Series-wound Dynamo Machines.
Year: 1885

page_text: Series-wound Dynamo Machines.
We WILLIAM HENRY ALLEN and RICHARD WRIGHT both of York Street Works,
Lambeth in the County of Surrey, Engineers, and GISBERT KAPP, of Warley in the
County of Essex, Engineer do hereby declare the nature of our invention for
"IMPROVEMENTS IN THE METHOD OF EXCITING THE FIELD-MAGNETS OF SERIES-
WOUND DYNAMO MACHINES" to be as follows:-
Our invention consists in improvements in the method of exciting the field-
magnets of series wound dynamo machines, whereby the current and the electro-
motive force given by such machines can be automatically regulated to suit special.
requirements.
It is well known that the electromotive force produced at the terminals of a
series wound dynamo having wrought iron or cast iron field magnets depends on
the current flowing through the dynamo and outer circuit. If the resistance of the
outer circuit be increased beyond a certain limit the current passing through the
field-magnet-coils becomes weakened to such an extent that it is insufficient to
maintain the magnetization of the field magnets and the machine ceases to be self
exciting.
It is also well known that for this reason the ordinary series wound dynamo
machine is not suitable for use on circuits where a constant or nearly constant
electromotive force must be maintained whilst the current varies from nothing to
maximum,
Our improved method of exciting the magnet coils of a series wound dynamo
overcomes this difficulty and enables us to maintain the electro-motive force at the
terminals of the dynamo or at any point of the circuit constant or nearly constant
whilst the current in the circuit may be varied from nothing to maximum. This
result we obtain by joining the terminals of the series coils (in addition to the
usual connection with arinature and outer circuit) with an auxiliary source of
electricity the nature of which we shall presently describe.
We make the connections in such way that the current circulating through the
series coils is the sum of the current given by the dynamo and of the auxiliary
Current We arrange the auxiliary current to be of an intensity just sufficient to
magnetize the field magnets to such a degree that the dynamo when running on
open circuit shall give the required electromotive force. If now the outer circuit

page_text: be closed through a certain resistance a current will flow, which, added to the
auxiliary current in the series coils, increases the magnetizing power of the latter
sufficiently to maintain the external electromotive force approximately or even
absolutely constant.
The source of the auxiliary current may be a primary or secondary battery, or a
separate small series wound dynamo, the terminals of which are connected to the
terminals of the series coils of the main dynamo, or we may mount an auxiliary
armature and commutator side by side with the main armature on the same spindle
and thus utilize the main field magnets for the production of the auxiliary current,
or we may provide the armature of the dynamo with auxiliary coils wound between
the main armature coils and connected in the usual way to a second or auxiliary
commutator, thus utilizing the armature core and the field magnets of the main
dynamo for the production of the auxiliary current.
The external electromotive force of our auxiliary source of electricity should be
at least equal to the difference of potential which must be maintained between the
terminals of the magnet coils in order to produce a magnetising current sufficiently
strong to give the desired electromotive force in the main dynamo when the latter
is working in open circuit; but some times we prefer to employ an auxiliary source
of electricity having a slightly higher electromotive force and to introduce sufficient
resistance between the auxiliary source of electricity and the terminals of the
magnet coils to bring the auxiliary current down to the proper intensity.
We sometimes make this resistance adjustable either by hand or automatically
by any well known method' and by these ineans we can vary within certain limits
the electromotive force of the dynamo whilst the same is at work.
If it is desirable that the current given by a series wound dynamo should be kept
approximately constant or if it is required that the current. should not be increased
to any dangerous extent whilst. the resistance in the external circuit varies from'
maximum. to zero, we also employ an auxiliary, source of electricity to excite the
field magnets, but we reverse the connection between the, armature. field magnet.
coils and outer circuit, so that the current circulating through the series coils is the
difference between the auxiliary current and the main current. Any abnormal or
dangerous increase of the main current weakens therefore the magnetising, power
of the series coils and thus reduces the electromotive force of the dynamo as the
resistance in the outer circuit is reduced.

page_text: Improvements in the Method of Exciting the Field-magnets of
Series-wound Dynamo Machines.
