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.