Improvements in Dynamo Electric Machines.
Year: 1892

page_text: Date of Application, 2nd Nov., 1892
Complete Specification Left, 31st Aug., 1893-Accepted, 2nd Feb., 1894
PROVISIONAL SPECIFICATION.
Improvements in Dynamo Electric Machines.
HENRY CHITTY, Engineer Holmewood, 5 Bolton Gardens Chiswick, W. I do
hereby declare the nature of this invention to be as follows:-
This invention has for its object the combination of various known or previously
described methods or principles of construction of electric motors, and dynamos,
for the purpose of obtaining the maximum possible output for a given weight at
relatively slow speeds, and the application of various devices for making such
combination most effective.
A machine constructed according to this invention has an armature of the ring
or cylinder type, slotted or grooved on its circumference for the reception of the
conductors, as in the well known Pacinotti armature.
Such an armature if used in a two pole field, has the disadvantage that the
intensity of magnetic induction in the airspace cannot exceed about 8,000 lines
per sq. centimetre owing to saturation of the iron in the armature.
There is the further difficulty that if the airspace is small excessive distortion
of the field is induced by the current in the armature, whilst any increase of
airspace is attended by a corresponding increase in the weight and cost of the
magnets.
This tendency to distortion is reduced by subdividing the poles so as to form a
multipolar machine.
If the same total polar area, and depth of airspace is retained, the distortion
is inversely as the number of poles. The total weight of copper required on the
magnets will be less than if the reduction of distortion had been effected by
increasing airspace, while the amount of iron in the magnets is much reduced.
It is possible moreover to increase the intensity of induction in the airspace,
without saturating the armature.
These advantages are of course to a certain extent common to all multipolar
machines, but unless a slotted armature is used, the magnets are necessarily heavy
and require much energy for excitation.
It is one of the disadvantages of multipolar machines that the heat generated in
the iron of the armature by Foucault currents, and Hysteresis, is greater than in
two pole machines at the same speed, and the same strength of field in the airspace.
This is still further increased if the field is pushed to the utmost to obtain high
output at slow speed, it is therefore necessary to provide means for getting rid of
the extra heat.
It is the object of this invention to show how a practical multipolar machine
may be made of very light weight, without heating, sparking, or mechanical
weakness.
According to this invention the armature is made up as usual of thin iron plates
mounted on a spider or equivalent support and separated at frequent intervals by
spacing strips which by keeping the plates apart allow free passage of air from the
interior outwards.
This is much more effective with a slotted armature than in one in which the
conductor covers the whole circumference, since in the former case the air can pass
freely between the conductors, and it is unnecessary to leave gaps for the purpose.
It is important that the spacing pieces should not block up the air ways and to
avoid this U shaped pieces of thin metal closely surrounding the conductor may be
used with advantage.

page_text: In order to obtain the full benefit of this ventilation, spaces are left in the
polepieces corresponding to the spaces in the armature, and to accomplish this the
magnets are best made of plates, or sheets, cut, stamped, or forged, to the required
form and held together by bolts, spacing pieces being inserted at intervals,
corresponding. In this way the current of air can flow without obstruction from
the interior of the armature to the outside of the magnets.
Where the slots are very narrow, so as not to exceed about one and a half times
the depth of the airspace the polepieces may consist of plates of considerable
thickness, but when the slots are wider than this, it is necessary to use thin plates
or sheets, in order to avoid Foucault currents in the polepieces, or in certain cases
cast iron may be used.
The magnets are in the form of a ring surrounding the armature; having
projections on which coils are slipped, after being wound on a former.
In order that the projections on the armature may leave the edge of the pole-
pieces smoothly without causing irregularities of torque polar extensions are added
after the coils have been placed in position. These extensions may consist of iron
strips projecting tangentially from the polepiece edges, tapering preferably from
the centre to the edges, and may be held together by wedges drawn inwards by
nuts and bolts, but any other arrangement may be used that will have the effect of
keeping the induction in the field from varying as the slots approach or recede from
the polepieces.
The armature is wound by any of the methods in which the conductors under
different field poles are connected in series since the ordinary gramme winding is
quite unsuitable for multipolar machines, especially when the airspace is small.
In order that the size and weight of the magnets may be kept small the depth
of airspace should not exceed about one eightieth of the diameter of the armature,
and the greater the number of poles the smaller should the airspace be.
Since the reduction of airspace tends to make the field between the poletips,
which is available for the reversal of current, weak, under ordinary circumstances
the lead would be variable, and sparking would result. This is obviated by using
carbon brushes and giving no lead. Instead of carbon brushes, brushes of copper,
or other metal may be used with carbon brushes set a little in advance.
Dated this 2nd day of November 1892.
HENRY CHITTY.
COMPLETE SPECIFICATION.
Improvements in Dynamo Electric Machines.
I, HENRY CHITTY, 5, Bolton Gardens, Chiswick, London, 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 :-
The object of my invention is the construction of a dynamo-electric machine of
comparatively little weight, and capable of operating so as to furnish a large output
of electricity without excessive heating.
For this purpose, I provide in both the armature and the field magnets passages
for ventilation so arranged that, when the armature revolves, there is free
passage for air into it at either end or at both nds, and out of it at a number of
points in its circumference, and that there is also free passage for the air through
the field magnets or their pole pieces. The dynamo to which my invention is
applied may be bipolar or multipolar; I prefer it to be multipolar, as being capable
of giving a larger output for a certain weight of machine.
Fig. 1 of the accompanying drawings is a section and Fig. 2 is an end view,
shewing the application of my invention, by way of example, to a dynamo having
8 poles.

page_text: b
The armature is of the slotted kind, having a number of passages a from its
hollow interior. In line with these there are corresponding passages 6 through the
field magnets, which, as shewn in Fig. 10, may be made up of a number of iron
sheets bolted together with distance pieces between groups of the sheets. Or the
magnets may be cast rings as shewn in Fig. 11, thickened at intervals to form
distance pieces k.
A bipolar dynamo, having my invention applied to it, is shewn in end view in
Fig. 3, in transverse section on line fg by the left half of Fig. 4, and in transverse
section on line hi by the right half of Fig. 4. A half plan is shewn in Fig. 5.
In these figures b indicates the ventilation passages through the pole pieces of
the field magnets, which passages are in line with the passages through the
armature. In this case, as in that of the multipolar dynamo, I employ an armature
of the slotted kind because I thus easily get large and clear ventilation passages.
With a smooth cored armature such passages cannot readily be obtained as may
be understood on referring to Figs. 6 and 8 which shew respectively transverse
sections of a slotted and of a smooth cored armature of corresponding dimensions,
and to Figs. 7 and 9 which are partial developments of their respective surfaces.
It will be seen that in Fig. 7 the ventilation passages, indicated by the black colour,
are almost entirely free, whereas in Fig. 9 they are covered by the conductors,
except at certain intervals giving only small passage for air.
In consequence of. the ample ventilation and cooling provided for as I have
described, I am enabled to work a dynamo thus constructed with stronger magnetic
fields and at higher voltages and current densities, also, if desired, at greater speed
than a dynamo not thus provided with means of cooling; and hence from a given
weight of machine I can obtain a much higher output.
I have above described my invention as applied to dynamo machines, but, obviously
it is applicable with like advantage to electric motors.
Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
A dynamo electric machine having an armature with ventilation passages left at
intervals between its conductors and between groups of the plates of which it is
built
and field magnets with corresponding ventilation passages through their
pole pieces, substantially as, and for the purpose set forth.
up,
Dated this 29th day of August 1893.
ABEL & IMRAY,
Agents for the Applicant.