Improvements in Field-magnets for Continuous-current Dynamos.
Year: 1894

page_text: Date of Application, 7th July, 1894 -Accepted, 11th Aug., 1894
COMPLETE SPECIFICATION.
Improvements in Field-magnets for Continuous-current Dynamos.
We, WILLIAM HENRY ALLEN, RICHARD. WILLIAM ALLEN, both of York
Street Works, Lambeth, in the County of Surrey, Engineers, and CHARLES
CESAR HAWKINS, of 15, Great College Street Westminster, in the County of
Middlesex, Electrical Engineer, 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 :-
1
Our invention consists in improvements in field 'magnets for continuous-current
dynamos, and is specially applicable to those in which the air gap between the iron
of the poles and the iron of the armature core is comparatively small. It is well
known that in such dynamos the effect of the current flowing, in the wires, on, the
surface of the armature is to alter the distribution of the field, and to crowd the
magnetic lines in increasing density towards the "trailing" corners or edges of
the poles; and unless precautions are taken, this crowding of the lines may become
so excessive as to cause saturation of the iron either in the trailing pole corners, or in
the portion of the 'armature immediately opposite to these corners. To lessen this
distortion of the field, each of the poles has in some dynamos been divided into two
portions by an air-gap in the form of a slot running parallel to the length of the
armature, the object of this being to prevent as far as possible the passage of lines
from the one half 'pole piece into the other half. In order however to secure a
still more uniform distribution of the field when the armature. is carrying large
currents, it is "advantageous to divide the, pole-piece by more than one slot, and
this object is attained by our improved 'forms of field magnets by which the pole is
divided into more than two entirely separate portions, and the magnetic lines are
conveyed into or out of these by portions of the yoke which for the whole or part
of their length are also entirely, separate.
Fig. 1, shows one half of our improved form of magnet for a two-pole dynamo
with double horse-shoe field, in which the pole N is divided into four slabs A, B, C, D,
of which A is magnetically continuous with the portion of the yoke E; B with
the two portions of the yoke F, F¹; C with the two portions G, G; and D with the
portion H. The portions of the yoke in the above form of field magnet may be
conveniently made by castings of steel or iron, united along the line X Y to
wrought-iron slabs; or the several portions A and E, F and one half of B, F' and
the other half of B, G and one half of C, G' aud the other half of C, D and H
may respectively be cast in one; or B, F, and F¹ may be cast in one, and
likewise C, G, and G. In Fig. 1, the magnetising coil is shown wound on the pole
lettered A B C D; if two magnetising coils are to be used, one on each of the
poles, then for the second half of the magnet the arrangement of Fig. 1, may be
repeated in precisely the same way, on the other side of the line O P. If however
only one magnetising coil is to be used, and it is further desired to continue the
division of the magnetic circuit right through to the other pole we use a slightly
different form for the second half of the magnet which attains the same end as
above described, i.e. the form as shown in Fig. 2. where the corresponding portions
are indicated by the same letters as in Fig. 1. In both Figs. 1, and 2. the portions
of the poles A, B, C, D, are separated, each from the others, by intervening
air-gaps, 9, 9.
In other cases it may be more convenient to wind the magnetising coils not on
the poles but on the yoke, and Fig. 3. illustrates our improved form of field-
magnet applicable to such cases. In this form the separate portions of the
poles A, B, C, D, and A', B', C¹, D¹³, may be conveniently made by castings of

page_text: steel or iron, these being united on each side either by a common wrought-iron slab.
forming the yoke, or by separate slabs..
The same forms of magnet may also be adapted to multi-polar continuous-current
dynamos, when it is desired to divide their magnetic circuits into more than two
separate portions. If the magnetising coils are to be wound on the poles, the forms
illustrated in Figs. 1, and 2, are applicable; if the magnetising coils are to be
wound on the yokes the form shown in Fig. 3. is applicable. In both cases the only
modifications necessary are such as will readily occur to any electrician, or as affect
the particular method by which the several portions are supported and retained in
their proper places relatively to one another, as for example by distance-pieces of
gunmetal or other non-magnetic materials.
In all cases the mention of air-gaps as separating the portions of the poles or
yokes in the foregoing description is not to be held to exclude the use if it be
desired of dividing pieces of gunmetal or other non-magnetic material, separation
by such non-magnetic materials being for our purposes equally as good as by air.
Having now particularly described and ascertained the nature of our said inven-
tion, and in what manner the same is to be performed, we declare that we do not
claim broadly the separating of the magnet or pole pieces by air-gaps into separate
portions in order to neutralise the effect of armature currents, but what we
claim is.-
1. In continuous-current dynamo-electric machines the improved form of field-
magnet substantially as and for the purpose hereinbefore described and illustrated
in Figs. 1, and 2, by means of which a division of the magnetic circuit into more
than two portions, separated by air-gaps, may be obtained in the pole-pieces, and
continued throughout the whole or part of the yoke.
2. In continuous current dynamo electric machines the improved form of field
magnet substantially as and for the purpose herein before described and illustrated
in Fig. 3, by, means of, which a division of the magnetic circuit into more than two
portions, separated by air-gaps, may be obtained in the pole-pieces, and continued
throughout the whole or part of the yoke.
n
16L..1- A D 1904