Popular Science Monthly/Volume 56/February 1900/What Makes the Trolley Car Go II
|WHAT MAKES THE TROLLEY CAR GO.|
IF the successful operation of a street-railway car by mechanical power depended wholly upon the ability to produce a motor of sufficient capacity to do the work, the problem would be an easy one to solve, and would have been solved long before the advent of the electric motor. Mere ability to furnish the necessary power, however, is not enough to meet the requirements. As already shown, the mechanism must be light, strong, compact, simple, and so well protected that it can not be injured except under abnormal conditions. In addition, speed-controlling devices must be provided whereby the velocity may be changed at will and in the shortest possible time, and with as nearly absolute precision as possible. This controlling mechanism must also be so arranged that the direction of motion may be varied with the greatest certainty and as rapidly as may at any time be required. The way in which these
results are accomplished in an electrically operated car can be understood from Figs. 18 and 19, which are line drawings, in a simplified form, of an ordinary trolley car. Fig. 18 is an elevation showing the outline of the car body and the wheels in broken lines, while the motors and the wires through which the current is conveyed thereto are drawn in solid lines. Fig. 19 is a plan in which the outline of the car floor and the platforms is represented in broken lines, the solid lines being the motors and connecting wires.
In almost every instance railway cars are provided with two motors, as shown at M M in these two figures. This arrangement is adopted not because one motor can not furnish all the power required, but simply for the purpose of making the equipment more reliable. Everything of human make is liable to fail; hence if only one motor were used there would be more or less liability of its giving out at a critical moment, and then the car would be helpless. If two motors are provided, should one give out the car would not be disabled, for the remaining machine would be able to run it to its destination. In order that this result may be successfully accomplished, each motor is made of sufficient capacity to run the car without being overtaxed, unless the load is abnormally large; but even under the latter conditions the machine will in ninety-nine cases out of a hundred withstand the strain. Some roads, in small towns, where the traffic is light and the expense must be kept down to the lowest point, use single-motor cars, so as to effect a saving in first cost. This course, however, is very seldom followed, except in places where there are no heavy grades or where there is very little probability of the loads becoming excessive, except at rare intervals. If the cars are provided with a single motor, when one becomes disabled from any cause it has to wait until overtaken by the car behind it, so that it may be pushed by the latter to the end of the road.
The electric current for operating the motors is generated in a power house that is located at some convenient point along the route. The current is conveyed to the moving cars by means of a trolley wire, which is marked T in the drawings. Unless the road is very small and operates but a few ears, this wire will not be sufficient to carry all the current, hence in most cases there are a number of supplementary wires, which are called feeders. These wires are carried along on poles, and at proper intervals are connected with the trolley wire T. The electric current passes from the trolley wire through the motors on the car, and thence to the rails R, and through these, and also through the ground, back to the power house. The exact path of the current is as follows:
From the trolley wire, through the trolley pole t, to the fixture on top of the ear which holds the latter. From this fixture, as shown by the heavy full line, the current passes to a, which is a switch located under the car hood overhanging the platform. From this switch the current passes to a similar one, marked b,
Figs. 18, 19.—Outline Elevation and Plan of Electric Railway Car, showing Location of Motors, Controlling Switches, and Connecting Wires.
located in a like position at the other end of the car. These two switches are called emergency switches; they are provided simply as a safety device, and are used only when the main switches get out of order and the motorman can not turn the current off in the regular way. From the last hood switch b the current passes to the bottom of the car, where it enters the wire d d at the point c. This wire d d, as will be seen, runs in both directions, and ends in the stands C C. These latter are the controlling switches, and are provided with a handle h, by means of which the current is turned on or off from the motors, and is directed through them in such a way as to make the car run in whichever direction may be desired. From the controllers C C several wires are run under the car, as shown at e e e. These wires are generally bunched into one or two cables, but they are kept separate from each other by means of strong insulating coverings. Four wires lead into each motor, and three or four into each of the boxes marked G G. If the motors were required to run in one direction only, then two wires would be sufficient to convey the current to them; but as they have to run in either direction, at least three wires are necessary, but in almost every case four are used, as the results obtained thereby are more satisfactory. The boxes G G are called rheostats, and are simply devices through which the current is run so as to reduce the speed of the car, and also for the purpose of graduating the strength of the current that passes through the motors in the act of starting. These rheostats are very seldom in use when the car is in motion, because it is a waste of power to pass the current through them. After the current has passed through the motors it enters the ironwork, and thus gets into the car wheels and finally to the track.
