Field of the Invention

The present invention relates to an electromagnetic device which, in particular, has application in power factor correction and power system filtration. More particularly, the present invention relates to a partial core power inductor comprising a partial core member and a first magnetically non-permeable member wherein the first magnetically non- permeable member is configured to introduce a coupled resistance characteristic to the partial core power inductor.

Background

The presence of reactive, typically inductive, loads in electrical distribution systems causes a non-unity power factor. Inductive loads include motors, transformers, ballasts and many other devices. Correcting for this non-unity power factor can improve the efficiency of the power systems. This is because the presence of reactive power in an electrical distribution network, or any electrical network, results in greater current being required to be supplied without having additional real power output. Greater current in the network leads to greater resistive losses and higher device rating requirements. Therefore it is advantageous to improve the power factor to unity, or near unity. In some cases it is preferable that the power factor is improved to between 0.95 lagging and 1 , or, unity. US6259614 shows a method of power factor correction using a switch. In this method a MOSFET switch is used to control the power flow in the network in connection with a sensor. This arrangement can improve the power factor of the network but will result in greater control requirements and may require a skilled person to operate or check the operation of the system. MOSFET based systems have losses in the order of 2.5 to 3% and therefore generate significant amounts of heat as power levels rise above 100 kVA or so.

US5387821 shows a method of passive power factor correction in which a capacitor is connected in parallel with the inductive load. The capacitor counteracts the reactive current created by the inductive load. The capacitor must be sized to match the reactive power of the inductive load. In some cases it may be necessary to have a plurality of capacitors which may be switched into the circuit to provide the required amount of power factor correction.

Capacitors connected in parallel with a power system to improve the power factor will form a natural resonance with the power system. This is due to the typical power system being inductive. This natural resonance may occur at any frequency, but will typically be in the audio frequency spectrum. Therefore it is usually necessary to treat the capacitor with some form of filtration to limit or minimise unwanted effects due to the presence of harmonics and natural resonances of the system. In addition, due to harmonic voltage distortion being prevalent in modern power systems, capacitors may draw significant harmonic currents due to having diminishing impedance at harmonic frequencies.

Harmonic voltage distortions on a power supply may be due to the connection of non-linear loads that generate a certain frequency spectrum of harmonic current such as variable frequency drives for motors. Harmonic currents flow from such non-linear loads into the supply system and develop harmonic voltages in proportion to the impedance of the system. Harmonic currents tend to flow in proportion to the fundamental current flow of the load and are not greatly influenced by the impedance of the power system. Therefore if the power system has a high impedance at the frequency of the harmonic current flow, the voltage developed across that point in the network may also be relatively high. It is therefore desirable to reduce the impedance of the power supply at harmonic frequencies, thus lowering the effective voltage that may develop. A shunt filter can be used to achieve this as well as improving the power factor at mains frequency (displacement power factor).

Objects of the Invention

It is an object of the invention to provide an electromagnetic device which will allow for improved filtration in power factor correction systems and/or shunt filters. In particular, a partial core inductance is used as the basis of this new device which preferably also enables reduction in weight and physical size. It is an object of the invention to provide partial core electromagnetic devices which will at least go some way to overcoming disadvantages of existing systems, or which will at least provide a useful alternative to existing systems.

Further objects of the invention will become apparent from the following description. Summary of Invention

Accordingly in a first aspect the invention consists in a partial core power inductor comprising:

one or more windings associated with a partial core member having a first and a second end;

a first magnetically non-permeable member provided at the first and/or the second ends of the partial core member; and wherein the first magnetically non-permeable member is configured to introduce a coupled resistance characteristic to the partial core power inductor.

In an embodiment the partial core power inductor may further comprise a first magnetically permeable member provided at the first and/or end of the partial core member and the first magnetically non-permeable member.

In an embodiment the one or more windings is adapted to conduct electricity along its length. In an embodiment the first magnetically non-permeable member is configured to provide physical support for the partial core and/or the one or more windings.

In an embodiment the first magnetically permeable member is adapted to tune a magnetising reactance of the partial core power inductor and the first non-permeable member.

In an embodiment the first magnetically permeable member may form a complete shell around the device.

In an embodiment the first magnetically permeable member may further consist in one or more composites or mixtures of magnetically permeable and/or magnetically non-permeable materials and electrically conductive and/or non-conductive materials, including but not limited to a cast magnetically permeable resin or other exotic material(s). .

