US 3777295 A
Magnetic particle cores suitable for phase shifting applications having a permeability substantially above 275 and core losses below 0.24 ohms per henry per unit of inductance at 1800 Hertz are produced by a novel combination of steps including one or more of the following: reduction of pre-powder anneal additives, selective distribution of magnetic particle sizes, adding flexibility to the electrical insulation applied to the magnetic particles, elevated pressure compacting, surface etching of the pressure compacted core after hydrogen anneal, and an oxidation heat treatment following surface etching. Surface etching and oxidation combine to increase the breakstrength and improve other properties of various permeability cores, for example, from 125 through 300 permeability.
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United States Patent [1 1 Laing, Alfred M.
[5 MAGNETIC PARTICLE CORE  Inventor: Alfred MTLain g, Butler, Pa.  Assignee: Magnetics, lnc., Butler, Pa.
 Filed: Apr. 3, 1972 21 Appl. No.2 240,858
Related US. Application Data [62} Division of Ser. No. 80,476, Oct. 16, 1970, Pat. No. 3,666,571, which is a division of Ser. No. 7l4,805, March 21, 1968, Pat. No. 3,607,462.
 US. Cl 335/297, 336/233, l48/l04  lm. CI. n01: 3/00  Field of Search", 225/297; 336/233; 148/104  References cited UNITED STATES PATENTS 6/1966 Opitz 148/104 X 3/1970 Copp l48/l04X Primary ExaminerGeorge Harris Attorney-Raymond N. Baker  ABSTRACT Magnetic particle cores suitable for phase shifting applications having a permeability substantially above pacted core after hydrogen anneal, and an oxidation heat treatment following surface etching. Surface etching and oxidation combine to increase the breakstrength and improve other properties of various permeability cores, for example, from 125 through 300 permeability.
10 Claims, N0 Drawings MAGNETIC PARTICLE CORE This application is a divisional application of application Ser. No.'80,476, filed'Oct. 16, 1970, now U. S.' Pat. No. 3,666,571, issued May 30, 1972, which was -a divisional application of application Ser. No. 714,805, filed Mar. 21, 1968, now'U.S. Pat. No. 3,607,462, issued Sept. 21, 197 1.
This invention relates generally to preparation of magnetic particles'for compacting-into'magnetic components and, more particularly, to high permeability particle cores exhibiting-low core losses inthe audio frequency and related frequency ranges, and-to methods of manufacturing such cores.
A basic process for manufacture of particle cores is disclosed in the-U.S.'Pat. to Bandur No.2,105,070. The process steps include the preparation of magnetic particles, electrically insulating the particles, compacting into a magnetic core, and annealing the core. This basic process has been capable of producing molybdenumcontaining permalloy particle cores having a penneability of about 125 and having core losses acceptable for audio-frequency applications.
Attempts to improve on this process have been numerous and continuous since its origin. The problem is to increase the permeability without destroying usefulness of the coredu'e to an increase in core losses. Successful improvements of core properties have generally resulted'from treatment of thecompactedcore, for-ex ample by boiling, solution treatment, and re-anneal of the compacted core as disclosed in the U.S. patents to Harendza-Harinxma No. 2,977,263 issued Mar. 28, 1961, No. 3,014,825 issued Dec. 26, 1961, and 'No. 3,132,952 issued May 12, 1964.
The primary objective of particle core improvement endeavor is to increase the permeability of such cores while maintainingcore losses within acceptable limits. Particle cores are used inelectrical circuits-operating at voice frequencies and related frequencies up to about 20,000 cycles per second where low core losses are a major consideration. Also these cores find certain applications at much higher frequencieswhichfurther accentuate the importance of low core losses. In the audio frequency and related freq'uen'cyranges, components approaching theoretical'perfect' reactance are desirable in order to'obtain high quality performance. In filters, for example, shorter cut-off, better defined resonance, and higher attentuationratios are realized'with high quality inductors. Quality ininductors can be assessed from the Q factor. The Q of'induc'tors isdefined as a ratio v of reactance to resistance: Q nf w+ 4c)* f =frequency incycles'per second L inductance inhenries, and
R wire resistance in ohms R Ac resistance (ohms-) due' to losses of the core, in-
cluding eddy curre'nt losses, hysteresis losses, and.
wellknown in the-art, effortsto dateto improve on metallic particle cores have resulted in commercially available cores with-acceptable corelosses having an upper limitof 200, and-slightly'higher, permeability. Special laboratory techniques may yield acceptable cores of'about 240 permeability; however, it has not beenv possible to produce a particle core having a permeability above 275 with core losses acceptable in the industry for the uses considered above.