We, WILLIAM HENRY ALLEN and RICHARD WRIGHT, both of York Street Works,
Lambeth, in the County of Surrey, Engineers, and GISBERT KAPP, of Warley, in
the County of Essex, Engineer, do hereby declare the 'nature of our invention and
in what manner the same is to be performed, ito be particularly described and
ascertained in and by the following statement and accompanying drawing (that is
to say).:--
Our invention consists in improvements in the method of exciting the field
magnets of series wound dynamo machines, whereby the current and the electro-
motive force given by such machines can be automatically regulated to suit special
requirements.
It is well known that the electromotive force produced at the terminals of a
series wound dynamo 'having wrought iron or cast iron field magnets depends on
the current flowing through the dynamo and outer circuit. If the resistance of the
outer circuit be increased beyond a certain limit the current passing through the
field-magnet-coils becomes weakened to such an extent that it is insufficient to
maintain the magnetization of the field magnets and the machine ceases to be self
exciting.
.
It is also well known that for this reason the ordinary series-wound dynamo
machino is not suitable for use on circuits where a constant or nearly constant
electromotive force must be maintained whilst the current varies from nothing to
maximum.
An improved method of exciting the magnet coils of a series wound dynamo
overcomes this difficulty and enables us to maintain the electro-motive force at the
terminals of the dynamo or at any point of the circuit constant or nearly constant
whilst the current in the circuit may be varied from nothing to maximum. This
result we obtain by joining the terminals of the series coils (in addition to the
usual connection with armature and outer circuit) with an auxiliary source of
electricity the nature of which we shall presently describe.
We make the connections in such way that the current circulating through the
series coils is the sum of the current given by the dynamo and of the auxiliary
current. We arrange the auxiliary current to be of an intensity just sufficient to
magnetize the field magnets to such a degree that the dynamo when running on
open circuit shall give the required electromotive force. If now the outer circuit
be closed through a certain resistance a current will flow, which, added to the
auxiliary current in the series coils, increases the magnetizing power of the latter
sufficiently to maintain the external electromotive force approximately or even
absolutely constant.
The source of the auxiliary current may be a primary or secondary battery, or a
separate small series wound dynamo, the terminals of which are connected to the
terminals of the series coils of the main dynamo, or we may mount an auxiliary
armature and commutator side by side with the main armature on the same spindle

page_text: and thus utilize the main field magnets for the production of the auxiliary current,
or we may provide the armature of the dynamo with auxiliary coils wound
between the main armature coils and connected in the usual way to a second or
auxiliary commutator, thus utilizing the armature core and the field magnets of
the main dynamo for the production of the auxiliary current.
The external electromotive force of our auxiliary source of electricity should be
at least equal to the difference of potential which must be maintained between the
terminals of the magnet coils in order to produce a magnetizing current sufficiently
strong to give the desired electro motive force in the main dynamo when the latter
is working on open circuit; but sometimes we prefer to employ an auxiliary source
of electricity having a slightly higher electromotive force and to introduce sufficient
resistance between the auxiliary source of electricity and the terminals of the
magnet coils to bring the auxiliary current down to the proper intensity.
We sometimes make this resistance adjustable either by hand or automatically
by any well known method and by these means we can vary within certain limits
the electromotive force of the dynamo whilst the same is at work. If it is
desirable that the current given by a series wound dynamo should be kept
approximately constant or if it is required that the current should not be increased
to any dangerous extent whilst the resistance in the external circuit varies from
maximum to zero, we also employ an auxiliary source of electricity to excite the
field magnets, but we reverse the connection between the armature field magnet
coils and outer circuit, so that the current circulating through the series coils is the
difference between the auxiliary current and the main current. Any abnormal or
dangerous increase of the main current weakens therefore the magnetizing power
of the series coils and thus reduces the electromotive force of the dynamo as the
resistance in the outer circuit is reduced.
In the accompanying diagrams (which are in illustration of our invention)
figures 1, 2, 3, 4, and 5, represent different modifications of our invention as
applied to series wound dynamo machines, if the condition to be fulfilled is that
the electromotive force at the terminals of the machine or at some point in the
external circuit shall remain as nearly as possible constant whilst the external
current may vary from zero to maximum.
