfortunately, needs food and rest
to maintain it not only during working hours but during the hours of
inactivity as well. In actual practice four horses out of every five in
a tramway stud are in the stables while the fifth is at work. Moreover,
the same stud has to be kept up, at a practically uniform cost, whether
the daily traffic be light or heavy. Thirdly, the 'vital principle' has
only a limited number of years during which--apart from sickness and
disease--it is effective for traction purposes.
[Illustration: Fig. 2. A typical electric tramway on the overhead
system.--The trolley standard carries the wires for supplying current
to the cars on both the up and down tracks. The driver has his left
hand on the controller handle and his right hand on the brake handle.
(Photograph reproduced by courtesy of Dick, Kerr and Company,
Limited.)]
Another disadvantage is that the pull which a horse can actually
exercise on a car is strictly limited and is only a small fraction of
the total power represented by the fodder which the horse consumes.
The strain upon a horse in starting a car or omnibus is so great that
a 'lover of animals' used to supply London omnibuses with appeals to
passengers not to stop the omnibus more often than was necessary,
especially on an incline. This was a recognition of the fact that the
horse cannot cope easily with the heavy strain at starting, and that he
requires assistance on heavy gradients.
It was not surprising, therefore, that on horse tramway systems
the speed was low, the cars of limited capacity, and the fares
comparatively high. The shortness of the journey which a tramway horse
was able to cover without fatigue also tended to limit the length of
routes.
On all these points electric traction was soon found to be distinctly
superior to horse traction. It was more economical in power; it was
able to maintain higher speeds with larger and more commodious cars;
and there was no narrow limit to the length of routes or the gradients
which could be surmounted. Consequently electric traction offered the
public an improved service at lower fares.
The whole of the power-producing plant for a typical electric
tramway system is concentrated at a generating station placed (if
possible) near the centre of the system. From this station runs a
network of electric mains to feed the lines with current at convenient
points. This concentration is a benefit on several grounds. A large
generating equipment is cheaper in first cost than a multitude of small
power-producing plants, and it is much more economical in operation.
If every car had its own power equipment, that equipment would need to
be powerful enough to haul itself and the loaded car up the steepest
gradient on the route. That is to say, the sum of the car capacities
would be equal to the sum of the maximum demands. But when the power
is obtained from a single stationary source we do away with the dead
weight of the power equipment on the car, and secure the very vital
advantage that the capacity of the stationary source need not be so
great as the sum of the maximum demands. In actual working it never
happens that all the cars are full of passengers and ascending the
steepest gradients simultaneously. While some are running up-hill,
others are going down-hill; while some are full, others are half full
or almost empty. The result is that the total demand for power at any
time is always very much less than the total of the maximum demands
made by each car; and the capacity of the generating station need be
sufficient to cope only with the smaller amount.
This advantage reduces the expenditure necessary upon boilers,
engines, and dynamos at the tramway generating station. And it is
enhanced by two valuable capabilities of the electric motor. The first
is its power of taking a heavy overload for a limited period without
injury. There is no difficulty about making an electric motor, whose
normal capacity is 20 horse power, give 40 horse power momentarily, 30
horse power for several minutes, and 25 horse power during the best
part of an hour. Applied to tramway work, this advantage means that the
rated capacity of the motor equipment of a car may be less than what
is required to haul a loaded car at an adequate speed up the steepest
gradient on the system. Such maximum demands, which only occur at
intervals with each car, can be met by the readiness of the electric
motor for overwork. The motors may therefore be reduced in size, saving
money in first cost and in the current consumed.
The second valuable peculiarity of the electric motor is that it gives
its 'maximum torque' at starting. That is to say, it exercises the
highest propulsive effort at the precise moment when it is required.
When horses are employed, they have to endure an abnormal strain in
overcoming the inertia of a stationary vehicle; everyone must have
noticed how horses have to struggle to start a car which they can keep
going at an easy trot once it has got up speed. The electric motor--to
use an apparent paradox--gives this abnormal pull as part of its normal
action. As the inertia of the car is gradually overcome, the speed of
rotation of the motor increases and its torque decreases, automatically
and precisely in accordance with the demands of the case.
The starting torque of a motor is such an emphatic phenomenon that
the driver of an electric car may, if he is careless and switches the
current on too suddenly, jerk any standing passenger off his feet, even
though the total weight of the car may be ten tons or more. Properly
employed, however, the electric motor gives an even and _rapid_
acceleration.