The present invention teaches novelprocedures for preparation of magnetic powders for magnetic components and, specifically, preparation of molybdenumcohtaining permalloy powders forproducing magnetic cores of higher permeability, lower core losses, increased mechanical breakstrength, more linear temperature characteristic, and better moisture resistance. These teachings enable commercial production of molybdenum-containing permalloy powder cores having a permeability substantially above 275 and having c'ore losses within industry specifications.
Cor e losses are reflected in the windings of a core as resistance losses andconstitute loss of energy in an inductor. Core losses, as used in the core manufacturing art, include eddy current losses'which vary directly with the square of the frequency, hysteresis losses which vary directly with the flux density and the frequency, and residual losses which'vary directly with the Residual loss Hysteresis loss Eddy current loss Total loss factor 'R effective (total measured) resistance minus DC resistance p. permeability L inductance (henries) f frequency (Hz) 8,, flux density (gausses) e eddy current loss coefficient a hysteresis loss coefficient c residual loss coefficient To meet industry standards, cores must have a total coreloss no higher than 0.240 units (ohms per unit permeability per unit inductance) at 1800 Hertz (cycles per second), with a core loss of 0.200 units at 1800 Hertz beingthe accepted average.
To meet these standards and improve magnetic and mechanical properties as well, several novel steps are combinedin the present invention. These steps will be 'described'in relation to production of novel high permeability molybdenum-containing permalloy powder cores with acceptable core losses; Previously, the highest permeability core, of the type discussed, available commercially was a 200 perm" core. The present teachings made the 300 perm core commercially aVailableJOf special significance is the fact'that these 300"perm cores have losses acceptable within industry standards for audio-frequency uses, i.e. below 0.240 units.
78.0-83.0% the balance iron.
molybdenum, nickel, and
Methods for pulverizing the alloy are known. In practice the metallic constituents of the alloy are melted together and additives which embrittle the alloy are made in the molten state. This treatment facilitates a fine crystalline structure in the solidified alloy that enables reduction to a fine powder byconventional rolling, grinding, and pulverizing techniques.
In departing from prior practice, the invention teaches selection of a variety of particle sizes to obtain an optimum packing factor, i.e. an optimum density of magnetic powder and optimum space for electrical insulation (electrically equivalent to air space) in the compacted product.
A typical particle size distribution to produce 300 perm cores in accordance with the process of the present invention is:
TABLE I Average Particle Size Sieve by Microns Mesh-size Weight 90 from l20 to +230 ll5% 65 from -230 to +400 2535% 37 (or less) -400 45-65% l to +230 covers particle sizes which will pass through a 120 mesh screen but will not pass through a 230 mesh screen using normal sieve practice.
The important concept discovered here is that the optimum packing factor to obtain high permeability and acceptable core losses is not obtained by use of single particle size screening but rather by selective screening and distribution of particle sizes. The high permeability acceptable core loss product of the invention can be obtained by selecting particles with about one part by weight having an average particle size of 90 microns, about three parts by weight having an average particle size of 65 microns, and about six parts by weight in which the average particle size is no greater than about 37 microns. These proportions can be broadened to emphasize certain core properties and changes in the electrical insulation also permit variation in these proportions. However, a larger overall-average particle size tends to increase both permeability and core losses while smaller overall-average particle size tends to'decrease both permeability and core losses. I
After pulverizing and sieving, the powder is annealed to relieve the strains induced during brittle practice,
that is during the production of the powder. To. prevent welding of the particles during this anneal, additions of non-agglomerating material must be blended with the powder. Such material must remain non-reactive or inert at powder annealing temperatures. In.theprior art, pre-anneal additions constituted about 0.3 to 1.0 percent by weight of the metal powder. An important discovery of the present invention relates to better use of the limited amount of distributed non-magnetic gap available in producing the higher permeability product of the present invention; that is, this space can be better used to provide more effective electrical insulation of the particles rather than being occupied by pre-anneal additives when proper steps are followed. By the procedures of the present invention, the pre-anneal additive is drastically reduced to about 0.02 to about, 0.'05 percent by weight of the metallic powder; preferably such additives are held below about 0.03 percent by weight. Typically ceramic clays such as talc or kaolin are added to prevent agglomeration; a preferred preanneal additive is powdered kaolin.
