Inductor devices – Plural part core
Reexamination Certificate
1997-07-18
2001-05-15
Donovan, Lincoln (Department: 2832)
Inductor devices
Plural part core
C336S083000, C336S234000
Reexamination Certificate
active
06232865
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a tubular core for a controllable inductor with a main winding surrounding the core and a control winding passing substantially axially through said core, said core being intended to receive a magnetic flux from said main winding running substantially axially therethrough and comprising a number of core rings stacked co-axially on top of each other and connected to a rigid unit.
Such a controllable inductor is previously known from for example the applicant's WO 94/11891. The definition of “controllable” is to be given such a wide meaning, that it also comprises the case that a control current which is constant over time passes through the control winding.
A controllable inductor of this type functions in conjunction with a capacitor as a so-called harmonic filter in connection with a high voltage station for converting direct voltage to alternating voltage, wherein its main winding is connected to the high voltage net, usually on the alternating voltage side. In such a controllable inductor the permeability of its core and thereby the inductance is adjusted with the aid of the cross-magnetization generated inside the core by usually causing direct current to run through said control winding, alternating current would however also be possible to use, wherein the inductance of the inductor may be adjusted to exactly that frequency an overtone generated in the high voltage net is having for an effective fade-out thereof while causing small energy losses in the inductor.
To keep the heat losses that arise in the core due to the magnetic flux of the main winding, at a low level, the different core rings are usually formed by a winding of a thin sheet in several turns outside each other, while such eddy current losses/unit volume are proportional to the square of the thickness of the metal that a certain flux density passes through. The core rings have thereafter been given a rigid form, usually by a vacuum pressure impregnation, producing one core at a time and thereby requiring several and expensive fixtures, usually a single fixture for each core ring, to get the rings circular and plane, which has not always been successful. The so formed rigid rings have thereafter been stacked on top of each other and glued together with the aid of an electrically insulating glue therebetween. Because of that some of these core rings have become somewhat obliquely while being cured, the gap between two successive rings might thereafter become uneven and on some places too big, which results in that at the latter use of the core in the controllable inductor the resistance against the magnetic flux will become larger at these places, causing the flux lines to run obliquely out into the air, which in turn results in oblique directions of the magnetic flux and increased eddy current losses in the core.
A further disadvantage with this known production technique is that the cores resulting from this technique will become relatively fragile for impacts and delicate to transport.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a core and a method for producing a core for a controllable inductor in accordance with the preambles to the respective independent claims, which core and method to a large extent eliminate the above described problems by previous known methods and resulting cores thereof.
This object is achieved according to the invention by a method by that uncured core rings having a fixed shape are put successively against each other and aligned with each other regarding inner and outer surfaces of adjacent rings, and that thereafter successive core rings are bound to each other to a rigid construction while curing the core rings to a rigid form. Thanks to that uncured core rings having a fixed shape are put successively against each other and that these core rings are first cured in conjunction with their binding to a rigid construction, there is no need for expensive fixtures, but above all it is possible to avoid that the rings warp and it is possible to obtain a small but uniformly wide gap between two successive rings, so that the above mentioned problem with local oblique directions of flux lines and thereof resulting eddy current losses will be solved.
According to a preferred embodiment of the invention the uncured core rings are put against each other by putting them on top of each other. By such putting of the core rings on top of each other the gravity may be utilized in order to facilitate the alignment of the core rings to each other.
According to another preferred embodiment of the invention uncured core rings having a fixed shape are thread on the outside of an inner envelope with substantially equal outer cross section form as the inner cross section form of said cores in such a way that said joints between successive core rings are overlapped in axial direction of a part of said envelope continuously extending in this direction on each side of the joint in question, and that thereafter successive core rings and said core rings and said envelope are bound to each other to a rigid walled construction while curing said core rings to a rigid form. Thanks to that the inner envelope is utilized for supporting the core rings in place during the binding itself of the core rings to each other and to the envelope, these core rings may be cured to rigid core rings firstly at the binding of said core rings to each other forming a rigid core. In this way there is no need for expensive fixtures, but above all it is possible to avoid that the rings warp and it is possible to obtain a small but evenly wide gap between two successive rings, so that the above mentioned problem with concentrations of flux lines and thereof resulting heat losses will be solved. Thanks to that the joints between successive core rings will be overlapped in axial direction of said envelope and that this envelope will be bound to the core rings also a very strong, walled core will be obtained, said core being stable and may easily be handled and transported without risk for deformations thereof. With “uncured” means that the core rings have not adopted any rigid form when they are thread outside an inner envelope. The words “the core rings are thread on the outside of an inner envelope” are of course also comprising the case that a relative movement between these rings takes place so that it is the envelope that may be regarded to be pushed into the core rings. The curing of the core rings may take place by them being imparted a rigid form by curing of some binding agent or similar penetrated thereinto, possibly by way of an increased temperature, or simply by way of providing each core ring with a rigid form by connecting it both to the envelope as well as to an adjacent core ring.
According to another preferred embodiment of the invention the method comprises the step of carrying out said binding on the surface by a binding agent over substantially the complete surfaces in question adjacent to each other of the core rings and the envelope. In this way a very strong walled construction will be obtained and it is easy to handle and to transport.
According to another preferred embodiment of the invention the core rings are produced by winding a metal sheet in several turns on top of each other, said sheet being applied with an insulating layer, said core having an external and internal cylindrical form substantially conically tapering against the two outer ends, and the core rings having a substantially conical form are produced by cutting said metal sheet obliquely across the winding direction in connection with the winding, so that the width of the sheet decreases successively. In this way core rings for reducing eddy current losses at the ends of the core will easily be produced in the requisite form without any requirements of subsequent working, applying of specially produced conical outer casings or similar.
According to another preferred embodiment of the invention an electrically insulating binding agent
Ekwall Olle
Elofsson Dan
Holmgren Tommy
Sandin Bjorn
Valdemarsson Stefan
Asea Brown Boveri AB
Donovan Lincoln
Dykema Gossett PLLC
Nguyen Tuyen T.
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