A represents the armature of the dynamo, which may be of the Gramme, Pacinotti,
Hefner Alteneck, or any other type suitable for the production of a continuous
current. We keep this armature revolving at a constant speed. C represents the
commutator, B² represents the positive brush, that is to say that brush where the
positive current leaves the commutator and B¹ represents the negative brush.
The field magnet coil is represented by the undulating line F2 F¹ and in all the
diagrams it is supposed that the direction of winding of this coil and the direction
of winding of the armature coils are such that B2 becomes positive as regards B¹, if
the current traverses the field magnet coil in the direction from F2 to F, as shown
by the arrow. We do not in the diagrams show any particular type of field
magnets or armature and we do not show any constructive detail appertaining to
any special type of dynamo, because our invention is applicable to all types of
continuous current dynamos. Those conversant with the construction of dynamo
machines will be able by the aid of the diagrams to apply our invention to any
given type of dynamo. L' is the positive and L' is the negative external lead,
and the lamps, or other devices requiring a constant difference of potential, are
represented by the small circles and are supposed to be coupled in parallel
connection across the main leads L, L'. In figure 1 the auxiliary source of
electricity is a primary or secondary battery a, the current from which passes
through a variable resistance R and through the field magnet coils F' F¹.
If the external circuit L' L² be open, the magnetizm of the field magnets is due
to the auxiliary current only, which must be of such strength as to magnetize the
field 'magnets to such a degree that the internal electromotive force of the armature
is equal to that which it is desired to maintain in the external circuit plus the
difference of potential between F2 and F". If now a current be permitted to flow

page_text: through the external circuit and armature A a certain loss of electromotive force
will be occasioned by the resistance of the armature and circuit and by the effect
of self induction in the armature coils. In order to obtain perfect regulation this
loss must be counterbalanced by an equal increase in the internal electromotive
force of the armature and this increase according to our invention is obtained by
an increase of current through the field magnet coil.
We find it advantageous to employ a battery having an appreciable internal
resistance and in some cases we add the artificial resistance R. The reason is as
follows:-
Supposing that the internal resistance of the battery and that of the connections
with the terminals of the series coil F F1 were inappreciable, then the difference of
electromotive force at these two terminals would be approximately equal to that of
the battery and therefore constant; whatever may be the current passing through
the armature. In this case the strength of the magnetic field and the internal
electromotive force of the armature would be constant, but the external electro-
motive force would on account of resistance and self induction, as mentioned above,
decrease with an increasing current. This arrangement would therefore be no
better than a magneto machine or a dynamo machine separately excited in the
usual way.
But if the circuit of the battery contains some appreciable resistance, we can by
a proper choice of that resistance and of the electromotive force of the battery, .
obtain almost perfect regulation. To determine these quantities we proceed as
follows: We determine by experiment or calculation from the electrical data of the
dynamo what difference of potential exists between the terminals F2 F1 of the field
magnet coil, if the dynamo is giving the maximum current and if it be running at
such a speed as to give the desired electromotive force in the external circuit, when
the auxiliary source of electricity, (in this case a battery) is removed.
The electromotive force of the battery we arrange to be equal to the difference
of potential thus determined. If we now connect the battery in the manner shewn
in diagram Figure 1 no current will pass through it and no alteration will be
produced in the working of the dynamo as long as we keep the external resistance
of the circuit unaltered. We next determine what exciting current is required to
produce such an electromotive force in the armature of the dynamo that, when.
working on open circuit, the difference of potential between the leads L² L' is
again maintained at the desired value. This determination is best made by an
experiment or by using the characteristic curve of the dynamo in a manner well
known to electricians. By dividing the electromotive force of the battery by the
minimum exciting current thus obtained, we find the resistance of the auxiliary
circuit, which is the sum of the internal resistance of the battery, the connections,
the resistance R, and the resistance of the field magnet coils. The latter is a
quantity already fixed and by deducting it from the figure found as here described
we determine the resistance of battery, connections, and of R.
"
In figure 2 the auxiliary source of electricity is a small series wound dynamo
kept running at a constant speed; a is the armature of this auxiliary dynamo, c its
commutator 62 6¹ are its brushes, f, f, is its field magnet coil and R an additional
resistance which may be variable either by hand or automatically.
