COUPLING OF DYNAMOS
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, R^1, 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: Fig. 689.--Coupling compound dynamos in series; short shunt
connection. The dotted lines indicate the changes that would be made for
long shunt connection.]
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.
[Illustration: Fig. 690.--Diagram showing method of coupling compound
dynamos in parallel.]
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.
[Illustration: Fig. 691.--Diagram showing another and better method
of coupling compound dynamos in parallel. With this arrangement the
idle machines are completely disconnected from those at work. The same
reference letters are common in both diagrams. S, S' are switches; F,
F' fuses; A, A' ammeters, which indicate the total amount of current
generated by each of the machines; AC, AC', automatic switches, arranged
for automatically switching out a machine in the event of the pressure at
its terminals being reduced through any cause; R, R,' are hand regulators,
inserted in the shunt circuits of each of the machines, by means of which
the pressures of the individual machines may be varied and the load upon
each adjusted. The pressure at the bus bars is given by the voltmeter V,
one terminal of which is connected to each of the bars; a second voltmeter
may be used, to give the pressure of any individual machine, by connecting
"voltmeter keys" to the terminals of each of the machines, or a separate
voltmeter may be used for each individual machine. The only essential
difference between figs. 690 and 691 is, that in fig. 690 the equalizer
is connected direct to the positive brushes of all the dynamos, while
in fig. 691 the equalizer is brought up to the switchboard and arranged
between the two bus bars, a switch being fitted for disconnecting it from
the circuit when the machine to which it is connected is not working.]
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.