IMPROVEMENTS IN DYNAMO ELECTRIC MACHINES
Year: 1882

page_text: LETTERS PATENT to Rookes Evelyn Bell Crompton of the Mansion House.
Buildings, Queen Victoria Street in the City of London Engineer and Gisbert
Kapp of Arc Works Chelmsford in the County of Essex Engineer, and in the
employment of the said Rookes Evelyn Bell Crompton, for an Invention of
PROVISIONAL SPECIFICATION left by the said Rookes Evelyn Bell
Crompton and Gisbert Kapp at the Office of the Commissioners of Patents
on the 10th October 1882.
ROOKES EVELYN BELL CROMPTON of the Mansion House Buildings Queen
Victoria Street in the City of London Engineer, and GISBERT KAPP of Arc Works
Chelmsford in the County of Essex Engineer and in the employment of the
said Rookes Evelyn Bell Crompton
"IMPROVEMENTS IN DYNAMO ELECTRIC
MACHINES
"
Our invention relates to improvements in dynamo electric machines and more
particularly to those dynamo electric machines which are required to maintain a
nearly constant electro motive force at their terminals where the external circuit
is joined on, under extreme variations of the external resistance in the circuit
worked by such machines
The ordinary dynamo machine in which the coils which excite the field magnets
are in series circuit with the generating armature does not fulfil this condition,
as its electro motive force is high when the external resistance is low keeps nearly
constant through a short range of increase in the external resistance and then
suddenly drops owing to the insufficient excitation of the field magnets. The
other form of dynamo machine in which the field magnet coils form a current of
high resistance coupled parallel to the external circuit which form is usually called
the shunt dynamo electric machine, acts in the contrary manner to that first
described, in that when the external resistance is low a smaller fraction of the
current passes through the field magnet coils hence the field magnets are

page_text: -
insufficiently excited and the electro motive force is correspondingly low, when
however the external resistance is increased more current then passing through
the magnet coils the magnets are better excited and the electro motive force rises,
reaching a maximum when the resistance of the external circuit is infinite, so
that the whole of the current generated by the armature then passes through the
magnet coils.
The second form however has an advantage over the first when the machines
are used for supplying current for incandescent lamps, in that the magnet coils not
being in series circuit with the main or external circuits no damage is done to
the lamps in the latter circuit from the magnet charge or extra current from the
field magnets.
The second form has however the disadvantage of causing increased liability to
spark and wear at the commutator and brushes when the external resistance is low
and the armature current great. This sparking being caused by the weakness of
the fixed magnetic field occuring at the same that the distorting influence of the
induced magnetic field in the rotating armature is at its maximum In our
improvements we so wind the field magnets as to combine the two methods in
such manner and with such result that the electro motive force, or speaking more
accurately the difference of potential between the terminals is maintained at one
level throughout, id, est, when the external resistance is low and the armature
current correspondingly high The heavy current then passing through the thick
wire coils on the field magnets excites them very fully. When however the
external resistance is high and the armature current correspondingly low the high
resistance coils coupled parallel to the external circuit still continue to supply
current sufficient to enable them to aid the excitation of the field magnets to
maintain the electro motive force practically constant as above described.
We call this winding of the field magnets compound winding, one method of
carrying out this compound winding is to take a dynamo electric machine having
an armature resistance of say 01 ohm and wind onto its field magnets next to the
core, sufficient turns of No 5. B. W. G. wire to give a resistance equal to that of
the armature or 0.1 ohm, or thereabouts and couple these coils in series circuit with
the armature in the ordinary manner, outside these coils we wind on sufficient
057" wire to give a resistance of 20 ohms or from that to 30 ohms. We couple
up this second winding of fine wire from brush. to brush. Such an armature we
prefer to drive at such speed that the electro motive force when the resistance of
the external circuit is infinite will be about 65 volts. It will be found that if the
external circuit be now closed through resistances varying from infinity down.to
0:75 ohms, so that the current in the armature varies from 2 amperes up to 80
amperes the electro motive force between terminals will remain nearly constant at
the above named 65 volts. Moreover that the magnet charge or extra current of
the short length of low resistance coils will be so small that 40 or 50 lamps may
be switched instantaneously out of the main circuit without producing any
momentarily hurtful increase of current in one lamp left in circuit.
This compound winding of the field magnets of dynamo electric machines has
the further advantage of reducing sparking and wear of commutator and brushes
to a minimum as it always maintains a due proportion between the strength of
the fixed magnetic field and the distorting influence of the induced field of the
rotating armature.
The objection above referred to as inherent in the shunt dynamo machine is thus
removed.
Another method of carrying out the compound winding is to wind the thick
and thin wire coils on short tubes of vulcanized fibre, paper, asbestos sheet, or other
suitable material and slip these short tubes on to the magnet cores in alternate
order coupling them up as before directed these tubes may be fitted with flanges
of insulating or other suitable material so as to localize as far as possible any short
circuit or failure of insulation in the field magnets.