The subsequent powder anneal is held to a temperature of about 1250F. for about 1 7% hours in a nonoxidizing atmosphere, e.g. an atmosphere containing free hydrogen. Temperatures significantly higher than about l 250F. are avoided in order to eliminate agglomeration of the metal particles. With the present invention it is possible to avoid the 1400F. to l600F. powder anneals of the prior art without sacrificing electrical properties. In fact, a higher permeability low core loss product is obtained.
During the powder anneal the water of crystallization of the kaolin, which constitutes about 13 percent by weight of this pre-anneal additive, is driven off. The kaolin should be in the uncalcined condition before the powder anneal since it has been found that calcined kaolin is not as effective as standard kaolin in preventing agglomeration of the metal particles.
After annealing, the work product is sieved through a 50 to mesh screen to remove any lumps which may be formed; however, this does not change the analysis of the metal powder sizes which have been selected to produce the desired packing factor. This sieving merely breaks up loose lumping which may occur.
The magnetic powder is then electrically insulated utilizing a slurry including, by present practice, a silicate, an inert metallic oxide, and a ceramic clay. For example, a solution containing about 67 grams of sodium silicate, about 100 grams of milk of magnesia, and about 6000 cc. of deionized water is prepared to insulate about 50 pounds of powder. The electrical insulation is applied in a plurality of coats with the first coat ordinarily not. including a ceramic clay additive in order to utilize the pre-anneal non-agglomerating additive present with the metal powder. Subsequent coats after the first coating, utilize about 1400 ccs of the above solution with about 22 grams of powdered kaolin added. In preferred practice the electrical insulation is applied in four separate coats with intermediate dryings of the coatings being carried out at temperatures uy to about 315F. The total electrical insulation, dry weight, is less than about 0.4 percent by weight of the metal powder weight.
Highly beneficial results are obtained by the addition of a plasticizer coating during the insulation process. Plasticizer as used herein, refers to a material for imparting flexibility to the electrical insulation or a major ingredient of the electrical insulation, e.g. the metallic silicate in the disclosed insulation. As taught by the present invention, the plasticizer must maintain this capability of imparting flexibility to the electrical insulation. during processing steps up to and including the compacting step.
Preferably the plasticizer is added as a final coating to the insulated particles; however plasticizers exist which require intermediate coating for best results. Suitable plasticizers of the latter category include starches, sugars; or glycerin and a wetting-agent. A plasticizer suitable for addingas a final coatingis ammonium lignosulfonate-in aliquidcarrier.
The purpose of the plasticizer is to impart flexibility to the electrical insulation and-permit higher than usual pressures during compacting. of the particles while avoidingmechanical cracking of the electrical insulation. In accordance with'the teachings of theinvention, the plasticizer should maintain its capabilit'yof imparting flexibility during the temperaturesencountered in applying electrical insulation-andthose encounteredin compacting. Preferably the-plasticizer shouldbe driven off during the high temperature coreanneal or, atleast, not impair the insulation or leave a reaction product having reduced electrical insulation properties.-
After the insulation process,.including.the useof 'a Y plasticizer, the insulated powder is sieved through a 50 to 100 meshscreen to remove lumps and chips of'insulation. This sieving-is carried out without changing the. basic magnetic particle sieveanalysis.
The insulated powder is pressed into cores at a pressure which is. significantly higher. than that previously specified for molybdenum-containing permalloy powder cores-The compacting pressure taught by the present invention for the production of higher permeability cores is ordinarily in the rangeof about 135m 150 tons per square inch'and preferably .is about 140 tons per square inch. Theplasticizer'makes the insulation 'more'. flexible and reduces compacting friction.
Without a plasticizer, the core losses-increase considerably at the higher compacting pressures'taughtPA" portion of the decrease in core losses available with the present invention can be traced to the decrease in surface weldingstemmingfromthe decrease in'compacting friction. The reduction in core losses also stems from decreasing mechanical breakage of the electrical insulation between particles which apparently existed in the prior art practice. Further, the plasticizer reduces the amount'of lubricant needed in pressing.
However there are limits to the amount of plasticizer which can be safely used since mechanical breakstrength of the core decreases rapidly above certain low'level percentages. When ammonium lignosulfonate is used with an insulation containing sodium silicate, the amount of 'dry plasticizer should be about 0.06 percent by weight of the-metal powder weight.