The lines drawn in Fig. 18 to indicate the position of the wires in the car do not show their actual position, but only the general direction they follow. From the trolley base to the first hood switch the wire, as a rule, is run along the car roof on one side of the ventilator, and the wire leading from the first to the second hood switch occupies a corresponding position on the opposite side of the roof. From the last hood switch, h, the wire is run down one corner of the car body, being either within the car body, or, if not, so covered by moldings as to not be reached by the hands of passengers. The wires d and e are generally run under the car, and are firmly secured to it by means of suitable fastenings.
The controlling switches C C are provided with one and sometimes two handles, one of which is used to regulate the speed of the car and stop and start it, while the other is for the purpose of reversing the direction in which it runs. The handle h is for the purpose of regulating the speed, and by means of k the direction of motion is changed. Before h is moved from the inactive position k is turned so that the car may run either forward or backward, as may be desired; then, when h is moved, the car will start, and by varying the position of h the speed can be changed. If it is desired to reverse the car, h is brought back to the stop position, h is shifted to the reverse motion, and then h is again turned to the running position. When the controlling switch is provided with only one handle this is turned in one direction to run the car ahead, and in the opposite direction to run it backward, the graduations in velocity being obtained by placing the handle in positions intermediate between the stop position and the highest speed position.
As will be noticed, the wire d d branches at c and runs in both directions. Now, when the controller handles are both turned to the stop position the current from the trolley can get no farther than the ends of d in either switch, but if one of them is turned to the running position, the current at once passes to the wires in the cable e e e, and thus to the two motors. If the switches C C are in proper working order and there is no disarrangement of the wires leading to the motors or those within the latter, the current will obey the movements of the handle h, but under other conditions it may not. If such an emergency arises, the motorman reaches up to the hood and turns the safety switch a or b, and thus cuts the current off.
The force with which the motors turn the car wheels around depends upon the strength of the current; this is owing to the fact that the magnetic force is increased or decreased by variations in the current strength. If the current is doubled the magnetic force of the armature is nearly doubled, and so is that of the field magnet, therefore the pull between the poles is nearly four times as great. From this it will be seen that the force with which the car is pushed ahead can be increased enormously by a comparatively small increase in the strength of the current. If the current strength is doubled, the propelling force is practically quadrupled; and if the current is increased four times, the propelling force is made nearly sixteen times as great.
The speed at which the car runs depends upon the force that impels the current through the wire, and which is called electromotive force. The greater the electro-motive force, the higher the velocity. If the current passes from the wires in the cable e e e through each motor separately, and thence to the rails R, each machine will receive the effect of the whole electro-motive force of the current; but if after the current has passed through one motor it is directed through the other, then each machine will be acted upon by only one half the electro-motive force, and, as can be seen at once, the velocity in the first instance will be twice as great as in the last. This fact is taken advantage of in regulating the speed of the car, and controlling switches arranged so as to direct the current through the motors in this way are designated as belonging to the series parallel type, the name being given from the fact that when the car is running slow, the current passes through the two motors in series—that is, through one after the other; but when the motors are running fast, a separate current passes through each machine.
If, when a car is running, the controlling switch is turned to cut the current off, the effect will be that the speed will gradually
reduce; but if it is desired to effect a sudden stop, it becomes necessary to cheek the headway by means of a brake. For this purpose the hand brake ordinarily used on all types of cars is employed, but magnetic brakes are also used in some cases. Fig. 20 shows a car truck equipped with two motors and magnetic brakes, one on each axle. Looking at the front end of the truck, the brake is seen on the left side of the axle, between the motor bearing and the car wheel. The larger drum, on the right side, is the casing within which the gear wheel and pinion are inclosed. These magnetic brakes are operated by a current generated by the motors, and not by that of the main line. As was explained in the first article, an electric motor can be made to act as a generator of electric current by simply reversing the direction in which the armature revolves. If we do not desire to reverse the direction of rotation, the result can still be attained by reversing the direction in which the current passes through the armature coils. It is evident that the direction of a car motor can not be reversed at the instant that it is desired to have it act as a generator—that is, when it is desired to put the brakes on; hence the direction of the current through the armature is reversed.
