Page:Motors and motor-driving (1902).djvu/331

Rh of each half battery will be connected together to one brush of the motor, and the negative terminals connected together to the other. This will provide half the pressure of the whole battery, and as the speed of the motor at the same load is dependent on the pressure at which the current is supplied to it, the car will travel more slowly. Lower speeds are produced by similar groupings giving lower pressure. Where two motors are employed a smaller number of groupings of the cells may be adopted, or double the number of speeds obtained with the same number of cell groupings, as with each cell grouping the motors may be arranged either in series or parallel with one another. In this case the maximum speed will of course be with the cells in series and the motors in parallel, the lowest speed with the motors in series and the cells grouped so as to give the lowest pressure. Generally one of the lower speeds that is, with the cells arranged to give the lowest pressure will be used for starting; as the motor, owing to its taking a large current before its speed increases, should not be supplied with current at a high pressure. As the batteries also, when arranged in parallel groups, are able to yield a heavier current, the arrangement is mutually satisfactory to both batteries and motors. For reversing, the controller is brought into such a position as to send the current the reverse way through the motor.

An electric car does not require change-speed gear because it contains practically a gear in itself. It accommodates itself to the load and does its best at any given load, or by varying the electric pressure it may be caused to increase or diminish its power within limits. Thus a series or separately excited motor can develop proportionately more torque in overcoming an increased load than it was developing before the load was enlarged. This effect may be increased by arranging the field magnets with a large amount of iron in their cores, and working under ordinary circumstances with a small magnetic density. On heavy loads, as when going uphill, the increased magnetic intensity produced by the increased current passing