Page:Motors and motor-driving (1902).djvu/327

Rh enormous current at starting, which would be very liable to burn up the motors and injure the accumulators. In order to give different speeds and to enable the amount of current taken by the motors at starting to be reduced, arrangements must be made for connecting either the motors or the parts of the motors, or the two halves of the battery, or both in different groupings. It must be pointed out that more current can be put into an electric motor at starting than when running, owing to the fact that when running it produces a kind of back electric pressure.

If, then, we used on each of the motors at starting the full electric pressure our battery is capable of supplying, a great deal too much current would be forced through the motors and they would probably be burnt up. We may reduce the effective pressure applied to the motors by putting the two halves of the battery in parallel, when we should be working with a pressure of forty volts instead of eighty. We may also work further in the same direction by putting the motors in series, thereby doubling the effective resistance opposed to the current. To enable this to be done we want, however, to take several connections from the motors and several connections from the battery, and connect them by cables to a number of different points between which the required electrical connections can be established by means of an appliance termed a controller. This usually involves some such arrangement as is depicted in fig. 10, which shows the controller used in the Joel car. It consists of a non-conducting cylinder along which strips of metal are arranged, the cables uniting the different terminals of the battery and the motors being brought up to the flat springs which are shown pressing against the cylinder. Turning the cylinder round by means of the lever connected with it on the right produces electrical junction of such a kind as to group the motors and batteries in various ways according to the requirements for starting, for producing different speeds, forward and reversing in accordance with the principles above described.