Fig. 675.--Connections for two compound wound dynamos to run in parallel. The series fields of the machines are connected together in parallel by means of wire leads or bus bars, which connect together the brushes from which the series fields are taken. This is known as the equalizer and is shown by the line running to the middle pole of the dynamo switch. By tracing out the series circuits it will be seen that current from the upper brush of either dynamo has two connections to its bus bar. One of these leads through its own field, and the other, by means of the equalizer bar, through the fields of the other dynamo. As long as both machines are generating equally there is no difference of pressure between the brushes of either, but should the voltage of one be lowered, current from the other would flow through its fields and thereby raise the voltage, and at the same time reduce its own until both are equal. The equalizer may then be called upon to carry much current, but to have the machines regulate closely it should be of low resistance. It may also be run as shown by the dotted lines, but this will leave all the machines alive when any one is generating. The ammeters should be connected as shown. If they were on the other side they would come under the influence of the equalizing current and would indicate wrong, either too high or too low. The equalizer switch should be closed a little before the main switches are closed.
Ques. How should a shunt or compound machine be started?
Ans. All switches controlling the external circuits should be opened, as the machine excites best when this is the case. If the machine be provided with a rheostat or hand regulator and resistance coils, these latter should all be cut out of circuit, or short circuited, until the machine excites, when they can be gradually cut in as the voltage rises.
When the machine is giving the correct voltage, as indicated by the voltmeter or pilot lamp, the machine may be switched into connection with the external or working circuits.
Ques. In starting a shunt dynamo, should the main line switch be closed before the machine is up to voltage or after?
Ans. If the machine be working on the same circuit with other machines, or with a storage battery, it is, or course, necessary to make the voltage of the machine equal to that on the line before connecting it in the circuit. If the machine work alone, the switch may be closed either before or after the voltage comes up. The load will be thrown on suddenly if the switch be closed after the machine has built up its voltage, thus causing a strain on the belt, and possibly drawing water over the engine cylinder. On the other hand, if the switch be closed before the voltage of the machine has come up, the load is picked up gradually, but the machine may be slow or may even refuse to pick up at all.
Ques. Why does a shunt machine pick up more slowly if the main switch be closed first?
Ans. Because the resistance of the main line is so much less than that of the field that the small initial voltage due to the residual magnetism causes a much larger current in the armature than in the shunt field. If this be too large, the cross and back magnetizing force of the armature weakens the field more than the initial field current strengthens it, and so the machine cannot build up.
Ques. If a shunt dynamo will not pick up, what is likely to be the trouble?
Ans. The speed may be too slow; the resistance of the external circuit may be too small; the brushes may not be in proper position; some of the electrical connections in the dynamo may be loose, broken or improperly made; the field may have lost its residual magnetism.
Figs. 676 and 677.--Diagrams of ground detectors. Fig. 676, a ground detector switch suitable for mounting on a switch board. The two arms pivoted at their upper ends are connected with an insulating bar A and make contact at their lower ends with two brass strips and a contact button, which are connected to the bus bars and ground, respectively. When the arms are moved to the left, the positive bus bar is connected to the ground through the voltmeter V. In fig. 677 is another form of ground detector. This is known as a lamp ground detector. On a 110 volt system two ordinary lamps are connected in series, while the line connecting the lamps is connected to the ground through a snap switch S. When current is on, the two lamps will burn with equal brilliancy, but at a lower candle power. When the switch S is closed, if the two lines be clear, the brilliancy of the lamps will not be affected, but if there be a ground on the positive side, one lamp will burn brighter, the brightness depending on the resistance of the ground. If there be a dead ground, the lamp will burn to its full candle power.
Ques. What is the indication that the connections between the field coils and armature are reversed?
Ans. If the machine build up when brought to full speed, the connections are correct, but if it fail to build up, the field coils may be improperly connected.