In another aspect the invention consists in a partial core power inductor comprising:

one or more windings associated with a partial core member having a first and a second end; the one or more windings adapted to conduct electricity along its length; and a first magnetically non-permeable member provided at the first and the second ends of the partial core; and

wherein the first magnetically permeable member is adapted to tune a magnetising reactance of the partial core power inductor and the first non-permeable member.

In an embodiment, the first non-permeable member is configured to introduce a coupled resistance characteristic to the circuit component. In an embodiment the partial core power inductor further comprises a first magnetically permeable member provided at the first and/or second end of the partial core and non- permeable member. In an embodiment, the first non-permeable member is configured to provide physical support for the partial core member and/or the one or more windings. The present system uses partial core power inductor technology to achieve filtering devices and inductors with special characteristics. The stray magnetic fields that emanate from the ends of the partial core device are ordinarily considered undesirable. This is due to uncontrolled magnetic fields coupling with neighbouring equipment giving rise to induced eddy currents and unwanted related effects. However such stray fields can be controlled and harnessed by a first magnetically permeable member and a first magnetically non-permeable member to effectively produce unique features in partial core inductors. The unique features include a tuneable, frequency dependent resistance in the partial core power inductor along with tuneable inductance and a power to weight ratio that reduce the weight of comparable conventional technology by up to 50%. Along with that there is a commensurate reduction in physical size.

In an embodiment the first magnetic permeable member and the first magnetic non- permeable member are configured to provide a tuneable, frequency dependent resistance in the partial core power inductor along with tuneable inductance.

In an embodiment the first magnetic permeable member and the first magnetic non- permeable member are configured to provide a predetermined, and tuneable, coupled resistance characteristic and inductance. In an embodiment the first magnetically non-permeable member is a conductive or non- conductive material configured to physically support the partial core member and/or the one or more windings.

In an embodiment the partial core power inductor comprises a magnetically non-permeable, electrically conductive and/or non-conductive element. In an embodiment the electrically conductive element comprises non-ferrous material. In an embodiment the electrically conductive element is located at least at one of the first or second ends of the one or more windings. In an embodiment the first magnetically non-permeable member is an electrically conductive material connected at the ends of the partial core inductor. The material is chosen to have a specific electrical resistivity whereby if the resistivity of the conductive material is lower, the coupled equivalent series resistance will be higher, and is configured to tune the resistance coupled into the device by means of eddy current loss.

Preferably, the electrically conductive material in the first magnetically non-permeable member is adapted to provide a predetermined electrical resistivity that may be fixed or dynamic depending upon the temperature of operation.

In an embodiment the first magnetically non-permeable member comprises pure or alloyed metals which may be perforated, slit or laminated to obtain the predetermined electrical conduction and heat dissipative characteristic.

In an embodiment the first magnetically permeable member forms a C shaped section extending from the first end of the partial core member to the second end. In an embodiment the first magnetically permeable member comprises a plurality of magnetically permeable members.

In yet another embodiment the first magnetically permeable member forms a cap or end piece or extends in a plane substantially perpendicular (or radially) to the orientation of the partial core member.

In an embodiment the partial core comprises a magnetically permeable material. In an embodiment the system comprises a plurality of partial core members and associated one or more windings. In an embodiment the first magnetically permeable member comprises alloyed metals which may be perforated, slit or laminated to obtain the desired magnetic and conductive characteristic.

In embodiments the first magnetically permeable member in combination with the partial core member comprises a discontinuous magnetic path.

In an embodiment the first magnetically permeable member comprises a plurality of magnetically permeable members. In an embodiment at least one magnetically permeable member is located at the first and/or second ends of the partial core member and/or the one or more windings. In an embodiment the first magnetically permeable member is spaced apart from the partial core member(s). In an embodiment the first magnetically permeable member comprises a plurality of spaced apart magnetically permeable members. In an embodiment the first magnetically permeable member has a geometry wherein the magnetically permeable member would be magnetically saturated if completely or closely coupled to a magnetic core.

In an embodiment the partial core power inductor may further comprise one or more windings associated with one or more partial core members.