When a car is provided with magnetic brakes, the controlling switches are so made that when the handle h is moved back to the stop position it disconnects the motors from the trolley wire and at the same time connects them with the magnetic brakes in such a manner that they will act as generators and thus send current through the coils of the latter. In order that the force with which the brakes are applied may be graduated, the controlling switches are arranged so as to be moved several steps back of the point which in the ordinary type of switch would be the final stop position. When the handle h is placed on the first brake position the current generated by the motors is not very strong, and as a consequence the force of the brake is light, but sufficient to bring the car to a stop in a reasonable distance. If a quicker stop is desired the handle is moved to the second, third, or fourth brake position, thus increasing the retarding force as much as may be desired. Magnetic brakes are very desirable, as they save the car wheels, and furthermore afford an additional safety in cases where it is necessary to arrest the speed instantly.
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.
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.
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 h 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.
When two or more generators are used, it is necessary to provide means to prevent the current from dividing unequally between them; if this were not done, one machine might do nearly all the work, while the other one would be practically idle. The means employed to accomplish the result is simply an additional bus bar, which is called an equalizing bus. We will not undertake to explain the principle upon which this arrangement acts; it is sufficient to say that by such means the work can be distributed in amounts directly proportional to the capacity of the generators, so that if one machine is very much larger than the others it will take a portion of the load corresponding to its size. In order that these results may be attained it is necessary to properly adjust the several generators, and as no machine can be made to work with the accuracy of perfection, the work will not be distributed in true proportion for all conditions of load; thus if the generators are adjusted so as to each take its proper share when all the cars are in operation, one machine may do too much or not enough when only one half the number are running, but the excess or deficiency will not be more than a few per cent unless the adjustment is very defective.
Electric generators for railway work are made in all sizes, from those only large enough to operate four or five cars to others capable of furnishing sufficient current for thirty or forty or even more. Small generators are made so as to be driven by a belt running over a pulley mounted on the end of the armature shaft, or they may be arranged to be connected to the end of a steam-engine shaft, and thus become what is called direct connected machines. Large generators are almost invariably of the latter type. A machine of this class is illustrated in Fig. 23. The driving engine is shown at E, the cylinder being in the background and the crank toward the front, the shaft being clearly seen at S, while F is the
fly wheel. The generator is mounted directly upon the engine shaft, between the bearing at the crank end and the fly wheel. The large ring marked G is the field magnet ring, and at D D D the field coils are shown. These coils are equally spaced all the way around the circle. The commutator is marked C, and the commutator brushes are located at B B. The armature can not be seen very well, as it is covered by the brush holders and their supporting frames, but it is located within the ring G in the position designated by A. This machine is one of a number used to operate the roads of Troy, N. Y., and is of about one-thousand-horsepower capacity, which is enough to furnish all the current required to run sixty or seventy cars.
The switches a a and b b, shown in Fig. 22, and the bus bars A B, are mounted upon a large panel, made of marble or slate, called a switch board. These switches are sufficient for the purpose of turning the current on or off any track or for connecting and disconnecting the generators, but for the successful operation of the plant it is necessary to have other devices by means of which the strength of the current may be ascertained, and also the electro-motive force. It is necessary to provide each generator with means for varying the electro-motive force of the current it generates, otherwise the load could not be properly equalized between the several machines. All these different devices are located upon the switch board, so as to have them in an accessible position. A railway switch board, arranged for four generators and a large number of distributing circuits, is shown in Fig. 24. The four generator switches are shown at a a a a, and the circles marked R, directly under them, are the devices by means of which the electro-motive force of the current is regulated. These devices are called field regulators, from the fact that their office is to regulate the strength of the field magnets of the generators, making them stronger to increase the electro-motive force and weaker to reduce it. The part seen upon the front of the switch board is not the regulator proper, but only the handle and the contact points over which this swings. The instruments marked A A A A are for measuring the strength of the current of each individual generator, and are called ammeters. The instruments marked V V V V are for the purpose of indicating the electro-motive force of the currents of the several generators, and are called voltmeters. Ag is an ammeter used to measure the strength of the total current, and Vg is a voltmeter that indicates the electro-motive force of the current passing out to the cars on the various lines. The ammeter Ag is not an actual necessity, for the strength of the total current can be ascertained by adding the readings of the four instruments connected with the generators, but it is a convenience, as it saves the trouble of performing the addition. The voltmeter Vg, however, can not be regarded in this light; in fact, its presence is decidedly serviceable, for it indicates the average electro-motive force of all the generators; therefore if any one of the instruments V V V V is higher or lower it shows at once that the generator to which it is attached is out of adjustment and not doing its proper share of the work. The switches b b b, means of which the current is turned on to the several external circuits, are shown at the extreme end of the switch board.
