The present invention relates to a DC inductor, and particularly to a DC inductor having at least one permanent magnet arranged in the core structure of the inductor.
A major application of a DC inductor as a passive component is in a DC link of AC electrical drives. Inductors are used to reduce harmonics in the line currents in the input side rectifier system of an AC drive.
The use of permanent magnets in the DC inductors allows minimizing the cross-sectional area of the inductor core. The permanent magnets are arranged to the core structure in such a way that the magnetic flux or magnetization produced by the permanent magnets is opposite to that obtainable from the coil wound on the core structure. The opposing magnetization of coil and permanent magnets makes the resulting flux density smaller and enables thus smaller cross-sectional dimensions in the core to be used.
As is well known, permanent magnets have an ability to become demagnetized if an external magnetic field is applied to them. This external magnetic field has to be strong and applied opposite to the magnetization of the permanent magnet for permanent de-magnetization. In the case of a DC inductor having a permanent magnet, de-magnetization could occur if a considerably high current is led through the coil and/or if the structure of the core is not designed properly. The current that may cause de-magnetization may be a result of a malfunction in the apparatus to which the DC inductor is connected.
Document EP 0 744 757 B1 discloses a DC reactor in which a permanent magnet is used and the above considerations are taken into account. The DC reactor in EP 0 744 757 B1 comprises a core structure to which the permanent magnets are attached. The attachments of the permanent magnets are vulnerable to mechanical failures since the permanent magnets are merely attached to one or two surfaces. Further the core structures in EP 0 744 757 B1 are fixed to a specific current or inductance rating leaving no possibility of expanding said rating using the same core structure and dimensioning.
One of the problems associated with the prior art structures relates thus to a possibility of modifying the same core structure for different current levels or purposes.
An object of the present invention is to provide a DC inductor so as to solve the above problem. The object of the invention is achieved by a DC inductor, which is characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea of providing a core structure that can be easily modified for different current levels. The core structure of the invention comprises a supporting member, which supports one or more permanent magnets and produces a magnetic path for the magnetic flux or magnetization of the permanent magnets. Further, the core structure includes one or more magnetic gaps formed by one or more magnetic slabs. Modifications to the properties of the DC inductor can be achieved by modification of these slabs.
An advantage of the DC inductor of the invention is that the same basic core structure can be used for different ratings. The length of the at least one supporting member can be changed, which allows changing the number of permanent magnets used. The supporting member further affects the inductance of the inductor and can be varied to achieve a desired inductance value. Further, the one or more magnetic slabs that are in the core structure can be modified in various ways. The magnetic slabs are used to provide magnetic gaps to the main magnetic path. The length of this gap can be adjusted with differing slabs having different properties. Further the slab can be used to provide non-uniform magnetic gaps providing differing properties for the DC inductor.
Thus the present invention gives the possibility of using basic core structure that can be modified depending on the application. This leads to considerable savings in production of inductors, since only the commonly used forms of the inductor core need to be specifically structured for the intended use.
In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which
The DC inductor of the invention comprises at least one coil 14 inserted on the core structure and one or more magnetic gaps 12, 13. The coil is typically wound on a bobbin and then inserted on the core structure in a normal manner. Alternatively, the coil can be wound directly to the core without a bobbin. The gaps are formed on the main magnetic path, by which it is referred to the magnetic path the magnetic flux of the coil flows. In the present invention, at least one of the possibly multiple magnetic gaps are formed by using magnetic slabs. In the embodiment of
Since magnetic slabs are used in the invention to create magnetic gaps, i.e. air gaps, the length and shape of the air gap so created can be varied by changing the dimensions and shape of the slab. Non-magnetic materials can also be used together with the magnetic slab(s) to support the slab(s) and to form the magnetic gap(s) to the core structure. Non-magnetic materials include plastic materials that have a similar effect in the magnetic path as an air gap. The magnetic gaps in a core structure are situated such that the gaps are used to direct or block magnetic flux in order to aid to suppress the demagnetization effect upon the permanent magnets. In addition, different magnetic gap dimensions affect differently the total inductance of the DC inductor. However, a larger air gap decreases the numerical value of the inductance of the inductor, but at the same time makes the inductance more linear while a smaller magnetic gap has the opposite effect.