In an embodiment the first magnetically permeable member at least partially surrounds or encircles the partial core member(s) and/or the one or more winding(s). In an embodiment the first magnetically permeable member forms a cylinder, or a portion thereof, associated with at least one of the first and second ends of the partial core member and/or the one or more windings. The magnetically permeable member is adapted to tune the winding resistance, capture stray magnetic fields and/or dissipate energy. In an embodiment the first magnetically permeable member comprises alloyed metals which may be perforated, slit or laminated to obtain the desired magnetic, conductive and heat dissipative characteristic.

In an embodiment the conductive element is located between the partial core member and the magnetically permeable member.

In an embodiment the partial core power inductor further comprises a plurality of magnetically permeable members as herein described and/or conductive members. In an embodiment the first magnetically permeable member is conductive.

In an embodiment one or more windings and/or partial core members are associated with the first magnetically permeable member. In an embodiment two or three windings are associated with the magnetically permeable member. In an embodiment two windings are arranged on separate partial cores, the partial cores adjacent to each other or three windings are arranged in a similar triangular arrangement. In an embodiment the two or three windings are each connected to one phase of a single, two or three phase power system. In an embodiment the first magnetically permeable member may enclose the first and/or second ends of the partial core member(s). In an embodiment the first magnetically permeable member forms a complete or an incomplete magnetically conductive path around the partial core member(s).

In an embodiment the first magnetically permeable member encircles at least one end of the first and/or second end of a grouped winding.

In an embodiment the first magnetically permeable member extends above and/or along any portion of the axis of the grouped winding. The position of the first magnetically permeable member is predetermined by the influence it has on the electrical and magnetic properties of the component.

In yet another embodiment the first magnetically permeable member substantially encircles at least one end of the grouped windings. In an embodiment the magnetically permeable surround extends along only part of the partial core member and/or the one or more windings. In an embodiment the first magnetically permeable member substantially surrounds or encircles a second magnetically permeable member and/or the conductive element.

In an embodiment the magnetically permeable member is perforated or slit or laminated. In an embodiment the magnetically permeable member is not continuous or full band along its perimeter.

In an embodiment the first magnetically permeable member extends along a portion of the axial length of the one or more windings and/or core. In an embodiment the magnetically permeable member extends along between 70% and 5% of the core or winding, and more preferably between 40% and 15% of the core or winding and most preferably between 30% and 20% of the core or winding. In an embodiment the magnetically permeable member is substantially parallel to the axis of at least one of the cores. In an embodiment the magnetically permeable member extends parallel to the core with a first end coincident with the axis of the core and a second end beyond the end of the core. In an embodiment the first magnetically permeable member may extend beyond the end of the partial core member parallel to the axis of the partial core power inductor. In an embodiment the partial core power inductor provides a frequency dependent resistance in-line with an inductance. In an embodiment the frequency dependent resistance of the system is tuneable by the position of the second magnetically permeable member or partial core member with respect to the windings.

In an embodiment the position of the magnetically permeable member(s) is also able to tune the device resistance characteristic.

In an embodiment the second magnetically permeable member is a substantially co-axial member. In an embodiment the second magnetically permeable member has substantially constant cross section, or is a cylinder. In an embodiment the one or more windings extends from the first to the second end of the second magnetically permeable member. In an embodiment the second magnetically permeable member forms a partial core member. In an embodiment the first magnetically permeable member is spaced apart from a first end of the second magnetically permeable member.

In an embodiment the first magnetically permeable member is spaced along the axis of the second magnetically permeable member at a distance from an end of the one or more windings. In an embodiment the first magnetically permeable member is a substantially planar member. In an embodiment the first magnetically permeable member extends in a radial plane from the axis of the second magnetically permeable member.

In an embodiment the first magnetically permeable member is spaced around the circumference of the second magnetically permeable member. In an embodiment the first magnetically permeable member extends along the axis of the second magnetically permeable member each side of the end of the second magnetically permeable member. In an embodiment the first magnetically permeable member surrounds a portion of the one or more windings. In an embodiment the first magnetically permeable member is discontinuous or does not form a continuous band around the second magnetically permeable member. In an embodiment the first magnetically permeable member covers a distance of up to 30% of the coaxial length of the one or more windings and is arranged substantially at the end of the one or more windings. In an embodiment the partial core power inductor comprises a third magnetically permeable member at a second end of the one or more windings. In an embodiment the third magnetically permeable member has any one or more of the embodiments of the first magnetically permeable member.

In a further aspect the invention consists in a circuit comprising a partial core power inductor as described in any one of the aspects herein.