page_text: Or in large machines the field magnet cores may consist of several rows of
cylindrical or suitably shaped bars and the thick and thin wire coils before described
may be wound on to the bars in alternate order.
In every case the resistance of the. thick wire coils must be kept exceedingly
low, and in order to get the required exciting effect the cores of the field magnets
must be of increased size so as to admit of the increased bulk of coils necessitated
by the large sectional area of the wire.
This compound method of winding the field magnets can be applied to the
field-magnets of almost any type of direct current machine so as to produce the
results and for the purposes herein before mentioned.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Rookes Evelyn Bell Crompton and Gisbert Kapp in the Great Seal
Patent Office on the 9th April 1883.
ROOKES EVELYN BELL CROMPTON of the Mansion House Buildings Queen
Victoria Street in the City of London Engineer and GISBERT KAPP of Arc Works
Chelmsford in the County of Essex Engineer and in the employment of the
DYNAMO
"IMPROVEMENTS IN
said Rookes Evelyn Bell Crompton
MACHINES"
ELECTRIC
Our invention relates to improvements in dynamo electric machines and more
particularly to those dynamo electric machines which are required to maintain a
nearly constant electro motive force at their terminals where the external circuit is
joined on under extreme variations of the external resistance in the circuit worked
by such machines.
The ordinary dynamo machine in which the coils which excite the field magnets
are in series. circuit with the generating armature does not fulfil this condition ns
its electro motive force is high when the external resistance is low, keeps nearly
constant through a short range of increase in the external resistance and then
suddenly drops owing to the insufficient excitation of the field magnets. The other
form of dynamo machine in which the field magnet coils form a current of high
resistance coupled parallel to the external circuit which form is usually called the
shunt dynamo electric machine acts in the contrary manner to that first described
in that when the external resistance is low a smaller fraction of the current passes
through the field magnet coils hence the field magnets are insufficiently excited
and the electro motive force is correspondingly low-when however the external
resistance is increased more current then passing through the magnet coils the
magnets are better excited and the electro motive force rises reaching a maximum
when the resistance of the external circuit is infinite. So that the whole of the
current generated by the armature then passes through the magnet coils.
The second form however has an advantage over the first when the machines are
used for supplying current for incandescent lamps-in that the magnet, coils not
being in series circuit with the main or external circuits no damage is done to the
lamps in the latter circuit from the magnet charge or extra current from the field
'magnets.
The second form has however the disadvantage of causing increased liability to
spark and wear at the commutator and brushes when the external resistance is low
and the armature current great. This sparking being caused by the weakness of
the fixed magnetic field occurring at the same time that the distorting influence of
the induced magnetic field in the rotating armature is at its maximum. In our
improvements we so wind the field magnets as to combine the two methods in
such manner and with such result that the electro motive force or speaking
more accurately the difference of potential between the terminals is maintained.at
one level throughout, id est, when the external resistance is low and the armature
current correspondingly high, the heavy current then passing through the thick
wire coils on the field magnets excites them very fully. When however the external
resistance is high and the armaturo current correspondingly low the high resistance
coils coupled parallel to the external circuit still continues to supply current
sufficient to enable them to aid the excitation of the field magnets to maintain the
electro motive force practically constant as above described.
We call this winding of the field magnets compound winding. One method of
carrying out this compound winding is to take a dynamo electric machine having