The inventionincludes-discovery of a step to maintain desired mechanical breakstrength of the finished product. The co-action of this step, which will be described later, offsets any weakening effect on the cores caused by the plasticizer so that cores with mechanical breakstrength equivalent to prior art cores without plasticizer can now bemade notwithstandingthe use of a plasticizer.
After pressing, the cores are annealed between about l000F. and about 1500F., preferably about 1250 F. for approximately-40 minutes ina non-oxidizingatmo-v sphere, for example, an-atmosphere containing pure hydrogen. The cores'are quenched in water after removal from the annealing furnace.
Practice of the process of the invention described thus far consistently produces cores within the normal tolerance range for 300 permeabilitycores, however face etching step: By surface etching is meant removal of the skin effect resulting from present-day compacting. techniques used in commercial production of molybdenum-permalloy cores. i
The cores are surface etched subsequent to the hydrogen anneal which follows pressing. The cores should not be surface etched prior to this anneal. In general, chemical etching is preferred inorder to avoid adding any mechanical-strains to the particles. A typical etchingpractice utilizes a 50 percent nitric acid solution with an etching time of 20 seconds, plus or minus 5 seconds with temperature maintained at F. 5F. An alternate etching procedure is approximately 3 minutes in40 Baume nitric acid with temperature maintained at 80F i 5F.
Surface etching can cause a slight decrease in permeability but this decrease is limited to about 0.5 to about 5 percent of the core permeability. Typically, a 302.1 permeability core may be reduced to 300.6 permeability and a 323.3 permeability core may be reducedto 317.3. However, corelosses decrease at a much greater rate than permeability; decreases in core losses up to about 50 percent-are typical. For example, the above 302.1 permeability core had a AR/uL value of 0.117 before surfaceetching, This core loss was reduced to 0.0972 by surface etching. The above 323.3 permeability core had a AR/uL value of .413 units before etching which was reduced to 0.203 units, more than 50 percent, by surface etching. In brief, while the permeability may be decreased as-much as 5 percent by surface etching, the-core losses are reducedas much as 50 percent.
While 300 permeability cores with core losses within accepted standards can be produced consistently in production utilizing the above steps, the invention also includes discovery of a novel step in the treatment of compacted cores which further improves electrical and mechanical properties ,of molybdenum-containing permalloy'particle cores. This step co-acts with other steps in the production of 275 and higher permeability cores- For example, this step helps increase the mechanical strength of a high permeability core. offsetting any weakening effect of a plasticizer coating. This step also acts to offset the effects of the higher pressures used in producing 275 and higher permeability cores by decreasing core losses which could result from such higher pressures. However this novel step also improves the mechanical and electrical properties of lower permeability cores as well.
In accordance withthe invention, after annealing of the powder cores in a non-oxidizing atmosphere such as hydrogen, and after the surface etching, the cores are heat treated in an oxygen-containing atmosphere, for example, air. The order of these two heat treatments, that is the hydrogen anneal and the air heat treatment, cannot be reversed without loss of the benefits obtained by annealing in hydrogen, followed by surface etching,.followedby oxidation. It is believed that a hydrogen anneal subsequent to the heat treatment in air reduces the bonds formed during oxidation.
Theoxidation step should be carried out at a temperature between about 600F. and about 1000F. for an interval of about 10 to 15 minutes. A preferred oxidation treatment is applied at about 850F. for about 15 minutes. It should be understood that this oxidation treatment has a time-temperature relationship, that is, a longer period of time, for example, 1 Va hours at a lower temperature, for example about 225F., can be utilized to provide similar oxidation, but the time involved is uneconomical and can have slightly detrimental side effects on other properties. In general, within the above limits, the improvement in break-strength is greater at higher temperatures.
Oxidation increases the breakstrength of the core as much as 75 percent depending on the particular core, decreases total core losses as much as T 25 percent (chiefly a decrease in eddy current losses), and markedly decreases the effects of moisture on a core.
Certain benefits of oxidation heat treatment are more pronounced with higher permeability cores. A 300 permeability toroidal core, unoxidized, having a 1.06 inch outer about 0.5 inch inner diameter, and 0.44 inch height, breaks at a 170 pounds of force per square centimeter of radial cross sectional area. An otherwise identical core, oxidized at about 850F. for 12 minutes, breaks at 260 pounds per square centimeter, an increase in break-strength of 50 percent. The breakstrength, however, of a 1 15 to 135 permeability core of similar size shows an increase in breakstrength of about 10 percent when treated in the same fashion.