In an embodiment the circuit is a power factor correction circuit. In an embodiment the partial core power inductor is electrically connected in parallel to a reactive load. In an embodiment the reactive load is capacitive or inductive. In an embodiment the partial core power inductor is connected in series with a capacitance or capacitor.

In a further aspect the invention consists in a shunt filter adapted to electrically connect to a load, the shunt filter circuit comprising:

a first component, the first component having one or more windings associated with a partial core member; the partial core member having a first and second end; the one or more windings adapted to conduct electricity along its length;

a first magnetically non-permeable member provided at a first and second ends of the partial core member;

a first magnetically permeable member extending from the first end of the partial core member to the second end,

wherein the first and second magnetically permeable members form a discontinuous magnetic path. In an embodiment the shunt filter comprises a capacitance electrically coupled to the first component.

In a further aspect the invention consists in a method for providing a partial core power inductor, the method comprising;

providing one or more windings being associated with a partial core member having a first and a second end; the one or more windings adapted to conduct electricity along its length; and

providing a first magnetically non-permeable member at the first and the second ends of the partial core member; the first magnetically non-permeable member configured to provide physical support for the partial core member and/or one or more windings; and

wherein the first magnetically non-permeable member is configured to introduce a coupled resistance characteristic to the partial core power inductor. In another aspect the invention further comprises a partial core power inductor comprising: a partial core member having a first and a second end; and

a first magnetically non-permeable member provided at the first and/or second ends of the partial core member;

wherein the first magnetically non-permeable member is configured to introduce a coupled resistance characteristic to the partial core power inductor.

In another aspect the invention further comprises a partial core power inductor having a partial core member wherein the partial power core inductor is an inductor for power factor correction.

In another aspect the invention further comprises a device and/or circuit comprising a partial core power inductor as described in any one of the aspects herein.

In an embodiment the first magnetically permeable member forms a C shaped section extending from a first end of the partial core to the second end.

The disclosed subject matter also provides a device suitable for incorporation with power factor correction and harmonic filtration schemes which may broadly be said to consist in the parts, elements and features referred to or indicated in this specification, individually or collectively, in any or all combinations of two or more of those parts, elements or features. Where specific integers are mentioned in this specification which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated in the specification.

Further aspects of the invention, which should be considered in all its novel aspects, will become apparent from the following description.

Drawing Description

A number of embodiments of the invention will now be described by way of example with reference to the drawings in which:

Figure 1 is an image of an embodiment of the device showing the device (a) in use and (b) in exploded form,

Figure 2 is a cross-section of Figure 1 showing the stray flux paths coupling to the various elements of the device,

Figure 3 is a photograph of an embodiment of the device of Figure 1 , Figure 4 is an equivalent circuit for one winding of the device of Figure 1 ,

Figure 5 is a graph of the total series resistance of the device of Figure 1 compared to conventional inductor technology,

Figure 6 is a circuit diagram showing a use of the device of Figure 1 applied in a three phase reactive power compensation circuit, and

Figure 7 is a schematic of a first embodiment of a power factor correction and harmonic filter circuit containing shunt impedances.

Detailed Description of the Drawings

Throughout the description like reference numerals will be used to refer to like features in different embodiments.

The use of a partial core (i.e. between air core and full core) transformer technology is unusual in power applications because at low frequencies a large number of turns is required to create sufficient reactance to minimise the transformer magnetising current. However as it is described herein there are applications where partial core technology can provide a significant power to weight advantage compared to conventional full core technology in the described embodiments. The improvements described herein harness the properties of partial core inductance. The stray field effect of partial core, open core or air core inductors is harnessed in such a way as to provide the benefits of partial core technology applied to power inductors that may be applied to filtration devices in power systems.

An open or partial core device has a stray magnetic field that emanates along the length and from the ends of the device. In contrast stray fields are minimised by the use of a full core or a core with a small air gap, to increase the inductance and minimise the influence of stray fields coupling to neighbouring metallic objects. As described herein the arrangement of magnetically permeable material in the region outside the ends of the winding or core allows the stray fields to be controlled and harnessed. This provides a variable parameter that allows frequency dependent effects, along with a significant reduction in core weight and physical size of the device compared to a full core inductor of the same inductance and power rating. A partial core inductor requires a winding of an increased number of turns to achieve the same inductance as a full core device due to increased magnetic circuit reluctance. This increased number of turns reduces the flux density in the core and therefore a core of smaller cross section can be used in an equivalent application. In particular embodiments of the system a partial core power inductor is created 10 when the magnetically permeable partial core 2 and/or winding 1 are surrounded with one, two or a plurality of magnetically permeable members 5. These members may be referred to be based upon their approximate location or shape. For instance possible magnetically permeable members include an end piece and guard or halo. The magnetically permeable member 5 substantially completes surrounds or encircles the winding(s) 1 the magnetic circuit. However the partial core 2 and the member 5 do not fully complete a magnetic circuit.

