Title of Invention

"AN ELECTRIC CHOKE COMPRISING A COIL AND A FERROMAGNETIC METAL ALLOY CORE"

Abstract

An electrical choke comprising a coil and a ferromagnetic metal alloy core, the core consisting of an amorphous metal alloy characterized in that said core having a discrete gap, and comprising a non magnetic spacer located in an opening defined by said discrete gap, said discrete gap having a gap size determined by the thickness of said spacer and a distributed gap, the amorphous metal alloy of the core being partially crystallized and having an annealed permeability in the range of 200 to 1000.

Full Text

The present invention relates to an electrical choke. This invention relates to a magnetic core composed of an amorphous metallic alloy and adapted for electrical choke applications such as power factor correction (PFC) wherein a high DC bias current is applied. 2. Description Of The Prior Art; An electrical choke is a DC energy storage inductor. For a toroidal shaped inductor the stored energy is W=l/2 [(B2Aclm)/(2µoµr)], where B is the magnetic flux density, AC the effective magnetic area of the core, lm the mean magnetic path length, and µo the permeability of the free space and µr the relative permeability in the material. By introducing a small air gap in the toroid, the magnetic flux in the air gap remains the same as in the ferromagnetic core material. However, since the permeability of the air (µ~1) is significantly lower than in the typical ferromagnetic material (µ -several thousand) the magnetic field strength(H) in the gap becomes much higher than in the rest of the core (H=B/(i). The energy stored per unit volume in the magnetic field is W=1/2(BH), therefore we can assume that it is primarily concentrated in the air gap. In other words, the energy storage capacity of the core is enhanced by the introduction of the gap. The gap can be discrete or distributed. A distributed gap can be introduced by using ferromagnetic powder held together with nonmagnetic binder or by partially crystallizing an amorphous alloy. In the second case ferromagnetic crystalline phases separate and are surrounded by nonmagnetic matrix. This partial crystallization method is achieved by subjecting an amorphous metallic alloy to a heat treatment. Specifically, there is provided in accordance with that method a unique correlation between the degree of crystallization and the permeability values. In order to achieve permeability in the range of 100 to 400, crystallization is required of the order of 10% to 25% of the volume. The appropriate combination of annealing time and temperature conditions are selected based on the crystallization temperature and or the chemical composition of the amorphous metallic alloy. By increasing the degree of crystallization the permeability of the core is reduced. The reduction in the permeability results in increased ability of the core to sustain DC bias fields and increased core losses. A discrete gap is introduced by cutting the magnetic core and inserting a nonmagnetic spacer. The size of the gap is determined by the thickness of the spacer. Typically, by increasing the size of the discrete gap, the effective permeability is reduced and the ability of the core to sustain DC bias fields is increased. However, for DC bias excitation fields of 100 Oe and higher, gaps of the order of 5-10 mm are required. These large gaps , reduce the permeability to very low levels (10-50) and the core losses increase, due to increased leakage flux in the gap. For power factor correction applications in power equipment and devices there is a need for a small size electrical choke with low permeability(50-300), low core losses, high saturation magnetization and which can sustain high DC bias magnetic fields. SUMMARY OF THE INVENTION The present invention provides an electrical choke having in combination a distributed gap, produced by annealing the core of the choke, and a discrete gap produced by cutting the core. It has been discovered that use in combination of a distributed gap and a discrete gap results in unique property combinations not readily achieved by use of a discrete gap or a distributed gap solely. Surprisingly, magnetic cores having permeability ranging from 80 to 120, whh 95% or 85% of the permeability remaining at 50 Oe or 100 Oe DC bias fields, respectively are achieved. The core losses remain in the range of 100 to 150 W/kg at 1000 Oe excitation and 100 kHz. The present invention relates to an electrical choke comprising a coil and a ferromagnetic metal alloy core, the core consisting of an amorphous metal alloy characterized in that said core having a discrete gap, and comprising a non magnetic spacer located in an opening defined by said discrete gap, said discrete gap having a gap size determined by the thickness of said spacer and a distributed gap, the amorphous metal alloy of the core being partially crystallized and having an annealed permeability in the range of 200 to 1000. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiments of the invention and the accompanying drawings in which: Figure 1 is a graph showing the percent of the initial permeability of an annealed Fe-based magnetic core as a function of the DC bias excitation field; Figure 2 is a graph showing, as a function of the DC bias excitation field, the percent of the initial permeability of an Fe-based amorphous metallic alloy core, the core having been cut, and having had inserted therein a discrete spacer having a thickness of 4.5 mm; Figure 3 is a graph showing, as a function of the DC bias excitation field, the percent of initial permeability of an Fe-base core having a discrete gap of l.25 mm and a distributed gap; and Figure 4 is a graph showing, as a function of discrete gap size, empirically derived contour plots of the effective permeability for the combined discrete and distributed gaps, the different contours representing permeability values for the distributed gap DETAILED DESCRIPT1ON OF THE INVENTION The important parameters in the performance of an electric choke are the percent of the initial permeability that remains when the core is excited by a DC field, the value of the initial permeability under no external bias field and the core losses. Typically, by reducing the initial permeability, the ability of the core to sustain increasing DC bias fields and the core losses are increased. A reduction in the permeability of an amorphous metallic core can be achieved by annealing or by cutting the core and introducing a non magnetic spacer, In both cases increased ability to sustain high DC bias fields is trad'id for high core losses. The present invention provides an electrical choke having in combination a distributed gap, produced, by annealing or by using ferromagnetic powder held together by binder, ind a discrete gap produced by cutting the core The use in combination of the distributed and discrete gaps increases the ability of the core to sustain DC bias fields without a significant increase in the core losses and a large decrease of the initial permeability. These unique properties of the choke are not readily achieved by use of either a discrete or a distributed gap solely. In Figure I there is shown as a function of the DC bias excitation iield the percent of initial permeability for an annealed Fe base magnetic core. The core, composed of an Fe-B-Si amorphous metallic alloy, was annealed using an appropriate annealing temperature and time combination Such an annealing temperature and time can be selected for an Fe-B-St base amorphous alloy, provided its crystallization temperature and or chemical composition are known For the core shown in Figure 1, the composition of the amorphous metallic alloy was FegoBuSi9 and the crystallization temperature was Tx=507 °C This crystallization temperature was measured by Differential Scanning Calorimetry (DSC). The annealing temperature and time were 480 3C and thr, respectively and the annealing was performed in an inert gas atmosphere. The amorphous alloy was crystallized to a 50% level, as determined by X-ray diffraction. Due to the partial crystallization of the cors, its permeability was reduced to 47 By choosing appropriate temperature and time combinations, permeability values in the range of 40 to 300 and higher are readily achieved. Table 1 summarizes the annealing temperature and time combinations and the resulting permeability values. The permeability was measured with an induction bridge at 10 kHz frequency , 8-turn jig and 100 mVac excitation. TABLE (Table Removed) As illustrated by Figure 1, 80% of the initial permeability was maintained at: 53 Oe while 30% of the initia; permeability was maintained at 100 Oe. The core loss was delermined to be 650 W/kg at 1000 Oe excitation and 100 kHz. Figure 2 depicts, as a function of the DC bias excitation field, the percent of the initial permeabiJity of an Fe b-ise amorphous core, the core having been cut with an abrasive saw and having had inserted therein a discrete plastic spacer having a thickness of 4 5 mm The initial permeability of the Fe base core was 3000 and the effective permeabili'.y of the gapped core was 87. The core retained 90% of the initial permeability at 100 Oe, However, the core losses were 250W/kg at 1000 Oe excitation and 100 kHz. Figure 3 depicts, as a function of the DC bias excitation field, the percent of initial permeability of an Fe base core having, in combination, a discrete gap of 1.25 mm and a distributed gap. The amorphous Fe base alloy can be partially crystallized ujing an appropriate annealing temperature and time combination, provided its crystallization temperature and or chemical composition are known. The example shown in Figure 3 had a composition consisting essentially of FesoB11S19 and a crystallization, temperature Tx=507 °C, The annealing temperature and time were 430 °C and 6.5 hr, respectively and the annealing was perforraed in an inert gas atmosphere. This annealing treatment reduced the permeability to 300. Subsequently, the core was impregnated with an epoxy and acetone solution, cut with an abrasive saw to produce a discrete gap and provided with a plastic spacer of 1.25 mm, which was inserted into the gap Impregnation of the core is required to maintain the mechanical stability and integrity thereof core during and after the cut'iing. The final effective permeability of the core was reduced to 100. At least 70 % of the initial permeability was maintained under 100 Oe DC bias field excitation. The core loss was 100 W/kg