The instrument marked W, located between the a switches, is called a wattmeter, and its office is to indicate the amount of power furnished by the generators. This instrument is not always used, as it is a convenience but not a necessity. It can be seen at once that whether it is used or not, the amount of power required to operate the roads will be the same, but it is thought by most railroad managers that it is desirable, for then the relation between the coal consumed and the power developed can be seen; and if the showing is not as good as it should be, the engineer can remind the firemen that they are not exercising as much care in feeding the boilers as they should. Considered in this light, the wattmeter acts as a check to wastefulness on the part of the employees.
The instruments marked C C C C serve the same purpose in connection with the generators as the safety fuses do with respect to the car motors; they are electro-magnetic devices used to open
the generator circuits whenever the current reaches a strength that is sufficient to injure the machine. These devices are called circuit breakers. As will be noticed, there are four located directly above the four a switches, and, at the farther end of the board, a large number located directly above the b switches. The latter act to open the individual circuits when the currents flowing in them become too strong, and the former are controlled entirely by the current of the generator circuits. A circuit breaker is more reliable than a safety fuse, because it acts quicker. With the fuse the current must act for some time before it can melt the metal, as a sufficient amount of heat can not be generated instantly. With the circuit breaker, however, the action-is instantaneous, for as soon as the current reaches the predetermined strength the magnetism of the operative parts of the device becomes sufficiently strong to cause it to act. A circuit breaker is simply a switch that is arranged to be opened automatically by the action of a magnet, instead of by the hand of the operator. The switch part of the apparatus is held in place by a catch that is set much after the fashion of the catch in a mouse trap—that is, so that the least pressure will disengage it. A strong spring acts to throw the switch open, and as soon as the catch is tripped by the actuating magnet the force of the spring comes into action and the circuit is opened.
The circuit breaker is a very valuable apparatus, for it frequently happens that, through delays of one kind or another, a large number of cars concentrate at one point on the road, and, as all the motormen are anxious to make headway, they all start up at once at the first opportunity. If there were no circuit breakers at the power house the result would be that some of the generators would be greatly overloaded and perhaps disabled; but, owing to the presence of the circuit breakers, the actual result is that the circuit is broken, and then the motormen have to wait until the current is turned on again. If too many of them try to start their cars at the second trial the current will again stop. After two or three ineffectual efforts have been made to start all the cars together the motormen will conclude to go easy, and set a few in motion at a time. In this way the cars will become more evenly distributed along the line, and the demand for current at the point of blockade will reduce to the normal amount, or nearly so, and the running of the cars will continue without further interruption, for the current drawn by the motors having been reduced to the average amount, the circuit breaker will cease to act.
The bus bars and all the connections between them and the generators and external circuits, as well as with all the instruments, are located behind the switch board. All these connections are so secured that they can not come in contact with each other except where contacts are required; care is also taken to prevent any connection being made with the iron framing that supports the marble slabs. The front of a switch board is generally very attractive, the surface being of highly polished marble, while all the switches and instruments are finely finished and, as a rule, of decidedly ornamental design.
The switch board might be looked upon as the fountain head from which the entire operation of an electric railway system is controlled. By the movement of one set of switches upon it the generators are thrown in or out of service, and by the movement of another set of switches the several branches of the road are rendered active or inactive.
Note.—The illustrations of railway generator and switchboard were made from photographs kindly furnished by the Westinghouse Electric and Manufacturing Company. For the photographs of the electric truck and car controller we are indebted to the courtesy of the General Electric Company.