Preferably both the first and second ends of the winding 1 are covered or surrounded by the guard 5. The use of the magnetically permeable material 5 enables the stray fields to be effectively captured or redirected. This causes a small increase in the device inductance. However, the addition of magnetically permeable member 5 significantly changes the resistance characteristic of the device. The size and shape of the magnetically permeable member 5 determines the amount of stray field absorbed. For instance the effect of the magnetically permeable member will reduce after a certain length of the winding 1 is covered.

The magnetically permeable member 5 provides a lower reluctance path than the surrounding material (e.g. atmosphere, air, liquid or gas). However, because the magnetically permeable member 5 is not directly connected to the core material 2 a variety of other paths are also possible for the magnetic flux. This means that the main field passes through the core 2 and the magnetically permeable member 5 is used to magnetically couple to, and/or absorb, the electromagnetic energy of the stray field. This can be dissipated through heat. Because the magnetic field is distributed over a significant surface around the ends of the winding 1 and/or core 2 the flux density at the core 2 ends is reduced and therefore a smaller volume of material in the member 5 is permissible without leading to saturation of the magnetically permeable member as compared to the partial core 2.

The geometry of the member 5 will vary its effect. Preferably the member 5 is substantially planar, and initially extends in a radial direction away from one end of the coil/partial core and then turns in a parallel direction towards the other end. It then turns back radially towards the coil/partial core.

A spacer 3 may be used to separate and insulate the winding from the magnetically non- permeable member 4. Figures 1 and 2 show the magnetically non-permeable member 4 which serves the following purposes; Mechanically support the partial core 2 and winding 1 within the device and between one another.

Space apart the partial core 2 and member 5. This means that magnetically non- permeable member 4 may be used to adjust the air gap to the partial core end which tunes the magnetising reactance Xm shown in the equivalent circuit of Figure 4.

Provide an electrically conductive path between the partial core 2 and member 5 so that they maintain an equal electrical potential.

Provide a thermally conductive path to enable heat generated within the partial core 2 to be conducted to the member 5 where heat dissipation is more effective due to increased surface area.

Provide a means of controlling the resistance characteristic of the device by choosing a material of the required electrical resistivity.

Figures 1 and 2 show an embodiment of the system. Figure 1 shows an in use device 10 and an expanded device 10. Figure 2 shows a cut away view around one of the cores 2 of the device of figure 1.

The magnetically non-permeable member 4 is adapted to support and separate the partial cores and windings from member 5 and to contribute to the resistance characteristic Rc"of the device which is governed by the choice of material of member 4.

The magnetically permeable member 5 absorbs and dissipates as heat, a portion of these eddy currents produced by the stray field. The position, or extent, of the magnetically permeable member 5 in relation to the winding(s) 1 (and sometimes the core(s) 2) can be used to control the amount of stray field absorbed.

The magnetically permeable member 5 can be used to shield the stray fields from coupling to neighbouring metallic objects including adjacent devices 10. This enables closer proximity of one device to another without influencing the electrical characteristic of the devices.

The magnetically permeable member 5 can be chosen to have desired characteristics of magnetic permeability and electrical resistivity. For instance ferrous sheet metals or cast ferrous metals of particular alloys may be used. In other embodiments metals may be configured in laminated sandwiches or in cast compositions with perforations or slits to obtain the desired magnetic, conductive and heat dissipative characteristic. Such slits or perforations may be provided to enhance the magnetic properties of the first magnetically permeable member in relation to the electrical characteristics of the device, or they may assist the device to dissipate heat more freely.

