Year: 1887

page_text: Date of Application, 9th May, 1887
Complete Left, 9th Feb., 1888
Complete Accepted, 23rd Mar., 1888
We, ROOKES EVELYN BELL CROMPTON, and JAMES SWINBURNE of Arc Works,
Chelmsford, Electrical Engineers do hereby declare the nature of this invention to
be as follows:-
Our invention relates to improvements in and connected with dynamo electric
machines, for use as generators or motors.
Our invention relates to getting large outputs out of comparatively small armatures,
so that a less costly dynamo may do any given work at the same efficiency.
Our invention relates to shaping the pole pieces and magnets of dynamos, and to
the arrangement and disposition of the iron and conductors of the armature, and
interpolar air space.
Our invention also relates to means or methods of attaching the armature to the
shaft, and to the ventilation of armatures of the kind known as the drum, for two or
four pole machines.
Our invention also relates to the collection of currents of dynamo electrical machines
& to the avoidance of sparking & of cutting of the commutators.
Our invention also relates to coupling the armature conductors of drum armatures
across the ends in such a way as to facilitate repairs & to allow ventilation.
The
Our invention refers chiefly to dynamos & motors with drum armatures.
present practice of dynamo makers is to wind armatures with little wire so that there
are few ampère turns even at full load so as to keep the air spaces between the
armature & the pole pieces as small as possible. We reverse this and make the ampère
turns round the armature comparatively large. We thus greatly increase the output
of the machine without increasing the field magnets much. The magnetising force
required on the fields is of course greater, but the output of the machine is so greatly
increased that that is of little consequence. By properly ventilating the armature we

page_text: are enabled to use a high current density so that the output is made much larger than
at present without increasing the air space very much. My object is mainly to get
as many ampère turns round an armature of given diameter as possible. By suitable
ventilation & by proper designing of the field magnets, I keep the air space as small
as compatible with such increased outputs thus keeping down the size of the
field magnets.
.
For small machines we prefer what are known as single horseshoe magnets. If
there are many ampère turns round the armature & the air space is not very large the
brushes have to be moved forward, or in other words, lead of the brushes is demanded.
If the armature is wound as usual in drum armatures there is sparking unless the
brushes are put in such a position that the electromotive force has just time to stop
the current in a section reverse it & bring it up to its normal value, while that section
is short circuited by the brush. The position of the brushes depends therefore on the
self induction of each section at the position of commutation on the time it is short
circuited by the brush or brushes, & on the induction or strength of field at the point
of commutation. When the brushes have lead they approach two of the four corners
of the magnet poles. These will be called for distinction the leading corners & the
other pair the trailing corners. When there is no current round the armature the
strength of field under the leading & trailing corners is practically the same, but when
there is a current the induction at the leading corner is lessened & at the trailing
corner increased. The induction due to the armature is easily calculated in many
cases. The action of the armature current may be divided into two parts one which
weakens the induction under the leading corners & strengthens
equally under the
trailing corners. This we call the cross magnetisation or cross induction and one
which opposes the induction due to the field magnets. This we call back induction.
The cross induction is equal & opposite under the two corners & is roughly independent
of the lead of the brushes within reasonable limits & the back induction varies really
as the angle of lead of brushes. As the induction under the leading corners is that
due to the field magnets less that due to the armature, it is advisable to have the
induction due to the field great to begin with. In small armatures this cross
magnetisation or cross induction due to the armature is of little importance but in
larger machines it becomes a very important factor. To get a very strong field I
use an armature which has great radial depth, that is to say it contains as much
magnetised iron as possible I magnetise this strongly & thus have a very strong field
under the pole pieces or in the air space. There is then a fairly strong field under the
leading corner, or close to it to reverse the current in the armature sections. If the
induction under the leading corners due to the field magnets were exactly or nearly
balanced by the back induction due to the cross magnetisation or armature current
the lead would be excessive, or there would be sparking. We therefore arrange in
such cases the poles so as not to embrace too great an angle. With a given total
induction through the armature the cross induction is less when the polar angle is
small, & the induction per square centimetre due to the field is greater.