Fig. 678.--Method of correcting reversed polarity in large shunt dynamo by transposing the shunt field leads, and then starting up the machine. As soon as the voltmeter registers any voltage, the dynamo may be stopped and the field leads restored to their original position, when it will be found that the residual magnetism in the pole pieces will usually bring the dynamo up to its polarity and proper voltage. This method has the disadvantages, of the uncertainty as to the machine building up, and that a temporary wire must probably be run from the switchboard to one terminal of the field circuit, which is usually connected to a terminal back of the dynamo frame, so that the flow of current through the field coils may be reversed. With dynamos having laminated field magnet cores of comparatively low residual magnetism, this method may suffice, but in the case of solid field magnetic cores it is not practical. A better method is to disconnect the shunt field leads and temporarily extend them to some other source of direct current. If the current be of higher voltage than the coils are designed for, as for instance 110 volt dynamo and available current 500 volt, caution must be exercised and a suitable resistance be provided to protect the coils. A 500 volt coil, however, may be supplied from 110 volt circuit, providing the field winding to be energized is equipped with a cut off switch having a discharge resistance, so that it may be used to close and break the circuit when the temporary leads have been connected. If the field windings be not so provided, a bank of lamps or some other non-inductive resistance must be connected across the leads between the field magnet coils and the point at which the circuit is to be opened and closed. This is to provide a path for the discharge of the induced electromotive force. The circuit should not remain closed more than a few seconds if the full voltage can be applied. It is well, however, to leave the current on long enough to run the machine up to about half speed and make sure, by means of a voltmeter, that the polarity has been corrected. When this has been ascertained the dynamo should be stopped and the field winding leads returned to their proper terminals. Then the voltage will be brought up in the right direction, provided the work has been done correctly.
This can be tested by connecting a voltmeter across the terminals of the armature, or by means of a magnetic needle placed at a short distance from one of the pole pieces in such a position that it does not point to the north pole. If the field coils be improperly connected, the current due to the initial voltage will weaken the field magnetism and thus prevent the machine building up, and when the field circuit is closed the voltmeter reading will be reduced, or the magnetic needle will be less strongly attracted.
Ques. What will be the result if the connections of some of the field coils of a dynamo be reversed?
Ans. If one-half the number of coils oppose the other half, the field magnetism will be neutralized and the machine will not build up at all; but if one of the coils be opposed to the others, the machine might build up, but the generated voltage will be low, and there will be considerable sparking at some of the brushes.
Ques. How may it be ascertained which coil is reversed?
Ans. In all dynamos there should be an equal number of positive and negative poles, and in almost all of them the poles should be alternately positive and negative. Therefore, if a pocket compass be brought near the pole pieces, and it show that there are more poles of one kind than the other, the indication is that one or more of the coils are reversed, and the improper sequence of alternation will determine which one is wrongly connected.
Ques. When a dynamo loses its residual magnetism, how can it be made to build up?
Ans. By temporarily magnetizing the field. To do this a current is passed through it from another dynamo, or from the cells of a small primary battery. Usually, this will set up sufficient initial magnetism to allow the machine to build up. The battery circuit should be broken before the machine has built up to full voltage.
Ques. What should be done if a dynamo become reversed by a reversal of its field magnetism due to lightning, short circuit, or otherwise?
Ans. The residual magnetism should be reversed by a current from another dynamo, or from a battery; but if this be not convenient, the connections between the machine and the line should be crossed so that the original positive terminal of the dynamo will be connected to the negative terminal of the line, and vice versa.
Fig. 679.--Method of correcting reversed polarity in compound wound dynamo. The polarity may be reversed without disconnecting or changing the wire. The figure shows two compound dynamos, and essential connections. The current from any machine connected to the equalizer bar by its equalizer switch will divide, a portion going through the series field winding of the other machines connected to the bus bar, the division being determined by the resistance of the different sets of coils. For instance, assume that No. 1 dynamo has had its polarity reversed and that No. 2 is running connected to the bus bar. The method of reversing the polarity of No. 1 machine is as follows: No. 1 machine should be at rest and then make sure that the circuit breaker and negative switch are open and that any other special connections to other machine or station lighting circuits are open. Then close the positive and equalizer switches, thus allowing a part of the current from the other dynamo to pass through the equalizer connection and through the series field winding of No. 1 machine in the usual direction, which will magnetize the magnetic core. If No. 1 machine be a large unit and No. 2 a small unit, it will be necessary to cut out the resistance of the shunt field circuits by means of the rheostat, if it be desired to maintain its bus bar voltage at its normal point. This will rob the series winding of any other machines which may be connected to the bus bars and will lower the voltage slightly. No. 1 machine is then brought up to full speed when it will be found to have recovered its correct polarity. The positive switch may be readily opened, watching the bus bar voltage closely as it will rise when the current is restricted again to the series field winding of the other machines. The dynamo will then be ready to cut in with the other machines as soon as the voltage has been brought up to the proper point, or it may be shut down until required.