The electrically conductive element acts to couple a resistance (Rc") to the device. The resistivity of the electrically conductive element and its physical size and shape determine the magnitude of this resistance (Rc") and thus control the magnitude of eddy currents into the electrically conductive element. The larger the portion of the electrically conductive element encountering stray magnetic fields, the greater the eddy currents and therefore the lower the resistance Rc". The induction of eddy currents in the electrically conductive element give rise to losses dissipated as heat. The electrically conductive element may be thermally coupled to the end piece, another form of heat sink, or a further magnetically permeable member, to enable the dissipation of heat. The electrically conductive element may be thermally insulated from the winding and the core. The electrically conductive element is preferably a substantially planar element extending radially from the axis of the core (or a shared axis between the cores). The electrically conductive element may be a electrically conductive metal sheet or be formed by a winding, or plurality of windings or wires.

In one embodiment the member 5 acts as a shield member surrounding the end portion of the windings and the core. In the embodiment of Figures 1 and 2 the member 5 extends partially over the windings 1 and surrounds the end piece. Preferably the member is arranged so as to be on the outside of the windings, that is to say the windings pass between the partial core member and the member 5.

The member 5 can capture stray magnetic fields and couple them with neighbouring coils, increasing Xm in a similar manner to that of the end piece. The member 5 also controls the stray magnetic field through the winding ends which tunes R _s as shown in Figure 4.This is because stray magnetic fields predominate towards the ends of the winding. The stray field may cut or pass through the windings and give rise to a form of proximity effect. The effect of the field passing through the winding increases the resistance R _s of the winding. The winding resistance is therefore dependent on (increasing with) frequency. The member preferably is adapted to control how the magnetic fields cut through the windings and therefore control the frequency dependent ac resistance R _s of the winding. The member 5 can also encourage the stray field to pass through the winding 1 therefore increasing the resistance of the winding 1.

It is also possible to form the magnetically permeable member 5 from a plurality of individual or connected magnetically permeable members. These may be shaped or spaced apart so as to substantially encircle the core(s) 2 and windings (1 ). The magnetically permeable member(s) 5 may therefore not be continuous but can be formed in a plurality of sections, or have slits or gaps. In some embodiments these may be used to create or encourage particular absorption or to improve heat dissipation.

The member 5 is preferably positioned near to the partial core 2 (although insulated 3 separated 4 therefrom). The separation determines the intensity of the field through member 4. A closer proximity to member 5 will produce a lower resistance Rc' A closer proximity of member 5 will therefore dissipate more heat. The member 5 has a preferable thickness related to the skin depth of the flux in its material. Depending on the magnetic field strength, and the member 5 material, the thickness of the member 5 will determine the extent of stray field capture and the how well the member 5 couples a lower resistance Rc'. In a multi-phase device with multiple partial core inductances combined into a partial core power inductor 10 the material of the member 5 is chosen to effectively capture stray magnetic field and improve magnetic coupling between the windings 1 . This magnetic coupling is balanced with the conductivity of the member 5 material. Where the member 5 has a high conductivity it will enable the generation of stronger eddy currents within the material, giving rise to a lowering of Rc' and more heat production. Therefore the design of the member 5 involves a trade-off between these characteristics depending on the desired electrical features.

Due to the ability of the device or system to dissipate significant heat by having a magnetically permeable material coupled to, but separate from the core 2 and windings 1 , very high levels of harmonic voltage distortion can be tolerated by the device for short periods of time, or for longer periods of time by the inclusion of additional cooling systems such as heatsinks, forced air or liquid cooling.

The magnetically permeable member 5 is preferably used to also connect the stray magnetic fields emanating from around the ends of the cores associated with two or a plurality of further windings 1. Windings 1 are each wound on separate magnetically permeable cores 2 so that the inductance of the windings does not cancel out each other, as in a three phase configuration for example. A benefit of this effect is that the member 5 is wrapped around or encircles all legs of the device 10 but does not induce any coupled current around the perimeter of the member 5. This is because there is no net magnetic field passing through the axis of the member 5.

When three windings are combined as shown in Figure 1 the alternating currents flowing through each winding, or leg of the device, are phase shifted by 120 degrees therefore affecting the electrical characteristic of the device 10 due to magnetic coupling between the three windings 1 and cores 2. This allows member 5 to cover the multiple windings without acting as a short circuit loop for a single winding. This increases the effective inductance of each winding. A similar system can be arranged in a two phase system at 180 degree phase shift. The plurality of windings allows an arrangement in which no net current flows in the magnetically permeable member surrounding the windings. This avoids a short circuit turn forming.