The position of commutation is likely to come close to the leading corners & this
gives considerable back induction especially when there are many ampère turns on
the armature. This may be lessened by winding so that each coil on the armature is
wound so as to include little more than the polar angle instead of being wound right
across. Or it might be lessened by making the polar angle nearly two right angles
so that the magnets almost embrace the armature. If a single limb magnet were
made in this way there would be excessive magnetic leakage from one leg to the other. I
therefore keep the legs a reasonable distance apart but bring the pole pieces close together
on the far side. If the armature conductors are connected right across this prevents
excessive lead & back induction. For large machines 1 prefer double horse shoe
magnets, because single horseshoe must have large pole pieces & these pole pieces
admit of excessive cross induction.
The pole pieces of the double horse shoe magnets are made to offer as little
magnetic resistance as possible to the induction due to the field magnets but to oppose
as great resistance as possible to the cross induction. The section of iron connecting

page_text: one horseshoe with the other is made as small as possible. The horseshoes may be
absolutely separated, but we prefer to make the internal surface of the pole pieces
practically continuous to avoid heating of the armature bars.
This part of our invention is also applicable to machines with cylinder armatures
in fact in such machines the cross induction with a given number of ampère turns is
even more serious, especially in large machines. The armature we prefer is made up
of thin discs of soft iron. In very small armatures it is unnecessary to provide
ventilation. The spindle is then made of steel or of manganese steel & has a key-
way & key. The discs have also a hole to fit & a key way & are strung on.
The spindle may have a gunmetal, or non magnetic bush which may be cast on to
insulate it magnetically from the discs.
In larger armatures the spindle may have a gunmetal or other casting with longi-
tudinal ridges which fit into notches in the discs or the spindle itself may be made
with longitudinal ridges so that the discs are strung on the spindle itself. There are
then ventilating spaces between the ridges. There are then clear spaces right
through the armature and the commutator connections act as a fan drawing air right
through the centre of the armature. The discs may be spaced apart at intervals to
allow a current of air from the internal space outwards radially. The armatures may
be built of iron wire or tape but we prefer discs. The connections over the ends of
the drum armatures are made in different ways according to the size of the armatures
& the dimensions of the conductors.
In a large armature if the conductors are deep radially & narrow we use end con-
nections arranged in two layers as is sometimes usual in drum armatures. As these
end connections are commonly used it is however difficult or impossible to remove
any one connection for repair without taking the whole armature to pieces & there is
no efficient ventilation for the armature. According to our invention the conducting
path is as follows starting from one end of a conductor. First inwards towards the
axle. Then away from the armature head in a direction parallel to the axis. Then
by a curved path to the other conductor in a plane at right angles to the axis. The
connectors thus form two layers. The parts in each layer may be curved but we
prefer especially in large armatures to make the inner parts radial & curve the outer
parts as much as then is necessary. The connectors are made so that they can be
disconnected at the central part so that the conductor with the radial part of the
connector can be withdrawn without interfering with others. The other part of the
connector can then be drawn outwards parallel to the axis. The radial part of
the connector may be formed by bending the conductor itself in. The connectors are
so arranged as to leave ventilation space between them & the spindle. At one end of
the armature the commutator' sections themselves may form part of the connectors
but we prefer to connect them separately so as to admit of ventilation. We prefer to
make the joints screwed & soldered so as to be easily repaired even without resoldering.
For resoldering, amalgam of copper, or other cold solder may be used if desired. The
layers are insulated by washers of insulating material between the inner & outer parts
of the connectors.
When the conductors are broad & not deep radially we prefer plate connectors for
which letters patent No. 15484 have been granted me.
In low tension large current machines such as are used for electro deposition, plates
or the other connectors may be used at one end & at the other another form of con-
nection. Thus suppose an armature has 24 outside turns or 12 convolutions each
consisting of 8 bars, coupled up so as to avoid heating. If each convolution embraces
one third of the periphery there will be 12 plates at the commutator end arranged in
four layers of three plates each. At the pulley end there are also four layers but
each layer consists of 3 sets of bars each set of bars consisting of 8 lying in the same
plane. If the coils embrace nearly half the periphery there are six layers of two
layers each at the commutator end & 6 layers of 16 bars each at the pulley end.
