Improvements in Dynamo Electric Machines of the Inductor Type.
Year: 1897

page_text: Date of Application, 23rd Nov., 1897-Accepted, 8th Jan., 1898
COMPLETE SPECIFICATION.
Improvements in Dynamo Electric Machines of the Inductor Type.
We, THE BRITISHI THOMSON-HOUSTON COMPANY LIMITED, of 83 Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by Charles P.
Steinmetz, of Schenectady, in the County of Schenectady, State of New York,
United States of America) and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:
This invention relates to alternating current dynamos of the inductor type;
and has for its object to remedy some of the difficulties which have been met
with in designing satisfactory machines of this class. In inductor dynamos the
armature and field-magnet coils are stationary and the induction is produced
by a change in the path of the magnetic flux effected by the teeth or poles of the
rotating inductor. In consequence, in part of the magnetic circuit the magnetism
varies, and this part, to avoid eddy currents, must be laminated; while in the
other parts the magnetism is stationary and solid material may be used. It is
therefore of importance to avoid changes in magnetism in the solid part of the
structure, in order to avoid excessive heating and loss from eddy currents.
During the rotation of the inductor, the length of the path of the lines of
magnetic force, and thereby the reluctance, varies periodically in the laminated
portion of the armature, which tends to make the magnetic density vary periodi-
cally also. To keep this periodic variation of magnetic density from extending
into the solid part of the magnetic circuit, by the present invention an auxiliary
air-gap is inserted between the solid and the laminated part, of sufficient length.
to make the magnetic reluctance of the shortest and of the longest path across
the armature iron and auxiliary air-gap practically equal, and thus the magnetic
density at the point of entrance into the solid part of the magnetic circuit prac-
tically uniform.
Another feature of the invention consists in the use in an inductor dynamo
of a short-circuited or squirrel-cage winding surrounding the outer edge of the
armature iron. The magnetic reluctance of the air-gap between the inductor and
the armature, especially in uni-toothed single-phase machines, is less when the
inductor tooth stands in front of an armature tooth than when it stands in front
of a slot; thus the magnetism in the latter case tends to decrease and this induces
eddy currents in the whole magnetic structure. The use of the short-circuited
winding, for reasons more fully to be pointed out, tends to obviate this difficulty.
The accompanying drawings show a machine constructed according to this
invention, it being applied to a dynamo of the well-known Thomson inductor
alternator type; but it might equally well be applied to any other machine of the
same general class.
Fig. 1 is an end elevation, partly in section, of such a machine, and Fig. 2
is a side elevation, partly broken away for convenience of illustration.
In Fig. 1, A is the yoke or external portion of the armature core, B is the
inductor consisting of the usual yoke F and the laminated teeth b, b: CC are
the armature teeth, consisting, as is usual in such machines, of laminated iron,

page_text: the yoke E being ordinarily made of cast iron or mild steel. Surrounding the
inductor and supported by the yoke, are the field-magnet coils E' E', they being,
as is well understood, stationary. In the machine illustrated each of the armature
teeth is surrounded with a single coil D), but this is merely typical of any
selected type or connection of winding. G is the auxiliary air-gap which is
above referred to, separating the laminated armature iron from the yoke E. Be-
tween the yoke of the inductor and the supporting yoke E of the armature,
is the air-gap K, and the usual air-gap e is also located between the inductor
and the armature.
The action of this part of the invention will be best understood from Fig. 2.
The magnetic lines entering from the tooth in the upper part of the figure and
threading the coil, spread out in both directions in the iron of the armature,
reaching the yoke E at the points in a practically straight line from the tooth,
and at the point over the path y, which is curved and is considerably longer
than the path, having therefore higher magnetic reluctance. The magnetic
density at will be greater than at t. By the rotation of the inductor, at the
next moment the inductor tooth b will stand in front of the next tooth of the
armature and the path to the point will be approximately straight, while that
to the points will be curved and thus higher density will exist at the point t
than at the point s. To avoid these pulsations of magnetism in the yoke E is the
object of the air-gap G; the reluctance of this gap being very large as compared
with the reluctance of the armature iron as above pointed out, makes the total
reluctance of the path y practically equal to the reluctance of the path æ, and
thereby ensures practically a uniform distribution of magnetism in the yoke.
To assist the effect referred to there is also provided a short-circuited squirrel-cage
winding, consisting of the copper pins I (Figs. 1 and 2) which pass through the
iron of the armature near its outer edge and are connected upon each side of the
iron by the short-circuiting bands H also of low resistance metal. Any pulsation
or variation of the magnetic flux reaching the outside of the armature acts to
induce secondary currents in this short-circuited winding. These currents tend
to magnetize at the points of lower magnetic density, and to demagnetize
at the points of higher density, and thus cause a uniform distribution
of the magnetic flux at the outer part of the armature, that is, when the flux
enters the solid structure. The action of this winding is also beneficial in causing
uniform distribution of magnetism and in preventing pulsation of the magnetism
in the entire magnetic circuit due to the differences of reluctance in the main
air-gap of the machine caused by the revolution of the inductor. As already
pointed out, the reluctance is less when the inductor teeth stand in front of the
armature teeth, and greater when they stand in front of a slot, thus giving rise
to changes in the distributing of the flux, and inducing eddy currents in the
whole structure.
It has been proposed to remedy this to some extent by winding the field-
magnet coils E upon a solid copper ring or spool. While this is useful, the
construction above indicated acts in a more efficient manner, because it influences
directly the armature iron of the machine.
Having now particularly described and ascertained the nature of the said inven-
tion (as communicated to us from abroad) and in what manner the same is to
be performed, we declare that what we claim is: -
1. An inductor dynamo, comprising a revolving inductor and a stationary
armature, the latter having portions of laminated iron and a solid yoke, with an
auxiliary or additional air-gap in the armature iron.
2. An inductor dynamo having a revolving inductor and stationary armature,
the armature having on its outer edge a short-circuited or squirrel-cage winding.
3. An inductor dynamo, comprising a revolving inductor and a stationary
armature, the armature provided with an auxiliary air-gap in its circuit and a
short-circuited or squirrel-cage winding near its outer edge.

page_text: 4. In a dynamo, an auxiliary or additional air-gap between the laminated and
the solid parts of the magnetic circuit.
5. In an inductor dynamo, an air-gap between the laminated and solid parts
of the armature iron.
6. In a dynamo-electric machine of the inductor type, the combination of a
polar inductor, and an annular magnetic structure recessed to receive the armature
coils and having its magnetic resistance increased by an air-gap or gaps, of high
resistance compared with the reluctance of the armature iron.
Dated the 23rd day of November 1897.
For the Applicants,
J. C. CHAPMAN,
Chartered Patent Agent, 70 Chancery Lane, London, W.C.