Figure 4 shows an equivalent circuit which is developed from the Stienmetz transformer equivalent circuit and may be used to describe the operation of the device 7. This circuit diagram shows the following components (which are present for each of the windings of the device 7):

Rw - Winding ac resistance: Rw is the winding ac resistance and is equal to the dc winding resistance at OHz (DC) and increases with frequency due to skin effect in the conductor. The skin effect for aluminium and copper conductors is not very significant at power frequencies for the conductor sizes generally used in this work. Any type, size or shape of conductive winding material may be used to achieve the desired characteristic.

Rc - Core loss resistance; Rc approximates the core loss resistance and is the parallel combination of two magnetically induced effects in the core; eddy current loss and hysteresis loss. The effect of these losses is to couple a resistive burden across the magnetising reactance Xm. As frequency increases, the effect of the core loss resistance becomes more prominent and the magnetising reactance reduces relative to the value it would be without the loss resistance connected in parallel.

Rc'- Electrically conductive element loss resistance Rc' is due to eddy current loss in conductive materials coupled magnetically by the stray magnetic field emanating from the core. Such conductive materials include the windings 1 and member 5. This resistance is frequency dependent. There is also a frequency dependence due to skin effect in the conductive materials, but this effect is usually small at power frequencies with materials typically selected for practical devices.

Rc"- Electrically conductive element loss resistance Rc" is due to eddy current loss in conductive materials coupled magnetically by the stray magnetic field emanating from the core such as in the magnetically non-permeable member 4. Depending on the resistivity of the conductive material 4 the resistance may be small or very large to the point it has a practically negligible effect.

X _L - Leakage reactance; and X _M - Magnetising reactance. These inductances are determined by the number of turns and physical dimensions of the windings and the core. In particular, the present device introduces the term for winding stray field dependent resistance R _s which is a frequency dependent resistance in the winding and is controllable by the geometry of the winding itself. R _s has a significant frequency dependence which is not due to skin effect. R _s is a frequency dependent resistance akin to that produced by the 'proximity effect' caused by currents in the winding. Where stray magnetic flux cuts through the windings, particularly at the ends of a partial core or winding, it gives rise to longitudinal eddy currents in the winding conductor in the same manner as those produced by the classical proximity effect in power inductors and transformers. These eddy currents effectively diminish the cross sectional area of the winding conductor, increasing its resistance, and therefore directly impeding the flow of higher frequency currents through the conductor. Therefore R _s is not directly due only to the current in neighbouring windings, but predominantly due to the stray flux which acts in the same plane as the magnetic field produced by current in the winding. R _s is also influenced by the winding material type, size and shape. R _s is represented as a series resistive element in the equivalent circuit and may be a significant component of the total ac resistance of partial core devices.

Figure 5 shows a comparison of total series resistance for a typical power inductor and that of an embodiment of the device described herein. The devices compared in Graph 5 have a similar inductance and power rating so that the comparison of the total series resistance is meaningful. Due to the inherent nature of partial core technology as harnessed in the described embodiment, a reduction in total series resistance, typically in the order of 20% to 50% can be achieved along with a reduction in device weight in the order of 10% to 50%. The total series resistance at the main power frequency is comparable, however at higher frequencies the reduction of resistance in the embodiment ranges between 20% to 50% depending on the frequency and the specific design of the device. This observed characteristic is typical for the class of partial core power inductor devices where the total series resistance has been optimised to be at its lowest possible value. Such devices are most suitable for applications further described in this patent application as power factor correction chokes and series and shunt filter reactors.

By changing the physical parameters of the device as described in this application it is possible to increase the value of the series resistance by 300% and greater depending on the means of heat removal employed, and without creating more undesirable heat in the windings of the device. Resistance can be added by the placement of members 4 and 5 in relation to the partial core as well as by selecting the material used in these members. The physical nature of the members 4 and 5 mean that heat dissipated in these elements can be easily removed by convectional air movement, forced air movement or other means such as liquid cooling.

Figure 6 shows the use of the device in a three phase circuit. The circuits of Figure 6 may be applied as a shunt filter and power factor correction device 14 shown in more detail in Figure 7. The circuit shown in Figure 6 is a three-phase unit and therefore has three windings, one per phase. However two phase or single phase devices are possible as described above. In the three phase circuit a device as shown in Figures 1 and 2 may be connected with one winding 1 being connected to each phase of the circuit. Figure 6a shows a reactive compensation 60 for a lagging power factor circuit where each of the phases is connected in series with a capacitor. The circuit may also use a circuit breaker 63 and switching means 62 to control the operation of the compensation network. Figure 6B shows a reactive compensation 61 for a leading power factor circuit where the capacitor of Figure 6A has been removed. In embodiments of the system the reactive compensation can be controlled by disconnecting elements or switching between leading and lagging correction when required.