These bars are curved to allow ventilation of the armature. The commutators &
collectors may be as usual, but in many cases we prefer the following arrangement.
Each collector consists of two or more brushes arranged so that one is in advance of

page_text: the other & these brushes are not connected together in the usual way but each has its
own circuit round the field magnets of the dynamo so that for the current to pass
from one brush to the other it has to go along one wire round the field magnets &
back by another & this passage has a resistance many times greater than that of the
section or sections short circuited so that the large currents developed by short
circuiting the sections too long are avoided. Instead of taking the wires round the
field magnets, resistances may be inserted, but we prefer the first method as it involves
no wasteful resistances, if the machine is series or compound wound to begin with.
If the machine is shunt wound the same effect can be got at by dividing the main
leads part of the way-thus if two pairs of brushes are used each of the mains would
consist of the first part of their length of 2. cables. Instead of resistances suitable
self induction coils may be used. For instance the wires may be led round a core in
opposite directions.
In many cases, especially in large machines we prefer to use four or six poles. This
admits of greater simplicity in construction of the heads, and other advantages.
There are then four or six leading and trailing corners, and so on. The description of
my invention in most parts refers equally well to multipolar drum armature machines,
though mostly applicable to single horseshoe drum armature machines.
71

page_text: We, ROOKES EVELYN BELL CROMPTON and JAMES SWINBURNE of Arc Works
Chelmsford Electrical Engineers do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and ascertained
in and by the following statement :-
Our invention relates to improvements in and connected with dynamo electric
machines, for use as generators or motors.
Our invention relates to getting large outputs out of comparatively small
armatures, so that a less costly dynamo may do any given output at the same
efficiency.
Our invention relates to shaping the pole pieces and magnets of dynamos, and to
the arrangement and disposition of the iron and conductors of the armature, and
interpolar air space.
Our invention also relates to means or methods of attaching the armature to the
shaft, and to the ventilation of armatures of the kind known as the drum.
Our invention also relates to the collection of currents of dynamo electrical machines
& to the avoidance of sparking & of cutting of the commutators.
Our invention also relates to coupling the armature conductors of drum armatures
across the ends in such a way as to facilitate construction and repair & to allow
ventilation.
Our invention refers chiefly to dynamos & motors with drum armatures. The
present practice of dynamo makers is to wind armatures with little wire so that there
are few ampère turns even at full load. The object is to keep the air space as small
as possible so as to get a strong field between the field magnets and armature. We
reverse this practice in that we put a large number of ampère turns on the armature,
which demands a larger air space; but we still keep the field strong by suitably
designing the field magnets. The magnetising force required on the field magnets is,
of course, greater, but the output of the machine is so greatly increased that that is of
little consequence. By properly ventilating the armature we are enabled to use a high
current density in the conductors so that the air space does not become excessive. Our
object is mainly to get as many ampère turns round an armature of given diameter as
possible.
The application of our principle that the air space should be large, and the armature
ampère turns should be large leads to remarkable difference in the output of a
machine. Thus a drum armature of nine inches external diameter of core is, according
to present practice wound with about 3,000 ampère turns. According to our inven-
tion it can be wound with about 12,000 ampere turns, so that its output is increased
three or four fold. By ampère turns on the armature we mean half the number of
external conductors, that is to say, in a drum armature the number of convolutions,
multiplied by the current in each conductor, so that an armature with 100 convolu-

page_text: tions, or 200 conductors counted round the periphery, & giving 500 ampères would be
said to have 25,000 ampère turns. The limit of output in a small machine, such as
one with a nine inch arinature core depends on the ventilation of the armature, as only
a certain amount of heat per second can be got rid of by convection, & radiation.