The electromotive force of the auxiliary dynamo and the resistance of the
auxiliary circuit comprising the armature, a, the coil ƒ f, the coil F F¹ the
resistance R and that of the rest of the connections we determine in a manner
similar to that already described.
There is however this difference, that whereas the electromotive force of the
battery has been so chosen that there shall be no auxiliary current when the
dynamo is giving the full output, the electromotive force of the auxiliary dynamo
must be chosen sufficiently high so that there shall be an auxiliary current even
when the output of the dynamo is a maximum. This is necessary in order to
prevent the auxiliary dynamo having its polarity reversed. We do not consider
this arrangement as satisfactory as the one previously described or as the one

page_text: represented by figure 3, where the auxiliary source of electricity is a small shunt
wound dynamo. The direction of the currents is, as in the other diagrams,
indicated by arrows; a is the auxiliary armature, c its commutator; 69,61 are its
brushes and 8, 8, represents the shunt exciting coil on its field magnets. The
determination of the electromotive force of the auxiliary dynamo and of the
resistances in the auxiliary circuit we make in the same manner as already
described for diagram figure 1.
To avoid the expense and complication of having to provide a separate small
dynamo we sometimes adopt the arrangement represented by figure 4 where a
represents an auxiliary armature supposed to be mounted upon the same spindle
and revolving within the same field as the main armature A. The combination of
auxiliary armature a and field magnet coils F2 F1 forms a shunt dynamo which
may or may not contain a special resistance R; and the combination of main
armature A and field magnet,coils F2 F1 forms a series dynamo.
The electromotive force of the auxiliary armature and the resistances in the
auxiliary circuit we proportion in such manner that, when the dynamo is working
'an open circuit, the auxiliary current, which in this condition alone excites the field
magnets, shall produce a magnetic field of the strength necessary to produce in the
main armature A an electromotive force equal to the difference of potential to be
maintained between the main leads L L plus the loss of potential in the field
magnet coil F2 F¹. If now a current be permitted to flow through the external
circuit by the switching on some of the lamps 7, the exciting current in the field
magnet coil F F1 is the sum of this external current and of the auxiliary.current.
The strength of the field will thereby become somewhat increased and the
electromotive force, both in the main armature A and in the auxiliary armature a
will be increased to a certain extent. To obtain perfect regulation the increase
of electromotive force in the main armature A should be just sufficient to
counterbalance the loss of electromotive force occasioned by the resistance of the
armature and circuit and by the effect of self induction in the armature coils.
find it practically sufficient to determine the proportions of the auxiliary armature
and resistance with regard to two conditions only; namely, when the dynamo is
giving no external current as above explained, and when giving the maximum
current. In the latter case the exciting current through the coil F2 F has its
maximum value, which we determine either experimentally or by the use of the
characteristic curve of the dynamo in the manner well known to electricians.
We
In most cases the value thus found is the same as the maximum external current
itself and if this should not be the case we can alter the number of turns in the
field magnet coil by winding more wire on, or taking some wire off, so that the
maximum external current shall produce that exciting power which experiment or
the characteristic curve shows to be necessary. We next proportion the auxiliary
armature in such way that its electromotive force, when working in the field of
maximum strength, shall be equal to the difference of potential between the
terminals F F¹ of the series coils, when the maximum exciting current, is flowing.
This makes the arrangement right for the condition of maximum current.
make the arrangement right for the condition of open circuit, we determine either
experimentally or by using the characteristic curve of the auxiliary armature,
what resistance R is required to again maintain the difference of potential between
the leads L L at the desired value..
1:
To
Having thus satisfied the condition of constant electromotive force for the two
extreme cases we generally find that this condition is also very nearly, and
sometimes perfectly, fulfilled for intermediate values of the external current.
If we find however that the regulation is not as good as might be desired
throughout the whole range from no current to maximum, we may alter the
electromotive force of the auxiliary armature (by altering the number of turns of
wire upon it or by varying the size of the auxiliary armature, in the usual way
well known to electricians), and the auxiliary resistance so as to obtain better
automatic regulation. In tliese cases we sometimes arrange that there shall be an
ง

page_text: auxiliary current if the dynamo is giving the full output, so that the exciting
power in the field: magnet coil F F¹ may be somewhat greater than corresponds to
the maximum external current alone.