page_text: an armature resistance of say 01 ohm and wind on to its field magnets next to the
core sufficient turns of N° 5 B. W. G. wire to give a resistance equal to that of the
armature or 0.1 ohm or thereabouts and couple these coils in series circuit
with the armature in the ordinary manner. Outside these coils we wind on
sufficient 057" wire to give a resistance of 20 ohms or from that to 30 ohms. We
couple up this second winding of fine wire from brush to brush. Such an armature
we prefer to drive at such speed that the electro motive force when the resistance
of the external circuit is infinite will be about 65 volts. It will be found that if
the external circuit be now closed through resistances varying from infinity down
to 075 ohms so that the current in the armature varies from 2 amperes up to
80 amperes the electro motive force between terminals will remain nearly constant
at the above named 65 volts. Moreover that the magnet charge or extra current
of the short length of low resistance coils will be so small that 40 or 50 lamps may
be switched simultaneously out of the main circuit without producing any
momentarily hurtful increase of current in one lamp left in circuit.
This compound winding of the field magnets of dynamo electric machines has the
further advantage of reducing sparking and wear of commutator and brushes to a
minimum as it always maintains a due proportion between the strength of the fixed
magnetic field and the distorting influence of the induced field of the rotating
armature.
The objection above referred to as inherent in the shunt, dynamo machine is thus
removed Another method of carrying out the compound winding is to wind the
thick and thin wire coils on short tubes of vulcanized fibre, paper, asbestos sheet,
or other suitable material and slip these short tubes on to the magnet cores in
alternate order coupling them as before directed, these tubes may be fitted with
flanges of insulating or other suitable material so as to localize as far as possible
any short circuit or failure of insulation in the field magnets.
To illustrate this second method of carrying out our invention we will cite the
example of a dynamo machine having cast iron pole pieces but wrought iron
magnet cores. In a machine intended to give a current of about 300 (three
hundred amperes) and an external électro motive force of 100 (one hundred) volts,
the cores of the fixed electro magnets would be about 4" (four and one half
inches) in diameter and three such cores would be fixed in each pole piece making
in all 6 (six) cores. Upon each of these 6 (six) cores we place 4 (four) coils of
insulated copper wire of 0.160" diameter and two coils of insulated copper wire.
of 0.100" in diameter, making in all 24 (twenty four) thick wire coils which we
couple up in parallel and 12 (twelve) thin wire coils which we couple up in series.
The resistance of the 24 (twenty four) thick wire coils should be about 003 of
an ohm, the resistance of the thin wire coils should be about 24 ohms. Or we
place 6 (six) thick wire coils on each of the four outside magnet cores and 3 (three)
thin wire coils upon each of the 2 (two) centre magnet cores and connect all the
thick wire coils in parallel and all the thin wire coils in series as above.
Or in large machines the field magnet cores may consist of several rows of
cylindrical or suitably shaped bars, and the thick and thin wire coils before
described may be wound onto the bars in alternate order-In every case the
resistance of the thick wire coils must be kept exceedingly low and in order to get
the required exciting effect the cores of the field magnets must be of increased. size
so as to admit of the increased bulk of coils necessitated by the large sectional area
of the wire.
This compound method of winding the field magnets can be applied to the field
magnets of almost any type of direct current machine so as to produce the results
and for the purposes herein before mentioned.
Having now particularly described the nature of our Invention and the manner
in which the same is to be performed we would have it understood that what we
claim as our Invention is as follows:-
First. The use of two circuits on the field magnets of a dynamo electric machine
connected respectively in shunt and in series with the external circuit for the

page_text: purpose of keeping the electro motive force of the machine approximately constant
at constant speed when the resistance of the external circuit varies and so
proportioned as to effect this substantially in the manner herein described.
Secondly. The use in dynamo macliines of two circuits on the field magnets
arranged respectively in shunt and in series with the external circuit and having
resistances bearing respectively ratios to the resistance of the armaturo substantially
as herein described as and for the purposes herein described.
Thirdly. The various methods herein described of arranging the two circuits for
the field magnets whether the same be arranged on the same or on different cores.
substantially as herein described.
In witness whereof we the said Rookes Evelyn Bell Crompton and Gisbert
Kapp have hereunto set our hands and seals this Seventh day of April
One thousand eight hundred and eighty three
Witness to the signatures of
the said Rookes Evelyn Bell
Crompton and Gisbert Kapp.
ALFRED G BROOKES.
59 Chancery Lane.
London.
R. E. B. CROMPTON
GISBERT KAPP.
(L.S.)
(L.S.)