Ques. Can a dynamo be reversed by reversing the connections between the field coils and the armature?
Ans. No, for if these connections be reversed, the machine will not build up.
Ques. Will a dynamo build up if it become reversed?
Ans. Yes.
Ques. Then what is the objection to a reversed dynamo?
Ans. Since the direction of current of a reversed dynamo is also reversed, serious trouble may occur if it be attempted to connect it in parallel, with other machines not reversed.
Attention while Running.--When a dynamo is started and at work, it will need a certain amount of attention to keep it running in a satisfactory and efficient manner. The first point to be considered is the adjustment of the brushes. If this be neglected, the machine will probably spark badly, and the commutator and brushes will frequently require refitting to secure good contact.
Ques. What may be said with respect to the lead of the brushes?
Ans. The lead in all good dynamos is very small, and varies with the load and class of machine. The best lead to give to the brushes can in all cases be found by rotating the rocker and brushes in either direction to the right or left of the neutral plane until sparking commences, increasing with the movement. The position midway between these two points is the correct position for the brushes, for at this position the least sparking occurs, and it is at this position that the brushes should be fixed by clamping the rocker.
Fig. 680.--Method of taking temperature. In taking the temperature of a hot part, it is convenient to use a thermometer in which the scale of degrees has been etched on the stem. Bind this to the heated part, having first taken the precaution to cover the bulb with waste to prevent the radiation of heat and take the reading when the column of mercury has ceased to rise. The question which most often presents itself to the attendant is how hot can the various parts of a dynamo or motor become and yet be within the safe limit. The degree of heat can be determined by applying the hand to the various parts. If the heat be bearable it is entirely harmless, but if the heat become unbearable to the hand for more than a few seconds, the safety limit has been reached and the machine should be stopped and the fault located. Of course when the solder begins to melt at the commutator connections and shellac begins to "fry out" of the armature and an odor of burnt cotton begins to pervade the air, the safe limit has been far exceeded, and in most cases, as a matter of fact serious damage is the result. To be more definite, no part of the dynamo or motor should be allowed to rise in temperature more than 80 degrees F. above the temperature of the surrounding air, excepting in the case of commutators where no solder has been used to connect the leads. These can be allowed to rise to a still higher temperature.
Ques. How does the lead vary in the different types of dynamo?
Ans. In series dynamos giving a constant current, the brushes require practically no lead. In shunt and compound dynamos the lead varies with the load, and therefore the brushes must be rotated in the direction of rotation of the armature with an increase of load, and in the opposite direction with a decrease of load.
In cases where the dynamos are subjected to a rapidly varying or fluctuating load, it is of course not possible to constantly shift the brushes as the load varies, therefore the brushes should be fixed in the positions where the least sparking occurs at the moment of adjustment. If at any time violent sparking occur, which cannot be reduced or suppressed by varying the position of the brushes by rotating the rocker, the machine should be shut down at once, otherwise the commutator and brushes are liable to be destroyed, or the armature burnt up. This especially refers to high tension machines.
Ques. What should be done if the brushes begin to spark excessively?
Ans. First, look at the ammeter to see if an excessive amount of current is being delivered; second, see if the brushes make good contact with the commutator, and if the latter have a bar too high, or too low, and an open circuit.
Figs. 681 and 682.--Remedies for leakage of oil from self-oiling bearings. If there be sufficient space, a metal ring may be attached to the shaft as in fig. 681. With this arrangement the high speed of the shaft will carry the oil outside of the ring and throw it off in the oil reservoir. Another way is to insert a tin apron, as shown in fig. 682 at T, which will serve to drain the oil which may creep along the shaft, and also cut off the draft from the pulley which may suck the oil out of the bearing. Sometimes a tin fan is attached to the pulley, which tends to drive the oil back into the bearing, and which also assists in keeping the box cool.