We arrange for as complete ventilation as possible. Keeping the power spent in an
armature constant, the output of the machine may be increased by increasing the
radial thickness of the core, or by increasing the air space & decreasing the current
density in the armature, or by more strongly magnetising the armature. These all
need larger field magnets with more wire on them, and the size of the magnets and
their cost determine at what point it pays to increase the armature by going to the
next size of discs in use or making a longer armature & therefore a longer machine.
Machines with armature cores about 12 inches in diameter are, according to present
practice, commonly wound with about 8000 ampère turns; according to our invention
we wind them with about 24,000 ampère turns. Another reason for limiting the
ampère turns on the larger machines is the tendency to sparking. This tendency
depends on several things, and one object of our invention is to obviate it in large
machines.
Referring to fig. 1 of the drawings a is the end view of an armature, the whole
machine being shewn diagrammatically. b c are brushes, shewn for convenience,
pressing against the armature instead of against the commutator. The arrow shews
the direction of rotation as a dynamo. At full load the brushes come under the
corners c, d, of the pole pieces ef. We call these corners the leading corners, and we
call g
h the trailing corners. Suppose the arrows in the field magnets represent the
direction of the induction due to the field magnet coils. It is necessary, to prevent
sparking, that the sections under the brushes be shortcircuited while passing through
a field of force, so that the current in them dies away, reverses, and comes up to the
normal value just as the commutator section leaves the brush. With brushes of the
usual breadth the commutation at full load takes place close under the leading
corners. Putting thicker brushes makes very little difference, for reasons which need
not be discussed. In small machines, there is no difficulty from sparking. In large
machines the induction under the leading corners is decreased or, if badly designed,
reversed by the armature current. The armature acts as if it were wound with two
coils. One shewn by vertical lines forms a belt whose breadth is equal to twice the
displacement of the brushes. This belt opposes the induction due to the field coils, &
if it existed alone would magnetise the field in the opposite direction. The induction
due to this part of the armature coils we call the back induction.
The rest of the conductors may be taken as a belt or coil wound at right angles
to the first, the breadth of the belt in a large machine being rather greater than the
part covered by the pole pieces. This belt produces induction which opposes the field
magnet induction under the leading corners & increases it equally under the trailing
corners, passing round by the pole pieces as shewn by the dotted lines. This belt or
coil of wires does not affect the induction in the field magnets, as it makes its circuit
by the pole pieces. We call this the cross induction. As the effect of the cross in-
duction is to lessen or reverse the induction under c the field magnets & pole pieces
must be so calculated that the induction is not reversed or lessened so much that there
is sparking.
The theory of the relation between the armature induction and the field magnetising
force is already well known in cases where the effect of the armature current is
ignored, or a certain brush lead is assumed. The complete theory of the dynamo,
including the armature current action has not, so far as we know been published, we
will therefore explain as much as is necessary for the understanding of our invention.
First the maximum induction desired in the armature is settled. The fall of
magnetic potential or line integral of the magnetising force necessary is noted. The
induction needed in the air space is then obtained, &, when this and the thickness of
the air space are known, the difference of magnetic potential of the pole pieces is
determined. This is increased by an amount equal to that due to the back
induction.

page_text: The waste field is determined then, & the size of the field magnets & the ampère
turns needed are calculated in the usual way. This part is simply the reverse of the
calculations of characteristics which has been much discussed lately. The calculation
of characteristics is in itself useless, the reverse process of calculating out a machine
to give a certain output is what is wanted. The cross-induction under c & p is found
by multiplying the number of ampère turns by 1-256 and dividing the product by
twice the air space. When any of the iron is saturated, or strongly magnetised,
through which the induction passes, allowance must be made. The cross induc-
tion is greatest therefore under the leading & trailing corners, being zero half way
between them.
To prevent sparking it is therefore necessary to prevent too great cross induction,
or to make the forward induction in the air space great enough to leave a margin.
