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.
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.)