Improvements in Dynamo Electric Machines.
Year: 1896

page_text: Date of Application, 18th Aug., 1896-Accepted, 19th Sept., 1896
COMPLETE SPECIFICATION.
[Communicated from abroad by ROBERT LUNDELL, of Brooklyn, County of
Kings and State of New York, Electrician, and EDWARD HIBBERD
JOHNSON, of New York, County of New York and State of New York,
Electrician, both in the United States of America.]
Improvements in Dynamo Electric Machines.
I, HENRY HARRIS LAKE, of the Firm of Haseltine, Lake & Co., Patent
Agents, 45 Southampton Buildings, in the County of Middlesex, 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:-
This invention has reference particularly to direct current dynamo electrio
machines, and its object is to prevent shifting or distortion of the magnetic field in
machines of this type when working under full load.
It is well known to persons engaged in the designing of such dynamo electric
machines that the ampère turns due to the armature currents have a demagnetizing
and cross-magnetizing effect upon the poles of the field magnets. In other words,
the armature ampère turns which lie betweon the double angle of lead have a
demagnetizing effect upon the magnetic circuits, and the armature ampère turns
which lie directly under the poles have a cross-magnetizing effect or a tendency to
weaken the pole strength at that pole corner where the commutation takes place,
and to increase the pole strength at the opposite pole corner. This demaguetizing
effect can readily be compensated for by "compounding," but the cross-magnetizing
or distorting effect is not so easily overcome. Various methods have heretofore
been suggested for overcoming this objectionable feature, and the present invention
is directed to a simple method of accomplishing this object.
The invention will be fully understood by referring to the accompanying
drawing, in which,
Figure 1 illustrates in side elevation a well known form of compound wound
four-pole dynamo electric machine embodying the improvements in the field pole
pieces.
Figure 2 is an end view of one of the field magnet coils and the improved field
magnet poles surrounded thereby.
Figure 3 is an enlarged sectional view taken on the line, of Figure 2;
various magnetic conditions being also diagrammatically illustrated by the full and
dotted lines at the top of this figure.
A represents the armature of the machine, which in this instance is designed to
rotate from left to right. M represents the field magnet coils of a well known
form of compound wound field. The commutator brushes, four in number, are
represented by the letters C, C', C and C, Figure 1. The field maguet poles,
or rather the field magnet cores are divided as shown, into two distinct parts, a
and b, practically of the same cross section. The pole face of the part a has an
area only slightly larger than a cross section of the said part; whereas the pole
face of the part has an area of at least twice the same cross section. The part b,
as will be observed, is provided with a wedge-shaped polar projection p extending
laterally from the dotted line z-z, shown in Figures 2 and 3, and is so proportioned
that it covers about the same area as the pole face of the part a at its thickest
part, having a cross section at its base on the line z-z (Figure 3) about equal to
one half of the cross section of b.

page_text: The material used for the field magnet cores is cast steel, although these cores
may in some instances be constructed of sheet iron punchings. The induction
across the air gap is such that the field core bis saturated when the machine is
working under a light load, and super-saturated at full load. For example :-If
at light load the induction across the air gap is 7500 C. G. S. lines per square
centimeter under the field magnet core a, and 7000 per square centimeter under
the field magnet core b, it will be seen that the induction in b is about twice as
high as in a. In other words, b is saturated, whereas a is far below saturation and
capable of increasing the induction under its pole face as the magnetizing force
from the compound winding increases with the load. At full load the magnetizing
force upon the magnetic circuit through a is as follows; Shunt ampère turns +
series ampère turns on field cores - armature ampère turns between the double
angle of lead the armature ampère turns that lie under the pole face of b. The
sum of these forces is such that the induction under the pole face of a has increased
by a predetermined amount, see Figuro 3.
The magnetizing forces acting upon the magnetic circuit through at full load
are as follows:-Shunt ampère turns + series ampère turns on the field coils -
armature ampère turns between the double angle of lead + the armature ampère
turns that lie under the pole face of the part a. The sum of these magnetic forces,
although considerably greater than those acting upon the circuit through the
part a, are unable to materially increase the induction from the pole face of the
part 6 on account of the high saturation of said part and its extended polar pro-
jection p. The inductive effect across the air gap is represented diagrammatically
in Figure 3 by the beavy line A A A at no load and the heavy line A' A' A' at
full load. The fine line B B B represent diagrammatically the cross-magnetizing
effect of the armature, and the dotted line A' A'A' parallel therewith the resultant
inductive effect across the air gap at full load of a machine which is provided with
solid pole pieces of well known form.
This method of representing the inductive effects of the field magnets diagram-
matically by an irregular line A A A and the cross-magnotizing effects of the
armature by a fine diagonal line B B and the resultant effect thereof by a dotted
parallel diagonal line A A A is fully explained in volume 2. of Practical
Electrical Engineering" and reference is had to chapter XI, pages 72 to 81 of said
publication for the full explanation thereof.
The effect of this peculiar form of field magnet core with the polar projection p
is illustrated upon this diagram by the full irregular line A A' A', thereby
showing that the magnetic fringe or line of commutation between no load A A A
and full load A' A' A varies but little. In other worde, this diagram, shows that
with a field magnet core of well known solid form of construction the magnetic
fringe or line of commutation shifts between no load and full load to such au
extent that a shifting of the commutator brushes is required, while with the herein
described improvement the shifting of the line of cominutation or magnetic fringe
is scarcoly appreciable, the magnetic effect of the polar projection p being such
that the brushes may be allowed to remain constantly in one position for all
loads. If it should be required to run the machine in the opposite direction, it
would of course in that event be necessary to reverse the direction of the field cores
or poles.
Having now particularly described and ascertained the nature of the said inven
tion and in what manner the same is to be performed, as communicated to me by
my foreign correspondents, I declare that what I claim is :-
1. The method of preventing shifting of the point of commutation in a dynamo
electric machine having shunt and series coils, by increasing the magnetic field,
as the load increases, at the pole corner where the commutation takes place
without corresponding magnetic increase at the opposite pole corner, substantially
as described.
2. A dynamo electric machine having its magnetic field cores or poles

page_text: arranged to have saturated and unsaturated portions as and for the purpose
described.
3. For preventing the shifting of the magnetic field with an increasing load in
a dynamo electric machine, the employment of field coros or pole pieces having
low magnetic reluctance on the side where commutation takes place and high
magnetic reluctance on the opposite side of the pole.
4. In a dynamo electric machine a field magnet coil surrounding a field magnet
pole or core which is divided into two parts separated by an air space, one of said
parts having an inner field pole surface of greater area than the other for the
purpose specified.
5. In a dynamo electric machine a field magnet coil surrounding two field
magnet poles separated from each other by an intervening air space, one of said
field magnet poles having a wedge-shaped pole piece, the inner face of which
is of greater area than the inner face of the other pole piece, substantially as
described.
Dated this 12th day of August 1896.
HASELTINE, LAKE & Co.,
45 Southampton Buildings, London, W.C., Agents for the Applicant.