Several devices 10, 60, 61 as herein described or combinations may be required to treat the displacement power factor at mains frequency. The combined impedance of the several devices 10, 60, 61 can be adapted to allow a lowering of the supply impedance at target harmonic frequencies. The device can therefore act in conjunction with a capacitance as a shunt filter to the power system.

Figure 7 shows an embodiment of a power factor correction circuit for a single phase, but which can be generalised to a multiphase system. The power factor correction circuit 70 controls or adapts the relationship between a power source 100 and a harmonic load 12 or reactive loads 13. The harmonic load 12 may be a non-linear load and therefore introduces harmonics to the circuit 70. It is desirable to stop these harmonics from propagating into the power distribution network (e.g. connected to power source 100) as the harmonic currents may develop significant harmonic voltages across the supply. A harmonic trap uses a shunt impedance 14 (typically a tuned capacitance) to provide an alternative path for harmonic currents 19 relative to the impedance of the shunt and the supply. The balance of harmonic currents will propagate to the supply 10 from the harmonic loads 12.

Figure 7 shows a plurality 14 of devices or combinations of devices 10 are adapted to be switched 16 into a circuit parallel to a load 12, 13. The plurality of devices 14 is adapted to further reduce harmonic voltage distortion on the supply busbar 1 1. The reduction of voltage distortion lowers the heating effect in the circuit 14 and therefore enables the system to achieve operational stability. In embodiments of the circuit 14, thermal over current protection and over temperature monitoring would be provided for each device to limit harmful heating effects. The control of the switching 16 may be by a controller which has a measuring device in the circuit 70. For instance controller 102 may receive a measurement of a circuit characteristic from measurement device 101 or sensor 101. The controller may then send a control signal to switching means or switches 16 to control the operation of the shunt network. This may be advantageous where a number of harmonic loads 12, 13 are connectable to the power source 100 or to better match the power factor of the system. In embodiments on the system harmonic loads may have individual filters, or preferably a single filter is connected to a plurality of loads. The switching means or switches 16 allow one or more of the shunt impedances 10, 18 to be connected to the supply busbar 1 1.

In an embodiment the shunt filters 10, 18 are connected automatically to improve the displacement power factor at mains frequency. As loading increases, more shunt filters 10, 18 are connected which lowers the busbar 11 harmonic voltage. If there is little or no harmonic load present, the shunt filters will simply improve displacement power factor. The automatic control may use the sensor 101 and controller 102. Sensor 101 may be a control relay which senses the current and voltage prior to the series impedance and operates to switch in branches 10, 18 of the circuit to improve the true power factor to near unity apparent at the supply. In doing so, the voltage on the supply busbar 11 is maintained at the correct level and compensates any mains voltage (e.g. 50Hz or 60Hz) sag due to the reactance loading 12, 13. Variable reactive control ensures that the correct amount of reactive compensation is connected depending on the load, which has the added benefit of treating harmonic distortion on the supply and maintaining good voltage regulation at the load.

In the case where the power factor is leading due to the loads being capacitive in nature, a partial core power inductor 10, 20 of suitable rating can be switched into the circuit 11 to provide inductive reactive compensation without a series capacitance being required.

The partial core power inductor 10 herein described harnesses partial core inductor technology to produce a particular feature in the total resistance of the device as shown in the graph of Figure 5. This characteristic is useful in filtration devices because the reduction of resistance in the device at harmonic frequencies allows for a device with more efficacy at the target frequency. At the target frequency, having a lower resistance means that a greater filtration effect can be achieved whilst dissipating less power. Capacitors may be therefore utilised to treat both the power factor of a system and any harmonic distortion present by tuning the capacitor bank to target harmonic distortions and thus minimise the effect of these distortions on other equipment connected to the same supply. Embodiments of this approach have the benefit of being both cost effective and relatively efficient having losses typically in the order of 1 % or less.

Unless the context clearly requires otherwise, throughout the description, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention. The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features. Furthermore, where reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein incorporated as if individually set forth.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.