Page:Motors and motor-driving (1902).djvu/330

298 fourthly, no mechanical compensating gears are necessary, two series or separately excited motors acting as differentials to one another.

As regards the class of motor employed, the majority of electromobiles are propelled by series motors, a smaller number by compound motors, some very successfully by separately excited motors, and very few by shunt motors. The great majority of motors in actual use have four poles.

The principle of separate excitation has some advantages. It enables great torque or rotary pull to be obtained, without as much loss in the windings of the field magnets as is involved in the case of the series motor. In the separate excitation method of arranging, two or four cells are usually connected separately to the windings of the field magnets, the main battery supplying current to the armature. With the armatures in series an excellent differential gear is produced, while with the armatures in parallel, which only occurs at high speed, a steadying effect tending to keep the car straight results. This arrangement is employed in the cars of the Accumulator Industries, Ltd. The separately excited field motor, in addition to possessing, as already stated, certain of the advantages of the series motor, enables a very agreeable variation of speed to be obtained economically between the regular speeds by inserting resistance into the field magnet circuits. This acts like the accelerators on a petrol car.

According as one motor or two are employed, the groupings which the controller is arranged and designed to effect will of necessity vary also. Where a single motor is employed, unless it be compounded and used in some such way as in the Krièger system, differences of speed must be arranged for by grouping the battery cells in different arrangements. Thus a position of the controller will be arranged for which puts all the cells in series on to the motor. Running on the flat this will give the highest speed. The next lower speed will naturally be obtained by putting the cells in two bunches in parallel with one another; that is to say, the positive terminals