Figure 5 shews an arrangement which enables us to dispense with the separate
auxiliary armature a. In this case we utilize the core of the main armature A by
winding in between its own coils a comparatively small number of auxiliary coils,
which are connected to an auxiliary commutator c supposed to be mounted upon
the same spindle as armature A and main commutator C. We also sometimes
dispense with the separate resistance R (figure 4) by making the auxiliary coils on
armature A of rather smaller guage of wire than the main coils so as to increase
the resistance of the auxiliary circuit to the desired extent. The electrical data
of this arrangement we determine in the same manner as explained in the last
case.
It will be understood that the electro motive force created between the brushes 62
and b¹ depends upon the number of auxiliary coils wound in between the main coils
on the armature A, and by suitably determining this number we obtain exactly-
that amount of electromotive force which we require.
Figures 6 and 7 represent the application of our invention to cases. where it is
desired to maintain the current in the external circuit approximately constant,
whilst the resistance of the external circuit is being varied within certain limits, or
where it is desired to prevent any excessive increase of current taking place, if a
short circuit should accidentally occur between the main leads L'L'.
The lamps or other devices requiring a constant current are in the diagrams
indicated by the crosses marked . In figure 6 the auxiliary source of electricity
is a primary or secondary battery a, sending a current through a separate
resistance R which may be either automatically adjustable or variable by hand,
or which in some cases may be omitted altogether-and through the field magnet
coils in the direction shown by the arrows. The main current from the armature A
tends to pass through the field magnet coils in the direction from F¹ to F' that is
to say, it tends to demagnetize the field. The magnetizing current is therefore the
difference between the current passing through the battery and of the main or
external current. If through the short circuiting of single lamps or through a
short circuit occurring between the main leads L³ L' there should be a tendency to
unduly increase the main current, then the magnetizing current becomes weakened
and the internal electromotive force of the armature becomes lessened, thereby
checking the tendency of such undue increase in the external current.
We may use any of the arrangements shewn in figures 23 and 4 instead of the
battery a, the only difference being that we reverse the connection between the
brushes of the main armature A and the terminals F2 F1 of the field magnet coil
(shewn in figures 2, 3, and 4). We may also adopt the arrangement shewn in
figure 7, where we utilize the core of the armature A by winding a comparatively
small number of auxiliary coils in between the main coils of the armature, and
connect them in the usual way to an auxiliary commutator c supposed to be fixed.
on the same spindle which carries the armature A.and main commutator C. Since
in this case the auxiliary coils have to carry the main or external current as well
as the magnetizing current, we prefer to make them of somewhat stouter wire than
the main armature coils. When the external circuit is open the whole of the
auxiliary current passes through the field magnet coils in the direction from F
to F producing a imaximum difference of potential between the brushes B² B¹. If
now the external circuit be closed through a certain resistance, a current will flow
through it and through the auxiliary coils thereby reducing the difference of
potential between the auxiliary brushes 626¹. The consequence of this is that the
magnetizing current flowing from F' to F¹ becomes somewhat reduced thus also
reducing the difference of potential between the main brushes B, B. If now the
external resistance be further diminished, the action just described takes place again
thus further reducing the difference of potential between the main brushes B2 B₁
and consequently checking any undue increase in the external current..

page_text: We sometimes arrange the resistance R to be variable either by hand or
automatically for the purpose of adjusting the electromotive force or the current
within certain limits whilst the dynamo is at work.
Having now particularly described and ascertained the nature of our said
Invention, and in what manner the same is to be performed, we declare that what
we claim is :-
1. The combination with the field magnet coils forming part of a series wound
dynamo, of an auxiliary source of electricity in the proportions herein before
described and connected with the terminals of said field magnet coils and with
the external circuit in the manner hereinbefore described and illustrated in the
diagrams for the purpose of automatically regulating the external electromotive
force or the external current of said dynamo.
2. The resistance R in combination with the auxiliary source of electricity and
with the field magnet coils of a series wound dynamo, for the purpose of adjusting
the external electromotive force or the external current of said dynamo whilst the
same is at work.