Ques. What should be done if the current be excessive?
Ans. If the current exceed the rated capacity by more than 50 per cent., and continue for more than a few minutes, the main switch should be opened, otherwise the machine may be seriously injured.
Ques. How does an excessive current injure a dynamo?
Ans. By causing it to overheat which destroys the insulation of the armature, commutator, etc.
Lubrication.--The shaft bearings of dynamos may be lubricated by sight feed oilers or oil rings. The latter method is almost universally used. An oil well is provided in the hollow casting of the pedestals as shown in fig. 728. Oil rings revolve with the shaft and feed the latter with oil, which is continuously brought up from the reservoir below. The dirt settles to the bottom and the upper portion of the oil remains clear for a long period, after which it is drawn off through the spigot and a fresh supply poured in through openings provided in the top. The latter are often located directly over the slots in which the rings are placed, so that the bearings can be lubricated directly by means of an oil cup, if the rings fail to act or the reservoir become exhausted.
Fig. 683.--Imaginative view of a shaft showing its rough granular structure. In operation these minute irregularities interlock and act as a retarding force, or frictional resistance. Hence, the necessity for lubrication--a lubricant presents a thin intervening film against which the surfaces rub.
Ques. What kind of oil can should be used in filling the reservoir, or oil cups?
Ans. One made of some non-magnetic material such as copper, brass, or zinc.
If iron cans be used, they are liable to be attracted by the field magnets, and thus possibly catch in the armature.
Ques. What is the indication of insufficient lubrication?
Ans. The bearings become unduly heated.
Ques. What precaution should be taken with new dynamos?
Ans. They are liable to heat abnormally and for the first few days they should be carefully watched and liberally supplied with oil.
After a dynamo has been running for a short time under full load, its armature imparts a certain amount of heat to the bearings, a little more also to the bearing on the commutator end of shaft; beyond this there is no excuse for excessive heating. The latter may result from various causes, some of which are given with their remedies, as follows:
- A poor quality of oil, dirty or gritty matter in the oil;
- Journal boxes too tight;
- Rough journals, badly scraped boxes;
- Belt too tight;
- Bearings out of line;
- Overloaded dynamo;
- Bent armature shaft.
Ques. What is the allowable degree of heating?
Ans. It may be taken as a safe rule that no part of a working dynamo should have a temperature of more than 80° Fahr. above that of the surrounding air.
Accordingly, if the temperature of the engine room be noted before applying the thermometer to the machine, it can at once be seen if the latter be working at a safe temperature. In taking the temperature, the bulb of the thermometer should be wrapped in a woolen rag. The screws and nuts securing the different connections and cables should be examined occasionally, as they frequently work loose through vibration.
Fig. 684.--Diagram illustrating forces acting on a dynamo armature. In the figure the normal field magneto-motive force is in the direction of the line 1, 2, produced by the field circuit G, if there were no current in the armature. But as soon as the armature current flows, it produces the opposing force 3, 4, which must be combined with 1, 2 to give the resulting force to produce magnetism and hence voltage. The resultant 1, 5, if 3, 4 be large enough, does not differ much from the original force 1, 2. Or, expressed in a more physical way, the brushes E, F, rest on the commutator and all the turns embraced by twice the angle 6, 3, F, oppose the flow of flux through the armature core as well as all the turns embraced by twice the angle, 7, 3, E. The remaining turns distort the flux, making the pole corners at A and B denser, and at C and D rarer. So that all the effect is to kill an increase of flux, or voltage. This cross magnetism tends also to decrease the flow of flux, for the extra ampere turns required to force the flux through the dense pole tips are greater than the decreased ampere turns relieved by the reduction of flux at the other pole tips; this follows, since iron as it increases in magnetic density requires ampere turns greater in proportion than the increase of flux.
Instructions for Stopping Dynamos.--When shutting down a machine, the load should first be gradually reduced, if possible, by easing down the engine; then when the machine is supplying little or no current, the main switch should be opened. This reduces the sparking at the switch contacts, and prevents the engine racing.