This is a far more important point in tramway economics than it
appears to be at first sight. The superiority of the electric tramway
over the horse tramway depends less upon higher speed than upon the
fact that less time is wasted in stopping to pick up and set down
passengers. Time is the vital element in all transport, and it is
especially vital in connection with tramways, which have to stop
with great frequency. If the time which elapses between putting on
the brakes at each stop and getting up to full speed again can be
materially shortened, then the average speed of the tramway journey can
be materially raised. It is easy, by means of powerful brakes, to bring
a car to rest quickly; the electric motor enables speed to be regained
quickly. In this way a high average speed may be maintained in spite of
numerous stops; and, with larger cars, the electric tramway is able to
handle a larger volume of traffic in a shorter space of time than the
horse tramway.
The time lost in stopping is of so much consequence that, when electric
tramways were introduced, the old custom of stopping the cars at
any desired point was abandoned. Stopping places were arranged at
convenient points along the route, some of them being regular stops
and others optional at a signal from passengers desiring to alight or
to board the car. The public soon got used to walking a short distance
to a stopping place, although they did not, perhaps, appreciate how
much the collection of traffic at a reduced number of points tended to
improve the general tramway service.
A high average speed with numerous stops was, however, only one of
the improvements which the public derived from electric traction.
Tramway passengers expect to find a car not only at a convenient point
but within a convenient period of waiting. With electric traction
the service became much more frequent than with horse traction. It
is quite possible to run a horse tramway service profitably with
cars at intervals of fifteen to thirty minutes, if the passengers
are patient enough to wait and fill each vehicle. But with electric
traction the main item is the cost of the standing equipment--the
power house, mains, and overhead lines--and unless that equipment is
adequately utilised the revenue will not cover the standing charges.
A fifteen-minute service is, generally speaking, the lowest economic
limit on an electric tramway. Every tramway manager tries to attract
sufficient passengers for a more frequent service; and, as a matter
of fact, it was found that where there was sufficient population the
provision of a frequent and rapid service encouraged tramway travelling
so much that cars had to be run at far shorter intervals than had been
customary on horse tramways.
The increase of traffic brought with it the demand for larger as well
as speedier cars with a shorter 'headway' or interval between one car
and another. The capacity of a horse car is limited by the fact that it
is not convenient to harness more than two horses to a single vehicle.
But with electric cars there is no extraneous limitation to carrying
capacity. Large double-decked cars with seats for seventy passengers
are now quite common. In America it is a frequent practice to attach
'trailers' to the cars, making a short tramway train. Experiments have
recently been arranged on similar lines in London, for the handling of
the heavy traffic at rush hours. These instances show that electric
tramway capacity is flexible and may be adjusted to the density and the
fluctuating character of the demand.
Finally, it falls to be noted that the power consumed by a tramcar
is, roughly, proportional to the useful work which the car performs.
As already mentioned, it costs about as much to work a horse tramway
when the cars are empty as when they are full, since the main item is
the maintenance of the 'vital principle' of a certain number of horses
independently of the traffic. But with electric traction the motors
require less power when the cars are running light. And less current
for the motors means less current generated at the power station--that
is to say, less steam, less oil, less coal, less wear and tear. If more
current is demanded, it is because more passengers are being carried
and more revenue earned.
Reviewing the subject broadly, it is apparent that the adoption of
electric traction on a tramway is not so much a step in advance as
a beneficent revolution. The higher speeds with more frequent, more
comfortable, and more commodious cars have created a volume of traffic
far beyond what could have been handled with horse traction. The change
also led to a great increase in the length of tramway routes and to
the construction of new tramway systems. In 1898, when the electric
tramway movement began in earnest, there were 1064 miles of tramway in
the United Kingdom. Now there are 2562 miles, and the number of tramway
passengers is more than double the total of third class passengers on
the whole system of British railways. The number of tramway passengers
carried during 1909-10 (the last period covered by the published
official returns) was equal to about 62 times the estimated population
of the United Kingdom.
While the traffic has multiplied in this remarkable fashion, there
has been a heavy reduction in the fares charged. This has been made
possible by the economical features of electric traction. In the
old days a horse tramway had to spend about £80 to earn £100; an
electric tramway need spend only about £60. With this reduction in
the proportion of expenses to receipts, and with the greater volume
of business, it became feasible to stimulate traffic still further
by giving passengers much longer distances for their money. In fact,
electric traction proved so economical that people began to imagine
that there was no limit to the reductions which might be made with
financial safety. However, there is plenty of evidence that a limit
exists. In many cases it has been touched, if not passed, but the
public continues to clamour for all sorts of concessions. These demands
are a great compliment to electric traction, but they are a decided
embarrassment to the tramway manager who believes in a reasonable
margin between his total expenses and his total revenue.