Breakstrength measurements are made in accordance with the industry accepted Vertical Core Breakstrength Test. This is a mechanical test in which force is applied on diametrically opposite sides of a painted cores outer diameter with maximum tangential contact being made on both sides. The ramming force required to break the core is measured inpounds per square centimeter of a radial cross section of the core.
This test provides an important parameter for designating a mechanical characteristic, that is the breakstrength, of the product of the present invention. 1f the breakstrength measured in accordance with the Vertical Core Breakstrength Test is plotted versus crosssectional areas of a radial segment of cores in square centimeters a linear relationship is found to exist. The minimum acceptable value of this ratio of pounds of breakstrength to area of a radial segment in square centimeters, for a painted core, is 290. For example, a core with an outer diameter of 1.06 inches, an inner diameter of 0.580 inch, and a height of 0.440 inch has a radial section area of 0.635 square centimeters. The minimum accepted breakstrength of a painted core of this size is 184.15 pounds. The breakstrength factor of a core having this minimum breakstrength would be 184.15 divided by 0.635 which equals 290. The electrically insulating paint applied to the exterior as a final step in processing particle cores may be conventional, e.g. an enamel core paint with a thickness of roughly 7 to 12 mils.
All molybdenum-containing permalloy cores from 125 permeabilityv to 300 permeability show a substantial decrease in core losses after the oxidation heat treatment taught. The higher permeability cores show best results when annealed in the range of roughly 750 to 850F. for about 12 minutes. However with all cores of the type described, if the temperature of the oxidation heat treatment is allowed to rise above about 1000F., the cores will deteriorate with regard to losses.
Oxidation helps solve a problem of long standing in this art, that is the detrimental effect of humidity o'n permeability; see Stability Characteristics of Molyb denum Permalloy Powder Cores" by C. D. Owens, Electrical Engineering, March 1956, pages 252-255. Past efforts have been concentrated on finding and applying coatings and packings for cores which would from the point of view of practical handling problems and economics, have been at the limits of their capability for some time. The oxidation step taught by the invention helps to solve this problem in the core itself and, for the first time in this art, brings the humidity problem under practical and economic control.
Humidity decreases the permeability of a core. The oxidation treatment taught by the invention reduces this decrease in permeability by at least 50 percent in all cores and by greater amounts in the higher (300) permeability cores disclosed. For example, an unoxidized 300 permeability core, with conventional electrical insulating enamel coatings of about 7 to 12 mils totalthickness on its exterior surface, shows a change of 3.3 percent in permeability when exposed to percent relative humidity in air at F. for five days. Otherwise identical cores, oxidized between 575F. and 850F. for twelve minutes had a permeability change of 1.0 percent. Under the same conditions 200 permeability cores unoxidized showed a 2.2 percent change in inductance in this test while the oxidized cores showed an average change of l.1 percent in permeability.
Also the effect of changes in temperature on permeability, i.e. the change in permeability versus change in temperature characteristic, is made more linear. This is partially due, it is believed, to a reduction in the effect of the difference in coefficients of expansion between the paint on a core and the core itself, especially at lower temperatures. Evidently an oxidized core is better able to withstand the force of contraction of the paint on the core because of the increased breakstrength resulting from oxidation.
Improvement in breakstrength due to oxidation is especially beneficial with the higher permeability cores compacted from electrically insulated particles having a plasticizer coating. A plasticizer dry coating weight of 0.06 to 0.1 percent ammonium ligno-sulfonate significantly improves core losses but decreases the breakstrength of such cores slightly. Surface etching and heat treating in air cause equivalent or higher breakstrength than that experienced with conventional cores of the same size without a plasticizer.
The following table lists permeability and core losses, obtained in commercial production of 300 permeability core (normal tolerance :8 percent permeability), for the various standard core sizes, using the teachings of the present invention.
TABLE I1 OD(in.) lD(in.) Ht(in.) p. AR/uL 0.250 0.110 0.110 281 0.170 0.310 0.156 0.125 307 0.121 0.400 0.200 0.156 308 0.120 0.500 0.300 0.187 310 0.190 0.900 0.550 0.300 277 .130 1.060 0.580 0.440 298 0.128 1.300 0.785 0.420 292 0.130 1.410 0.880 0.412 298 0.146 1.570 0.950 0.570 308 0.210
In describing specific embodiments of the invention, detailed steps, values, and determinations have been set forth which not only enable practice of the invention but also provide guidelines for modification of the specific embodiments by those skilled in the art, therefore the scope of the invention is to be determined from the following claims.