The forward induction can be increased by taking a higher induction in the armature,
but we prefer to go up to 15,000 or 16,000, & not farther. The forward induction
in the air space is also increased by increasing the radial depth of the core. We also
increase it by lessening the polar area. This decreases the cross induction also, but
it increases the back induction also. Having shewn how these can be calculated, it
is only necessary to say that we design machines in accordance with these principles
so as to avoid sparking, & to give very large outputs..
We prefer to use armature discs with a central hole large enough to allow free
ventilation. The commutator connection acts as a fan, & draws air right through
the armature. We also allow radial passages so that the air goes out radially at
intervals.
The difficulties from sparking & crossinduction arise in large machines. In a 12"
disc with a 7" hole care has to be taken. We prefer what are known as single
magnets for use with drum armatures until we get to sizes where they become
inconvenient. We prefer to wind the armature in accordance with Letters Patent
granted to J. Swinburne No. 15484 of 1885 by which the back induction is practically
eliminated & the brushes need very little lead at full load. We sometimes make
single magnet drum armature machines with the pole piece of such section at e
that it is fairly saturated at no load by the induction due to the magnet coils.
As the cross induction passes through this section the iron becomes more strongly
saturated at full load, & this lessens the cross induction under c. To produce a
similar effect on the other side is impracticable, so the section at is left large.
This makes the induction under c & d unequal, but that is of no importance in a
drum wound armature with the conductors wound across diameters. With these
uneven field machines we prefer such windings.
For large output machines we prefer double magnets. We make these as shewn
in figures 2 & 3 so that the cross induction is practically halved. The parts i &; are
made small so that there is very small section available for the cross induction. Some-
times we separate the magnets there, so that there is no magnetic connection between
them. This gives rise to constructional difficulties, but in very large machines it is.
worth the extra expense. For machines with armatures of thirty or thirty six inches
diameter we prefer four pole machines. The radial depth is then considerably less,
and end connections are much easier to make. We do not intend to claim the use of
such magnets as shewn in figs. 2 & 3 in connection with Gramme wound armatures,
as it is not impossible that somebody may have blundered on such a form without
any intention of lessening crossinduction, and without using a large output
armature.
Sometimes we use what we call reversing poles which oppose the crossinduction
under the leading corners so that the brushes can be placed there without causing
sparking. This arrangement allows large machines with large output armatures to be
made with single magnets, which is often an advantage. A reversing pole piece is
shewn in fig. 4. It consists of a projection wound with a coil k which causes a local
induction circuit shewn by a dotted line. This opposes the cross induction under /&
assists it on the rest of the polar face of the field magnet. The surface is part of the
same circle as the bored part of the machine. Only one reversing pole is necessary

page_text: with an Alteneck wound drum, but with Swinburne's winding two are desirable, one on
each leading corner. The reversing pole may be detached from the field magnet as
shewn at m. The detached form is generally best on the open side of a single limb
armature so that it is not in the way of the field windings. We prefer to wind the
reversing pole in series with the armature.
The armature we prefer is made of discs or washers of thin sheet iron, soft & well
annealed. For small armatures we prefer to string these on a brass spider, but in
large machines we make the central hole smaller, and form the shaft or axle with
ridges to fit into notches in the discs. In both cases room is allowed for plenty of
ventilation, Fig. 5 shews the section of the shaft and a disc. For a machine of medium
size three ridges instead of four is sometimes better.
7"
Our invention also relates to methods of connecting the conductors across the
ends of drum armatures. Fig. 6 shews two conductors n o. One of them, o is cranked
inwards radially, and then bent so as to be again parallel to the axle at p. A strip of
copper connects n & p as shewn. The strips are insulated by lapping all, or overy
second with silk, or by mica sheets, or other insulating material. The conductor o is
cranked in till p is about half as far from the centre of the armature as o is. The
curve of is such that the various strips lie closely against one another. When the
turns are numerous two or more layers of strips are used. The individual strips are
thus narrower & thicker, so that there is less waste of room by insulation. The
conductors n & the parts p are then alternately long and short, in the case of two layers
of strips. Fig. 7 shews a pair of strips r&s there being two layers, n is short & q is
long, p is long to take s & there is a short end t to take r.