When the voltmeter almost indicates zero, the brushes should be raised from contact with the commutator. This prevents the brushes being damaged in the event of the engine making a backward motion, which it often does, particularly in the case of a gas engine. On no account, however, should the brushes be raised from the commutator while the machine is generating any considerable voltage; for not only is the insulation of the machine liable to be damaged, but in the case of large shunt dynamos, the person lifting the brushes is liable to receive a violent shock.
Ques. What attention should the machine receive after it has been shut down?
Ans. It should be thoroughly cleaned. Any adhering copper dust, dirt, etc., should be removed from the armature by dusting with a stiff brush, and the other portions of the machine should be thoroughly cleaned with linen rags. Waste should not be used, as it is liable to leave threads or fluff on the projecting parts of the machine, and on the windings of the armature, which is difficult to remove.
Ques. What attention should be given to the brushes and brush gear?
Ans. They should be examined and thoroughly cleaned. If necessary the brushes should be refitted and readjusted. All terminals, screws, bolts, etc., should be carefully cleaned and screwed up ready for the next run. The brush holders should receive special attention, as when dirty, they are liable to stick and cause sparking. All dirt and oil should be removed from the springs, contacts, pivots, and other working parts.
It is advisable at stated intervals to entirely remove the brush holders from the rocker arms, and give them a thorough cleaning by taking them to pieces, and cleaning each part separately with emery cloth and benzoline or soda solution.
Another point to which particular attention should be given is the cleaning of the brush rocker. This being composed wholly of metal, and the two sets of positive and negative brushes being only separated from it by a few thin insulating washers, it follows that if any copper dust given off by the brushes be deposited in the neighborhood of these washers, there is considerable liability for a short circuit of the machine to occur by the dust bridging across the insulation.
Ques. What further attention should be given?
Ans. It is a good plan, when the machine has been thoroughly cleaned and all connections made secure, to occasionally test the insulation of the different parts. If a record be kept of these tests, any deterioration of the insulation can at once be detected, localized and remedied before it has become sufficiently bad to cause a breakdown.
As a means of protecting the machine from any moisture, dirt, etc., while standing idle, it is advisable to cover it with a suitable waterproof cover.
Series and Parallel Connections.--When it is necessary to generate a large and variable amount of electrical energy, as must be done in central generating stations, apart from the question of liability to breakdown, it is neither economical nor desirable that the whole of the energy should be furnished from a single dynamo. Since the efficiency of a dynamo is dependent upon its output at any moment, or the load at which it is worked (the efficiency varying from about 95 per cent. at full load to 80 per cent. at half load), it is advisable in order to secure the greatest economy in working, to operate any dynamo as near full load as possible.
Under the above circumstances, when the whole of the output is generated by a single dynamo this can evidently not be effected, for the load will naturally fluctuate up and down during the working hours, as the lamps, motors, etc., are switched into and out of circuit; hence, although the dynamo may be working at full load during a certain portion of the day, at other times it may probably be working below half load, and therefore the efficiency and economy in working in such an arrangement is very low.
Ques. How is maximum efficiency secured with variable load?
Ans. It is usual to divide up the generating plant into a number of units, varying in size, so that as the load increases, it can either be shifted to machines of larger size, or when it exceeds the capacity of the largest dynamo, the output of one can be added to that of another, and thus the dynamos actually at work at any moment can be operated as nearly as possible at full load.
Ques. What should be noted with respect to connecting one dynamo to another?
Ans. It is necessary to take certain precautions (as later explained) in order that the other dynamos may not be affected by the change, and that they may work satisfactorily together.
Ques. What are the two methods of coupling dynamos?
Ans. They are connected in series, or in parallel.
In coupling dynamos in series, the current capacity of the plant is kept at a constant value, while the output is increased in proportion to the pressures of the machines in circuit.
When connected in parallel, the pressures of all the machines are kept at a constant value, while the output of the plant is increased in proportion to the current capacities of the machines in circuit.
Coupling Series Dynamos in Series.--Series wound dynamos will run satisfactorily together without special precautions when coupled in series, if the connections be arranged as in fig. 685.
The positive terminal of one dynamo is connected to the negative terminal of the other, and the two outer terminals are connected directly to the two main conductors or bus bars through the ammeter A, fuse F, and switch S. If it be desired to regulate the pressure and output of the machines, variable resistances, or hand regulators R, R1, may be arranged as shunts to the series coils as shown, so as to divert a portion or the whole of the current therefrom.
