ELECTRIC TRACTION ON MAIN LINE RAILWAYS
On tramways, automobiles, and 'third-rail' lines, the electric current
used belongs to the type described as 'continuous' or 'direct,'
because the flow is always in the same direction. The other type
of current is known as 'alternating,' as it flows backwards and
forwards many times per second. There are several kinds of alternating
current--single-phase, two-phase, three-phase, and polyphase--each
produced from generators designed in a particular way.
It is not possible to give any adequate account of these different
kinds of alternating current without going rather deeply into the
theory of electricity. The ultimate practical point is that in
transmitting alternating currents the circuits increase in number with
the phases. Thus, three-phase current requires three wires, two-phase
current three or four wires, and single-phase current a single circuit
like that of continuous current[1].
[Illustration: Fig. 12. Photograph of a train on the electrified
section of the London, Brighton and South Coast Railway. The overhead
wire is suspended from cables stretched between insulators, and
current is conveyed from it to the trains through a 'bow' which slides
along its lower side. The photograph is taken from the rear part of
the train. The front and rear cars are both equipped with electric
motors.]
Where current has to be conveyed economically over long distances, it
is generally done in the form of alternating current at high pressure.
For instance, the transmission from a tramway power station to the
sub-stations is almost uniformly by three-phase current at, say, 5000
volts. When it reaches the sub-station, it is 'transformed' down to
the working pressure of 500 volts and 'converted' from alternating to
continuous current by means of rotary machinery. The transforming is
done by a stationary piece of apparatus similar in principle to the
familiar induction coil. An induction coil takes current at a few volts
from a battery into its primary circuit and transforms it, by induction
in the secondary circuit, into current of high enough voltage to give a
long spark. A transformer can be designed to 'step-up' or 'step-down'
the pressure according to the requirements of the case.
So much explanation is necessary to give some account of the
alternating current railways on the Continent and thence of the
single-phase system on the London, Brighton and South Coast Railway.
The Morecambe and Heysham section of the Midland Railway is also
equipped on the single-phase system.
Most of the earliest electric railways on the Continent derived
their power from waterfalls and had to transmit it for a considerable
distance. Three-phase current at high pressure being adopted for this
purpose, the Continental engineers set to work to find some means of
utilising the high-pressure three-phase current directly. They did this
by carrying the three wires on poles alongside the railway track, and
using three 'bow' collectors (in place of trolley wheels) to convey
the current to transformers on the motor cars or locomotives. In these
transformers the current was brought down to working pressure and then
led to motors designed for three-phase current.
An immense amount of technical ingenuity was exercised in developing
this system; and when the Metropolitan Railway decided to follow the
District in electrifying its lines, a three-phase system was proposed.
As the Metropolitan and Metropolitan District companies share the
working of the Inner Circle, it was necessary that both should adopt
the same system. The result was that the question between three-phase
and continuous current working had to go to arbitration. After a
long discussion of masses of technical evidence, Mr Lyttelton, the
arbitrator, decided that the direct current system was better suited to
the conditions of traffic on an underground railway in London.
The wisdom of that decision will not be questioned now. Three-phase
motors do not give the rapid acceleration which is so urgently required
on suburban lines; there are complications in speed control; and
the necessity of having three overhead conductors is also a serious
drawback. For comparatively long-distance traffic with few stops,
however, the three-phase system is quite suitable. That is to say, it
is a possible solution of the main line problem.
The great simplicity and flexibility of the power supply arrangements
in the case of alternating current traction encouraged engineers to
find something better adapted to ordinary railway conditions than the
three-phase motor. Their problem was to find an arrangement which
required one overhead conductor instead of three, and also provided
a motor with the high starting torque and easy speed control of the
continuous-current motor. After much theoretical and experimental
work, they found it in the single-phase system, using a motor which is
similar in many respects to the continuous-current motor but capable of
being operated by alternating current.
On the advice of Mr Philip Dawson, the London, Brighton and South
Coast Railway Company decided to experiment with this system on the
double line connecting London Bridge and Victoria stations, about
9 miles long. Power is supplied to each track by a single overhead
conductor carrying current at 6000 volts. Transformers are placed on
the trains to bring the pressure down to 300 volts; the current is
then led through controllers to single-phase motors in much the usual
way. The reason for using so high a pressure on the overhead line is
not only economy in transmission. If lower pressures were used, the
heavy currents required for train propulsion would require a thicker
conductor and correspondingly heavier supports. At 6000 volts it is
possible for two double sliding bows to collect sufficient current
for a heavy train from a wire which is comparable in thickness to the
ordinary trolley wire of a tramway.
The power distribution arrangements, it will be noticed, are very much
simpler than with continuous current on the third-rail system. There
are no sub-stations with rotary machinery. Power is supplied direct
from the generating station to the overhead line and is transformed
down by stationary plant on the train itself. Single-phase traction
represents, in fact, power transmission for railway purposes reduced to
its simplest elements.
The overhead construction differs, however, in some important points
from the tramway standard. The supports, which are in both bridge and
bracket form, are stronger; the insulators are, owing to the much
higher pressure employed, more massive; and a different means of
suspension has been adopted. Each conductor is hung by links from two
steel cables stretched chain-wise between the supports. This method of
'catenary suspension' enables the bow to slide along the wire without
the jolts which are noticeable with a tramway trolley. Such smooth
running keeps the bow continuously at an even pressure on the wire--an
advantage which is of great importance at high speeds. The trains are
arranged on the multiple-unit system.
The full financial results obtained on this railway have not so far
been made public; but it is sufficient for our purpose to note that
the Company, after more than a year's full trial, extended the system
to the Crystal Palace and to Croydon. Further extensions are, it
is understood, contemplated over the suburban lines to Sutton and
elsewhere; and in course of time the conversion of the main line to
Brighton will be undertaken.
Here we touch upon the most interesting aspect of this demonstration of
electric traction on the single-phase system. The system was adopted
in the first instance because the third-rail system would lead to
complications and dangers which could not be permitted at crowded
railway termini shared by all kinds of traffic, suburban and main line.
But the advisers of the Company had also in view the possibility of
development beyond the range of suburban traffic. They therefore sought
a system which, while comparable to the third-rail continuous current
in the handling of suburban business, would be adaptable to main line
conditions, where infrequent stops and long runs at high speeds are the
rule.
The adoption of electric traction on such a route as the Brighton
main line would be a benefit in several ways. It would lead to a
faster express service, as the high overload capacity of the electric
motor enables it to take small account of gradients. It would also
lead to a more frequent service, as the electric system is free from
the conditions which force a steam railway to try to concentrate
traffic on a limited number of long trains. Further, it would, by
reducing the time lost in stopping and starting, bring the average
speed of stopping trains much closer to that of express trains. All
these improvements--assisted, probably, by lower fares--should lead
to a great increase in the volume of traffic, thus reproducing the
characteristic results of electric traction on suburban lines.
[Footnote 1: An admirable explanation of alternating currents will be
found in Mr Frank Broadbent's _Chats on Electricity_. (Werner Laurie,
1910.)]