The position of the motors with reference to the truck and car wheels is very well shown in [Fig. 20], and also the manner in which they are held in place. The covers of the openings through which access to the commutator brushes is obtained are removed from both motors, and in the forward one the top of the commutator and one of the brushes can be readily seen. The manner in which the motors are suspended from the truck is not the same in this figure as in those previously shown, but this is simply because the machines are not made by the same concern, and each manufacturer has his own design.
[Fig. 21] shows the appearance of the interior of the controlling switches C C, [Figs. 18 and 19]. It will be noticed that there are two upright shafts, the ends of which project above the top of the box. The handle h is placed upon the shaft to the left, and k on that to the right. The first is the main controller, and the other is the reversing switch. It will be noticed that the main controller shaft carries a number of circular segments of different lengths; these are so disposed that they come in contact with suitable stationary pieces as the handle h is turned around, and thus vary the path of the current through the motors and the rheostats in the manner required to effect the desired changes in the velocity of the car. The reversing shaft is also provided with a number of segments, but these are not so easily seen, although they can be discerned on close examination. The wires from the cable e e e and also wire d d are attached to the stationary pieces with which the segments carried by the two shafts make contact when the latter are moved around by the motorman. These wires can be seen back of the main switch shaft, and also above the board located at the lower left-hand corner. All these wires enter the controller through an opening in the bottom.
Fig. 21.—View of Interior of Car Controller.
In addition to the apparatus shown in Figs. [18] to [21], electric cars are provided with a safety fuse and a lightning arrester, the object of the latter being to protect the motors from the destructive effects of lightning strokes. The object of the safety fuse is to protect the motor from injury when the current becomes too strong. An electric current in passing through a wire generates heat, and the stronger the current the greater the heat. If the wire is large and the current weak, the heat developed may be insufficient to raise the temperature to a noticeable degree; but, on the other hand, if the wire is small or the current very strong, the heat generated may be capable of raising the temperature of the metal to the fusing point. In fact, the incandescent lamp operates upon this principle; the carbon filament is traversed by a current of a strength sufficient to heat it to a point where it becomes intensely luminous, and sometimes, through accident or otherwise, the current becomes strong enough to melt the filament, and then the light goes out. In an electric motor it is not necessary to raise the temperature of the wire to the melting point to do serious injury; in fact, if the heat is sufficient to char paper or cloth, the machine will be rendered useless until suitable repairs are made. The insulation of the wire coils is made principally of cotton, which is a very good electrical insulator in its natural state, but when carbonized by excessive heat it becomes a conductor. As soon as it becomes a conductor the current is no longer confined to the proper channel, but cuts through the insulation to find the shortest path through the machine. If safety fuses were not provided the danger of destroying the insulation of the motors and thus disabling the car would be decidedly great, for, as already said, the motors can not be stalled with an overload, the only effect produced being a reduction in the speed and an increase in current strength. Now, if there were no way of limiting the increase in current strength the motors, if greatly overloaded, would continue to operate until the insulation gave out. The safety fuse is simply a piece of wire of such size that it will be melted by a current that the motors can carry without being injured; hence when the current strength reaches a point where the safety of the apparatus is endangered the fuse melts and thus breaks the circuit and stops the further flow of current. Fuses are generally made of an alloy that melts at a low temperature, so that the molten metal may not set fire to anything upon which it may fall. These easily fused alloys are inferior to copper as electrical conductors, and on this account the fuse wire is as a rule much larger than that wound upon the motors, which fact makes its action somewhat mysterious to the uninitiated; but whatever its size may be, it is so proportioned that it will melt before the current rises to a strength that would injure the motor coils.
The manner in which the electric current generated in the power house reaches the motors is illustrated in [Fig. 22]. In this figure four tracks are shown, which may be taken to represent roads running in as many different directions. The three squares at the left side represent generators located in the power house. The circles a a a represent switches, by means of which the generators are connected or disconnected from the trolley lines. A and B represent heavy metallic rods, generally made of copper, with which the generators are connected by means of the switches a a a. These rods are called bus bars. The circles b b b b represent switches by means of which the current is turned on or off from the several tracks.
Fig. 22.—Diagram illustrating the Manner in which the Electric Current flows from the Generators to the Cars upon the Tracks.
Electric currents must always circulate in closed paths—that is, the current that starts out from a generator must return to it, and the amount coming back is the same as that which leaves. The action of an electric generator can be understood by comparing it with that of a water pump pumping into a pipe which runs around from the delivery end to the suction. With such an arrangement it can be seen that the action of the pump would be to keep the water in circulation, but the same water would be pumped through the pump and the pipe all the time. With an electric generator the action is the same, and in [Fig. 22] the current flowing along any one of the tracks follows the course indicated by the arrows. The currents pass out to the several tracks through the trolley wires T T T T, and return through the tracks R R R R. The bus bar A is connected with a plate D, which is imbedded in the ground, and is also connected with the ends of the rails R R R R. Suppose for a moment that the two lower generators are out of service, their switches a a being turned so as to disconnect them from the bus A, and, further, suppose that the three lower b switches are open, so that the current can only pass to the upper track; then the top generator will feed into the top road only. Tracing the path of the current under these conditions, we find that it will start from the upper side of the generator through the a switch to the B bus, and thence to the trolley wire at the top of the figure. On reaching the first car a portion of the current passes to the track R, the amount being dependent upon the speed of the car and the load. Why the whole current does not follow this path generally puzzles the layman, but the explanation is that the motors hold the current back, and only allow as much to pass through them as is necessary to perform the required work—that is to say, the current flowing through each car is not controlled by the generator or by the force of the current, but by the requirements of the motors. The amount of current delivered by the generator is governed by the demands of the motors. The current that does not pass through the first car goes on to the second one, and if there were more cars there would be current left in the trolley wire to supply them. After passing through the motors of the two cars the current returns through the rails R to the plate D, and thus to bus A, from which it enters the lower side of the top generator. It will from this explanation be seen that the action of the generator is simply to keep the current circulating. If two of the generators are connected with the bus bars A and B, the current required by the motors will be delivered by the two machines, and if the three generators are placed in service the current will be divided among them.