Series Dynamos in Parallel.--Simple series wound dynamos not being well adapted for the purpose of maintaining a constant pressure, are in practice seldom coupled in parallel; the conditions or working, however, derive importance from the fact that compound dynamos, being provided with series coils, are subject to similar conditions when working in parallel, which is frequently the case.
Ques. What may be said with respect to coupling two or more plain series dynamos in parallel?
Ans. The same procedure cannot be followed as in the case of plain shunt dynamos, for the reason that if the voltage of the dynamo to be coupled be exactly equal to that of the bus bars when connected in parallel, the combination will be unstable.
Fig. 685.--Diagram showing method of coupling series dynamos in series. R and R' are two hand regulators which are placed in shunt across the coil terminals to regulate the pressure and output of the machine.
Ques. Why is this?
Ans. If, from any cause, the pressure at the terminals of one of the dynamos fall below that of the others, it immediately takes a smaller proportion of the load; as a consequence, the current in its field coils is reduced, and a further fall of pressure immediately takes place. This again causes the dynamo to relinquish a portion of its load, and again occurs a further fall of pressure. Thus the process goes on, until finally the dynamo ceases to supply current, and the current from the other dynamos flowing in its field coils in the reverse direction reverses its magnetism, and causes it to run as a motor against the driving power in the opposite direction to that in which it previously ran as a dynamo.
Under such circumstances the armature is liable to be destroyed if the fuse be not immediately blown, and in any case it is subjected to a very detrimental shock. This tendency to reverse in series dynamos can be effectually prevented by connecting the field coils of all the dynamos in parallel.
Fig. 686.--Diagram showing method of coupling series dynamos in parallel. In the diagram A, A', are ammeters; F, F', fuses; S, S', switches.
Ques. How are the field coils of all the dynamos connected in parallel?
Ans. This is effected in practice by connecting the ends of all the series coils where they join on to the armature circuit by a third connection, called the "equalizing connection," or "equalizer," as shown in fig. 686.
Ques. What is the effect of the equalizer?
Ans. The immediate effect is to cause the whole of the current generated by the plant to be divided among the series coils of the several dynamos in the inverse ratio of their resistance, without any regard as to whether this current comes from one armature, or is divided among the whole. The fields of the several dynamos being thus maintained constant, or at any rate being caused to vary equally, the tendency for the pressure of one dynamo to fall below that of the others is diminished.
Shunt Dynamos in Series.--The simplest operation in connection with the coupling of dynamos, and the one used probably more frequently in practice than any other, is the coupling of two or more shunt dynamos to run either in series or in parallel. When connected in series, the positive terminal of one machine is joined to the negative of the other, and the two outer terminals are connected through the ammeter A, fuses F, F', and switch S, to the two main conductors or omnibus bars as represented in fig. 687. The machine will operate when the connections are arranged in this manner, if the ends of the shunt coils be connected to the terminals of their respective machines.
Shunt Dynamos in Parallel.--The coupling of two or more shunt dynamos to run in parallel is effected without any difficulty. This method of coupling dynamos is one that is very frequently used. Fig. 688 illustrates diagrammatically the method of arranging the connections. The positive and negative terminals of each machine are connected respectively to two massive insulated copper bars, shown at the top of the diagram, called omnibus bars, through the double pole switches, S, S', and the double pole fuses F, F'. Ammeters, A, A' are inserted in the main circuit of each machine, and serve to indicate the amount of current generated by each. An automatic switch or cutout, Ac, Ac', is also shown as being included in the main circuit of each of the machines, although this appliance is sometimes dispensed with. The pressure of each of the machines is regulated independently by means of the hand regulators R, R', inserted in series with the shunt circuit.
The shunt circuits are represented as being connected to the positive and negative terminals of the respective machines, but in many cases where the load is subjected to sudden variations, and when a large number of machines is connected to the bus bars, the shunt coils are frequently connected direct to these. In such circumstances this method is preferable, as by means of it the fields of the idle dynamos can be excited almost at once direct from the bus bars by the current from the working dynamos; hence, if a heavy load come on suddenly, no time need be lost in building up a new machine previous to switching it into parallel. The pressure of the lamp circuit is given by a voltmeter whose terminals are placed across the bus bars; and the pressure at the terminals of each of the machines is indicated by separate voltmeters or pilot lamps, the terminals of which are connected to those of the respective machines.
