THE MECHANISM OF AN ELECTRIC TRAMCAR: THE OVERHEAD SYSTEM
A rough idea has already been given of the elementary mechanism of
electric traction--the combination of generating station, of cars
fitted with electric motors, and of a sliding contact between the two.
It is in connection with the sliding contact that the ingenuity of
tramway engineers has been mainly exercised. Three distinct solutions
were evolved for tramway work, giving rise to three systems--(1)
the overhead or trolley system; (2) the conduit system; and (3) the
surface-contact system.
The first system is now almost universal in the United Kingdom.
Part of the London system is equipped on the conduit system; and
the tramways at Lincoln and Wolverhampton are constructed on the
surface-contact system. Beyond these cases the trolley holds the field.
In the United States and on the Continent there is a larger proportion
of conduit work, but from a practical point of view it would hardly
be necessary to mention either conduit or surface-contact if it were
not for the great engineering interest which they possess and for the
controversies to which they have given rise.
[Illustration: Fig. 3. Diagrammatic illustration of the general
arrangement of an electric tramway on the overhead system. At the
foot is shown the generating station which supplies alternating
current at high-pressure (for economy in transmission) to a
sub-station where it is 'transformed' to low pressure and 'converted'
in a motor-generator to continuous current for distribution to the
trolley wire from which each car takes its current. The course of
the current through the trolley pole and controller and thence to
the motors and back by the rails is indicated by arrows.]
The overhead system has conquered because it is cheapest in first cost,
cheapest to maintain, most economical in current, and most reliable in
action. Later developments in surface-contact traction have run it very
close on some of these points, but have not--for reasons which will be
explained--affected the established position of the overhead system.
In its essential features the overhead system has not altered very much
from the experimental line erected at the Paris Exhibition of 1881. The
slotted tube has been replaced by a solid copper wire; and the 'boat'
sliding within it has been replaced by a wheel or a bow pressed against
the lower side of the wire by means of a pivoted arm controlled by
springs. The sliding bow is common on the Continent, but it has been
adopted on only one British tramway--that at Sheerness. Its use for
electric traction on railways will be mentioned later, but as far as
British tramways are concerned the bow is the exception which proves
the trolley wheel rule.
The function of the trolley wheel is to collect current from the wire
along which it rolls. This current passes through insulated wires down
the trolley arm to the controller, which the driver of the car operates
by means of a handle. The controller, which is really a series of
electrical resistances, is analogous to a water tap. By its means the
current may be completely shut off from the motors, or allowed to flow
in varying degree as required by the speed of the car. In starting a
car, the driver moves the controller handle notch by notch, so as to
get a uniform rise in speed until the full current is allowed to pass
through the motors. With such a mechanism, supplemented by brakes, the
driver has the movements of the car under control.
In a four-wheeled car, each axle is driven by a motor. In a bogie car
(one with a set of four wheels at each end) the axles of the larger
wheels of the bogie are each driven by a motor; but not directly.
Considerations of space make it necessary to keep the motor as small
as possible, but if a motor is to be small and also powerful it must
rotate at a high speed. On the tramcar, therefore, the motor drives a
small toothed wheel which drives a large toothed wheel fixed to the
axle, thus effecting a reduction of speed between the motor and the
wheel.
The same considerations of space join with others in making two motors
on each car the general rule. And the use of two motors enabled the
tramway engineer to introduce a refinement into the method of control.
This refinement is known as the 'series-parallel system.' One of its
objects is to give a large 'starting torque' and so enable the car
to gain speed quickly. When the current is first switched on by the
controller it passes through the motors in tandem or in 'series,'
thus dividing the pressure of the current (analogous to a 'head' of
water) between them. The starting torque of a tramway motor (or the
turning moment which it exerts when current is first passed through
it) is dependent on the current but independent of the pressure. Thus
the tandem or 'series' arrangement, which passes the full current
through each motor, gives the maximum starting torque without an
undue consumption of current. After the car is well started, the next
movement of the controller puts the motors in 'parallel,' opening up
two paths for the current instead of one, so that each motor receives
the full pressure. The practical result is that there is a very rapid
acceleration at starting, with marked economy in current. If the motors
were kept in 'parallel' right through, twice as much current would be
required to get the same starting torque. It will be seen later how
valuable this arrangement for getting a rapid start, without excessive
current consumption, may be in improving the physical and economic
conditions of a tramway or train service.
After having passed through the motors and done its work, the current
is led to the wheels of the car and returns by way of the rails,
which are linked together by copper bonds so as to form a continuous
conductor. The passage of the current from the wheel to the rail is
indicated by sparks when the rails are rough or very dry and dirty.
Although the rails, like the overhead wires, are thus carrying current,
there is no danger of shock from them, as the electrical pressure
in them is only a few volts, at the outside, while the pressure in
the overhead wires is 500 volts. It is this difference of pressure
which--like the 'head' of water in a turbine--supplies the motive power
for the car.
Each car on a tramway system may thus be regarded as a bridge which
completes an electrical circuit. When the driver moves his controller,
current flows from the generating station at a high pressure, passes
through the controller, operates the motors, and returns to the
generating station at a low pressure. This typical circuit is completed
through every car, so that the demand on the generating station at
any moment is the sum of the demands of the cars at that moment. The
business of the engineer at the generating station is to maintain the
electrical pressure in the overhead wire at the normal level of 500
volts; and in order to do this on an ordinary tramway system it is
found convenient to divide the overhead wire into half-mile sections,
each of which has a separate main or 'feeder' from the generating
station. The passenger can detect the change from one section to
another by the click of the trolley wheel across the gap which
insulates one half-mile section from another. At the same spot he can
see the short square 'feeder-pillar' at the roadside (containing the
switches by which current can be turned off from that section) and the
cables which pass along the arm of the trolley standard and terminate
in the overhead wire.
On an extensive tramway system the power-supply arrangements become
more complicated. The central generating station remains the primary
source of power, but sub-stations are erected at convenient points
between the central station and the outskirts of the tramway area.
These sub-stations are secondary stations for the distribution of
electricity. They receive power at extra-high pressure (5000 volts or
more) from the central station; they contain special machinery for
reducing the pressure to 500 volts for distribution to the various
tramway feeders. The object of this arrangement is partly technical
but mainly economical. Electric power can be transmitted at a lower
cost in mains and with less loss of energy at high pressures than at
low. Consequently when the termini of tramway routes are several miles
from the generating centre, greater all-round efficiency is secured
by transmitting current at high pressure to a number of well selected
sub-stations.
[Illustration: Fig. 4. Photograph of a car on a conduit section of
the London County Council tramways. The centre line on the vacant
track indicates the slot rail through which the 'plough' on the car
passes to make contact with the conductors in the underground conduit.
(Photograph reproduced by courtesy of Dick, Kerr and Company, Ltd.)]