CHAPTER V
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.
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 The slotted tube has been replaced by a solid copper wire; and the 'boat' sliding within it has been replaced by a wheel or a bow pressed against the lower side of the wire by means of a pivoted arm controlled by springs.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.