Sometimes, to give more room for the end connections, we crank half of the bars in
and half of them out as shewn in fig. 8.
The connections as described so far have the drawback that a conductor cannot be
removed without the whole armature being taken to pieces; or at least a number of
end connectors must be removed. To obviate this, we make a joint in the cranked con-
ductor end, as shewn in fig. 9. The conductor n is bent up at the end, so that the joint
is clear of the neighbouring conductors, as shewn. The joint may be made by a screw
or by dovetailing and sweating, or by equivalent means. Where practicable the joint
is made without bending the conductor up at the end, but this is liable to cause
faults in the armature, as a joint generally takes up more room than a continuous
conductor.
When there is a joint in the cranked conductor end, as explained, the whole of the
end pieces can be detached if desired by unsoldering the outer joints. Any one bar
can also be taken out separately, & any one connector can be removed singly. The
piece u fig. 9 can also be taken out, by first removing the armature spindle, & then
drawing u inwards radially.
For collecting large currents we use collectors which cover a large arc of the
commutator. To prevent shortcircuiting of the sections under the collectors we
divide each into two or more brushes arranged one in advance of the other, & these
brushes are not connected together in the usual way, but each has its own circuit
round the field magnets of the dynamo, so that for the current to pass from one
brush to the other, it has to go along one wire round the field magnets, & back by
another, & this passage has a resistance many times greater than that of the section
or sections short circuited, so that the large currents developed by short circuiting the
sections too long are avoided. Instead of winding the wires from the brushes round
the field magnets resistances may be inserted, but I prefer the first method, as it
involves no wasteful resistance if the machine is series or compound. If the machine
is shunt wound the same effect can be got by dividing the main leads part of the
way. Thus if two pair of brushes are used each of the mains consists of two cables
for part of its length.
In many cases, especially in large machines we prefer to use four or six poles.
This admits of greater simplicity in construction of the heads, & other advantages.
The description of our invention in most parts refers equally well to multipolar
drum armature machines, though mostly applicable to drum armature machines.

page_text: For the purpose of drawing definite claims it may be advisable to draw some distinct
line between our large output machines and machines as now ordinarily constructed.
We therefore assume that ordinary machines have a number of armature ampere
turns equal to one hundred times the square of the diameter measured in inches.
As a rule dynamos do not come up to this, so by selecting one hundred and ten times
the square of the diameter in inches as the lower limit of the application of our
principle, or the lower limit of a large output armature we claim the application
of our principle & still leave a margin for variations in existing practice.
It is almost needless to remark that we use fans to ventilate our larger machines.
We include motors in the foregoing specification.
Having now particularly described and ascertained the nature of our said inven-
tion and what manner the same is to be performed, we declare that what we
claim is:
1. A dynamo machine having a number of armature ampère turns greater than one
hundred and ten times the square of the diameter measured in inches.
2. A large output' machine substantially as described.
3. The combination of a drum armature with field magnets substantially as described
with reference to Fig. 2 of the drawings for the purpose set forth.
4. The combination of a drum armature with field magnets substantially as described
with reference to Fig. 3 of the drawings for the purpose set forth.
5. An armature composed of discs mounted direct on the axle so as to leave
ventilating spaces substantially as described with reference to fig. 5 of the drawing.
6. The end connections for drum armatures substantially as described with reference
to Fig. 6 of the drawings.
7. The end connections for drum armatures with two or more layers of connecting
strips substantially as described with reference to Fig. 7 of the drawings.
8. A drum armature with some of the conductors cranked outwards substantially as
described.
9. A drum armature with its conductors and end connectors detachable substantially
as described.
10. A drum armature with some of its conductors jointed substantially as described
with reference to Fig. 9 of the drawings.
11. The collectors large currents each consisting of two or more brushes which are
electrically connected by conductors wound round the field magnets substantially as
described.
12. The collectors each consisting of two or more brushes in combination with
separate leads substantially as described.
Dated this Eighth day of February 1888.
JAS. SWINBURNE.