Fig. 687.--Diagram showing method of coupling shunt dynamos in series. The ends of the shunt coils may be connected to the terminals of their respective machine, or they may be connected in series as shown.
Ques. Describe a better method of parallel connection.
Ans. Better results are obtained by connecting both the shunt coils in series with one another, so that they form one long shunt between the two main conductors, the same as in fig. 687.
When arranged in this way, the regulation of both machines may be effected simultaneously by inserting a hand regulator (R) in series with the shunt circuit as represented.
Fig. 688.--Diagram showing method of coupling shunt dynamos in parallel.
Switching Dynamo Into and Out of Parallel.--In order to put an additional dynamo in parallel with those already working, it is necessary to run the new dynamo up to full speed, and, when it excites, regulate the pressure by means of a hand regulator until the voltmeter connected to the terminals of the machines registers one or two volts more than the voltmeter connected to the lamp circuit, and then close the switch. The load upon the machine can then be adjusted to correspond with that upon the other machines by means of the hand regulator.
Ques. In connecting a shunt dynamo to the bus bars, must the voltage be carefully adjusted?
Ans. There is little danger in overloading the armature in making the connection hence the pressure need not be accurately adjusted.
It is, in fact, common practice in central stations to judge the voltage of the new dynamo merely by the appearance of its pilot lamp.
Ques. How is a machine cut out of the circuit?
Ans. When shutting down a machine, the load or current must first be reduced, by gradually closing the stop valve of the engine, or inserting resistance into the shunt circuit by means of the hand regulator; then when the ammeter indicates nine or ten amperes, the main switch is opened, and the engine stopped.
By following this plan, the heavy sparking at the switch contacts is avoided, and the tendency for the engine to race, reduced.
Ques. What precaution must be taken in reducing the current?
Ans. Care must be taken not to reduce the current too much.
Ques. Why is this necessary?
Ans. There is danger that the machine may receive a reverse current from the other dynamos, resulting in heavy sparking at the commutator, and in the machine being driven as a motor.
Ques. What provision is made to obviate this danger?
Ans. Dynamos that are to be run in parallel are frequently provided with automatic cutouts, set so as to automatically switch out the machine when the current falls below a certain minimum value.
Dividing the Load.--If a plant, composed of shunt dynamos running in parallel, be subjected to variations of load, gradual or instantaneous, the dynamos will, if they all have similar characteristics, each take up an equal share of the load. If, however, as is sometimes the case, the characteristics of the dynamos be dissimilar, the load will not be shared equally, the dynamos with the most drooping characteristics taking less than their share with an increase of load, and more than their share with a decrease of load. If the difference be slight, it may be readily compensated by means of the hand regulator increasing or decreasing the pressures of the machines, as the load varies. If, however, the difference be considerable, and the fluctuations of load rapid, it becomes practically impossible to evenly divide the load by this means.
Under such circumstances, the pressure at the bus bars is liable to great variations, and there is also liability of blowing the fuses of the overloaded dynamos, thus precipitating a general breakdown. To cause an equal division of the load among all the dynamos, under such circumstances, it is needful to insert a small resistance in the armature circuits of such dynamos as possess the straightest characteristics, or of such dynamos as take more than their share of an increase of load. By suitably adjusting or proportioning the resistances, the pressures at the terminals of all the machines may be made to vary equally under all variations of load, and each of the machines will then take up its proper share of the load.
Coupling Compound Dynamos in Series.--Since compound dynamos may be regarded as a combination of the shunt and series wound machines, and as no special difficulties are encountered in running these latter in series, analogy at once leads to the conclusion that compound dynamos under similar circumstances may be coupled together with equal facility.
Ques. How are compound dynamos connected to operate in series?
Ans. The series coils of each are connected as in fig. 685, and the shunt coils are connected as a single shunt as in fig. 687, which may either extend simply across the outer brushes of the machines, so as to form a double short shunt, or may be a shunt to the bus bars of external circuit, so as to form a double long shunt.