In the embodiment of
The purpose of the supporting member is to support the permanent magnet 15 and simultaneously to provide a path for the magnetic flux of the permanent magnet. The flux generated by the coil senses the permanent magnet as a higher reluctance path and thus passes the permanent magnet via the magnetic slab 16. The magnetic flux of the permanent magnet on the other hand does not flow through the magnetic slab due to the reluctance encountered in air gaps, but flows through the coil 14 via the core structure and supporting member. The paths of magnetic fluxes are shown in
Since the supporting member is an element made of magnetic material, it can also be considered as a magnetic slab similarly to the slab 16. A magnetic gap may also be provided between the supporting member 17 and part 11d of the core structure. If so desired, the magnetic gap may be formed by a thin non-magnetic material piece inserted therebetween.
In
Since the permanent magnets are somewhat fragile and brittle quite easily from mechanical impacts, it is very advantageous to position them inside the core structure. It can be seen from
The permanent magnets are also strongly fastened to the core structure, since they are held in place from two opposing directions, i.e. above and below. The permanent magnets can be further glued or otherwise mechanically attached to the surrounding structure.
As seen from the
The magnetic slabs 16 are inserted in parallel fashion to the permanent magnets 15. The magnetic slabs are arranged in the main magnetic path, which means that slabs 16 are between the ends of the legs of the first U-shaped core and the base of the second U-shaped core. It is shown in
The structure of
The supporting members are extending from one leg of the core structure as shown in
The third embodiment described above is advantageous in that the upper and lower legs of the core can be made short while still holding multiple permanent magnets, since part of the permanent magnets are held outside of the core structure, but still inside supporting members giving protection and strong support against mechanical forces.
As with the other embodiments and their modifications, the supporting members can be further extended to accommodate more permanent magnets. Also the magnetic slab may be modified as described above.
In
In the embodiment shown in
The embodiment of
In
The permanent magnets are situated in
As with the previous embodiments, the supporting member is extendable to accommodate multiple permanent magnets. It is also shown in
In the modification shown in
In all of the above embodiments and their possible and described modification, the supporting members can be extended to hold more permanent magnets than shown or described. The number of the permanent magnets is not limited. Further the magnetic slabs in any of the embodiments or their modifications are modifiable. The slabs can be modified to have more or less magnetic gaps, which may be either uniform or non-uniform, depending on the intended purpose of the DC inductor. Magnetic gaps can also be provided at any joint between the supporting member and the core structure, the supporting member can thus also be considered as being a magnetic slab. Often it is more desirable to have multiple shorter magnetic gaps than one larger magnetic gap although the reluctance is defined by the total length of the magnetic gaps. This is due to the undesirable fringing effect of the magnetic flux which gets undesirable if magnetic gaps are too long.
In the above description, some shapes of magnetic material are referred to as letter shaped forms. It should be understood that a reference to a letter shape (such as āUā) is made only for clarity, and the shape is not strictly limited to the shape of the letter in question. Further while reference is made to a letter shape, these shapes can also be formed of multiple parts, thus the shapes need not to be an integral structure.
The above description uses relative terms in connection with the parts of the core structure. These referrals are made in view of the drawings. Thus for example upper parts refer to upper parts as seen in the corresponding figure. These relative terms should thus not be taken as limiting.
The term coil used in the document comprises the total coil winding wound around the core structure. The total coil winding can be made of a single wound winding wire or it can be made of two or more separate winding wires that are connected in series. The total coil winding can be wound on one or more locations on the core structure. The total coil winding is characterized by the fact that the substantially same current flows through every wounded winding turns when current is applied to the coil.
It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.
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