at 1000 Oe excitation and 100 kHz. Figures 1, 2 and 3 illustrate that in order to improve the DC bias behavior of an Fe base amorphous core while, at the same time, keeping the initial permeability high and the core losses low, a combination of a discrete and distributed gaps is preferred. The conventional formula for calculating the effective permeability of a gapped choke is not applicable for a core having in combination a discrete and a distributed gap. Figure -4 depicts, as a function of the discrete gap size, empirically derived contour plots of the effective permeability for a core having combined discrete and distributed gaps. The different contours represent the various values of the distributed gap (annealed) permeability Table 2 displays various combinations of annealed permeability and discrete gap sizes The corresponding effective permeability, percent permeability at 100 Oe and cors losses are listed, as wall as the cutting method and the type of the spacer material TABLE 2 (Table Removed) Core loss was measured at 1000 Oe excitation field and (00 kHz with the exception of * Excitation field 500 Oe ** Excitation field 850 Oe *** Excitation field 900 Oe Two different types of spacer material, plastic and ceramic, were evaluated No difference was observed in the resulting properties. Typically the magnetic core is placed in a plastic box. Since a plastic spacer car be used for the gap, the spacer can be molded directly into the plastic box, Several methods for cutting the cores were evaluated, including an abrasive saw, wire electro-discharge machining (wire edm), and water jet. All these methods were successful. However, there were differences in the quality of the cut surface finish, with the wire edm being the best and the water jet the worst. From the results in Table 2, it was concluded that the wire edm method produced core;; exhibiting the lowest losses and the water jet method the highest with all other conditions being equal. The abrasive method produced cores with satisfactory surface finish and core losses. From the above results it was concluded, that the finish of the cut surface of the core is important for achieving low core losses. Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that further changes and modifications may suggest themselves to one skilled in the art, all failing within the scope of the invention as defined by the subjoined claims. WE CLAIM : 1. An electrical choke comprising a coil and a ferromagnetic metal alloy core, the core consisting of an amorphous metal alloy characterized in that said core having a discrete gap, and comprising a non magnetic spacer located in an opening defined by said discrete gap, said discrete gap having a gap size determined by the thickness of said spacer and a distributed gap, the amorphous metal alloy of the core being partially crystallized and having an annealed permeability in the range of 200 to 1000. 2. An electrical choke as claimed in claim 1, wherein said gap size ranges in width from 0.75 mm to 12.75mm and said choke has an effective permeability ranging from 40 to 200. 3. An electrical choke as claimed in claim 1, having a core loss ranging from 80 to 200 W/kg at an excitation at 100 kHz and 80000 A/m excitation field, an effective permeability ranging from 40 to 200, and a retained effective permeability ranging from 50% to 95% at a DC bias field of 8000 A/m. 4. An electrical choke as claimed in claim 1, in which said amorphous metal alloy is an Fe-base alloy. 5. An electrical choke as claimed in claim 3. in which said amorphous metal alloy is an Fe-base alloy having an annealed permeability of 300, said gap size is, 1.25 mm, and said choke has an effective permeability of 100. 6. An electrical choke as claimed in claim 5, in which said core retains at least 75% of said effective permeability under a DC bias field of 8000 A/m. 7. An electrical choke as claimed in claim 5, having a core loss ranging from 80 to 100 W/kg at an excitation at 100 kHz and 80000 A/m excitation field. 8. An electrical choke as claimed in claim 1, in which said non magnetic spacer is composed of ceramic or plastic and molded directly into a plastic box containing said core. 9. An electrical choke as claimed in claim 1, said core being coated with a thin high temperature resin for electrical insulation and maintenance of core integrity. 10. An electrical choke as claimed in claim 1, wherein said distributed gap is produced by partially crystallizing said alloy. 11. An electrical choke as claimed in claim 1, wherein said discrete gap is produced by impregnating and cutting said partially crystallized core. 12. The use of an electrical choke as claimed in claim 1 for power factor correction applications. 13. An electrical choke substantially as herein described with reference to the accompanying drawings.

Documents:

690-del-1998-abstract.pdf

690-del-1998-assignment.pdf

690-del-1998-claims.pdf

690-del-1998-correspondence-others.pdf

690-del-1998-correspondence-po.pdf

690-del-1998-description (complete).pdf

690-del-1998-drawings.pdf

690-del-1998-form-1.pdf

690-del-1998-form-13.pdf

690-del-1998-form-19.pdf

690-del-1998-form-2.pdf

690-del-1998-form-3.pdf

690-del-1998-form-4.pdf

690-del-1998-form-6.pdf

690-del-1998-gpa.pdf