1. A pressure compacted magnetic core comprising magnetic particles and distributed non-magnetic gaps, the magnetic core having a permeability above 275 units and a core loss when operated at a frequency of 1800 Hertz no greater than 0.240 ohms per henry per unit of permeability with core loss being measured in accordance with the following formula:
R /uL ef aB f cf wherein R is the total AC core losses in ohms, equal to the effective resistance minus the DC resistance losses L is the inductance in henries u is permeability (above 275 units) T3,, is the flux density (20 gausses) fis the frequency (1800 Hertz) e is the eddy current resistance coefficient having a maximum value of 46.7 X v a is the hysteresis resistance coefficient having a maximum value of 1.3 X 10", and
c is the residual loss coefficient having a maximum value of 23 X 10 2. The magnetic core of claim 1 in which the magnetic particles comprise a molybdenum-containing permalloy consisting essentially of molybdenum, nickel, and iron.
3. The magnetic core of claim 2 in which the magnetic particles have the following particle size distribution:
about 1 part by weight average particle size about 90 microns,
about 3 parts by weight average particle size about 65 microns, and
about 6 parts by weight particle size average not greater than about 37 microns.
4. The magnetic core of claim 2 in which the magnetic metallic particles are separated by electrical insulation comprising the reaction product of a metallic silicate, an inert metallic oxide, a ceramic clay, and a plasticizer with the plasticizer comprising less than 0.1 percent dry weight of the magnetic metallic particles.
5. The magnetic core of claim 4 in which the electrical insulation includes sodium silicate, magnesium oxide and kaolin.
6. The magnetic core of claim 1 in which the permeability is at least 300.
7. The magnetic core of claim 1 having a toroidal configuration with its exterior coated with an electrically insulating paint and a mechanical breakstrength such that a ratio of the pounds of force required to break the core to the area in square centimeters of a radial cross-section of the core has a minimum value of 290, the breakstrength of the core being measured by applying the force to diametrically opposite sides of the outer diameter of the core.
8. The magnetic core of claim 1 characterized by improved resistance to humidity such that after conventional exterior electrical insulationcoating it exhibits a decrease in permeability of less than 2 percent when exposed to at least 95 percent relative humidity in air at 150F. for five days.
9. A pressure compacted magnetic core comprising magnetic particles and distributed non-magnetic gaps, the magnetic core having a permeability above 115 units and a core loss when operated at a frequency of 1800 Hertz no greater than 0.240 ohms per henry per unit of permeability with core loss being measured in accordance with the following formula:
R L ef aB f cf wherein R is the total AC core losses in ohms, equal to the effective resistance minus the DC resistance losses L is the inductance in henries y. is permeability (above 275 units) 13,, is the flux density (20 gausses) f is the frequency (1800 Hertz) e is the eddy current resistance coefiicient having a maximum value of 46.7 X 10' a is the hysteresis resistance coefficient having a maximum value of 1.3 X 10 and c is the residual loss coefficient having a maximum value of 23 X 10', and further characterized by improved resistance to humidity such that after conventional electrical insulation coating of its exterior surface it exhibits a decrease in permeability of less than 2 percent when exposed to at least percent relative humidity in air at 150F. for five days.
10. A pressure compacted magnetic core comprising magnetic particles and distributed non-magnetic gaps, the magnetic core having a permeability above units and a core loss when operated at a frequency of 1800 Hertz no greater than 0.240 ohms per henry per unit of permeability with core loss being measured in accordance with the following formula:
R L ef aB f cf wherein R is the total AC core losses in ohms, equal to the effective resistance minus the DC resistance losses L is the inductance in henries [L is permeability (above 275 units) B,, is the flux density (20 gausses) f is the frequency (1800 Hertz) e is the eddy current resistance coefficient having a maximum value of 46.7 X 10' a is the hysteresis resistance coefficient having a maximum value of '1.3 X 10', and c is the residual loss coefficient having a maximum value of 23 X 10 the magnetic corehaving a toroidal configuration with its exterior coated with an electrically insulating paint, and further characterized by improved mechanical breakstrength such that a ratio of the pounds of force required to break the core to the area in square centimeters of a radial section of the core has a minimum value of 290, the breakstrength of the core being measured by applying force to diametrically opposite sides of the outer diameter of the core.