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.

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.

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.

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.

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.

Compound Dynamos in Parallel.--Machines of this type will not run satisfactorily together in parallel unless all the series coils are connected together by an equalizing connection, as in series dynamos. The method of arranging the connections as adopted in practice, being illustrated in fig. 690. By means of it idle machines are completely disconnected from those at work.

Ques. How is the equalizer connected?

Ans. The equalizer is connected direct to the positive brushes of all the dynamos, a three pole switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working. The two contacts of the switch are respectively connected to the positive and negative conductors, while the central contact is connected to the equalizer.

Switching a Compound Dynamo Into and Out of Parallel.--If the characteristics of all the dynamos be similar, and the connections arranged as in figs. 690, or 691, the only precaution to be observed in switching a new machine into parallel is to have its voltage equal, or nearly equal to that of the bus bars previous to closing the switch. If this be the case, the new machine will take up its due share of the load without any shock.

Ques. How is a compound dynamo, running in parallel, cut out of circuit?

Ans. The load is first reduced to a few amperes, as in the case of shunt dynamos, either by easing down the engine, or by cutting resistance into the shunt circuit by means of the hand regulator, and then opening the switch. Previous to this, however, it is advisable to increase the voltage at the bus bars to a slight extent, as while slowing down the engine the load upon the outgoing dynamo is transferred to the other dynamo armatures, and the current in their series coils not being increased in proportion, the voltage at the bus bars is consequently reduced somewhat.

Equalizing the Load.--When a number of compound dynamos of various output, size, or make, are running together in parallel, it frequently happens that all their characteristics are not exactly similar, and therefore the load is unequally distributed, some being overloaded, while others do not take up their proper share of the work.

NOTE.--The action of an equalizing bar in equalizing the load on compound dynamos run in parallel may be explained as follows: The compound winding of a dynamo raises the pressure in proportion to the current flowing through it, and if, in a system of parallel operated compound dynamos without the equalizing connection, the current given by one machine were slightly greater than the currents from the others, the pressure of that machine would increase. With this increase in pressure above the other machines, a still greater current would flow, and so raise the pressure further. The effect is therefore cumulative, and in time the one dynamo would be carrying too great a proportion of the whole current of the system. With the equalizing connection, whatever the current flowing from each machine, the currents in the various compound windings are all equal, and so the added pressure due to the compound winding is practically the same in each machine. Any inequality in output from the machines is readily eliminated by adjusting the shunt currents by means of the shunt rheostats. When compound wound dynamos are operated in parallel, the equalizer bar insures uniform distribution among the series coils of the machines.

NOTE.--To secure the best results in parallel operation, dynamos should be of the same design and construction and should possess as nearly as possible the same characteristics; that is, each should respond with the same readiness, and to the same extent, to any change in its field excitation. Any number of such machines may be operated in parallel. The usual practice is to connect the equalizer and the series field to the positive terminal, though if desired, they may be connected to the negative terminal; both however, must be connected to the same terminal. The resistance of the equalizer should be as low as possible, and it must never be greater than the resistance of any of the leads from the dynamos to the bus bar. Sometimes a third wire is run to the switchboard from each dynamo and there connected to an equalizer bar, but the usual practice is to run the equalizer directly between the dynamos and to place the equalizer switches on pedestals near the machines. This shortens the connections and leads to better regulation. The positive and equalizer switches of each machine differ in pressure only by the slight drop in the series coil, and in some large stations these two switches are placed side by side on a pedestal near the machine. In such cases, the equalizer and positive bus bars are often placed under the floor near the machines, so that all leads may be as short as possible. If all the dynamos be of equal capacity, all the leads to bus bars should be of the same length, and it is sometimes necessary to loop some of them.

If the difference be small, it may be compensated by means of the hand regulator; if large, however, other means must be taken to cause the machines to take up their due proportion of the load.

If the series coils of the several dynamos be provided with small adjustable resistances, in the form of German silver or copper ribbon inserted in series with the coils, the distribution of the current in the latter may be altered by varying the resistance attached to the individual coils. The effect of the series coils upon the individual armatures in raising the pressure may be adjusted, and the load thus evenly divided among the machines.

Shunt and Compound Dynamos in Parallel.--It is not practicable to run a compound dynamo and a shunt dynamo in parallel, for, unless the field rheostat of the shunt machine be adjusted continually, the compound dynamo will take more than its share of the load.