US 3287677 A
Description (OCR text may contain errors)
OF MAGNETIC FLUID Filed May 25, 1964 11 R T AM Y! %w :u U Em G. R. MOHR HIGH FREQUENCY TRANSFORMER CORE COMPRISED DEVICE Nov. 22, 1966 CIRCULATING INVENTOR Glenn R. Mohr WWW Jim )9" ATTORNEY United States Patent 3,287,677 HIGH FREQUENCY TRANSFORMER CORE COM- PRISED OF MAGNETIC FLUID Glenn R. Mohr, Linthicum, Md, assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 25, 1964, Ser. No. 369,979 2 Claims. (Cl. 336-57) This invention relates to electric induction devices, and in particular to electrical inductors and transformers.
Presently, high kva., high frequency current transformers for induction heating employ low permeability air cores. Under matched conditions these air cores yield a coupling coeflicient of .7 to .8, and thus a secondary to primary kva. ratio of 20% to 25%. This ratio is much smaller under unmatched conditions. Magnetic materials having a much higher permeability than air could be employed to improve the secondary to primary kva. ratio. However, magnetic cores used in this application generally are subject to the following limitations.
The inductance of such a device is a direct function of the product of the core cross section and the core permeability. Because of the high frequencies involved in this application, a small inductance is required. The cross-section, permeability product must be kept low. For a core composed of a high permeability material the required cross-section becomes so small that insulation between windings becomes diflicult.
The inductance is also a direct function of the number of conductive windings. For this application involving high frequencies and a high permeability core, the low inductance required means fewer windings. If the number of conductive windings is allowed to approach unity, the turns ratio also approaches unity. Current transformers must have a large primary to secondary turns ratio.
Hysteresis losses, generated during inductive coaction between the conductive windings and the core material, cause hot spots in the core material. These hot spots are especially damaging at the high frequencies involved in the application.
It is, therefore, an object of this invention to provide an improved high frequency transformer or inductor.
Another object of this invention is to provide a core material having magnetic and fluid properties.
Another object of this invention is to provide a magnetic fluid to be employed as the core material in high frequency transformers and inductors and to have a permeability lower than that of core materials currently used by higher than the permeability of air.
Another object of this invention is to provide a magnetic fluid to be employed as the core material in high frequency transformers and inductors and which functions in a heat removing relationship within the transformer or inductor.
A further object of this invention is to provide a high frequency transformer which has a higher coupling coefficient and a corresponding higher secondary to primary kva. ratio than heretofore possible.
A further object of this invention is to provide a high frequency transformer or inductor of a smaller size than heretofore possible.
Further objects, features and advantages of this invention will become apparent to those skilled in the art upon further study of the specification.
Briefly, in accordance with this embodiment of the invention, these objects are attained by'circulating a magnetic fluid within a conduit means comprised of, a circulating or flow forcing means, a heat generating core Patented Nov. 22, 1966 zone, and a heat exchanger means. The magnetic fluid is continuously displaced from the core zone into the heat exchanger means by the circulating means. The magnetic fluid is heated while flowing through the core zone and releases this heat as it flows through the heat exchanger means. The particular value of the permeability of the magnetic fluid is suflicient to increase the coupling coefficient over that of air, but not high enough to require a core cross-section as small as that of high permeability materials previously used.
For a clearer understanding of the invention, reference may be made to the following detailed description and accompanying drawing.
The drawing illustrates magnetic fluid 1 contained within a conduit 2. A circulating device 3 forces the flow of magnetic fluid 1 through entrance port 4 into core zone 5 including flow path 6, flow path 7, which form closed magnetic path 8, out exit port 9, and into flow path 10 through a heat exchanger 11. Two conductive windings 12 are shown in this embodiment.
The magnetic fluid 1 is composed of small particles of a magnetic material suspended in an appropriate fluid. The magnetic material supplies the permeability necessary for induction action occurring in core zone 5. The fluid portion of the magnetic fluid 1 suspends and transports the magnetic material throughout the conduit means 2, and provides electrical insulation and mechanical lubrication between the particles of magnetic material.
The flow paths 6 and 7 form a path for the closed magnetic path 8. This magnetic path has a low reluctance due to the magnetic fluid 1 contained therein which expedites the establishment of the magnetic fields of the electric current in conductive windings 12. The conductive windings 12 must be disposed for inductive coaction with magnetic path 8 and accordingly may be wound around flow path 6 or 7, or both.
Heat is generated in the portion of magnetic fluid 1 which comprises the closed magnetic path 8. The sources of this heat are eddy currents and hysteresis losses incurrent during the inductive action. Copper losses in the conductive windings caused by the current therein may contribute a small amount of heat. At the power levels and frequencies involved in this application this heat is excessive and will damage the core zone 5 if not removed therefrom.
The excess heat is removed by circulating magnetic fluid 1 around a cooling loop described below. The heated magnetic fluid comp-rising the magnetic path 8 is dis-placed from the core zone 5 in the flowing action. This heated magnetic fluid flows through the heat exchanger which extracts the excess heat. The cooler magnetic fluid from the heat exchanger flows through the circulating device 3 which supplies the force causing the flow. The cooler magnetic fluid from the circulating device 3 displaces hot magnetic fluid from the core zone 5 thus completing the cooling loop around the conduit 2.
The use of magnetic cores in high frequency inductive devices results in certain limitations as stated previously. The insulation limitation due to the small size of the core cross section is overcome by employing the magnetic fluid 1. The permeability of the magnetic fluid 1 is less than the permeability of previously employed magnetic cores. For the same inductance, the magnetic fluid 1 allows a larger core cross-section. The unity turns limitation is also overcome by employing the magnetic fluid 1. The lower permeability of the magnetic fluid 1 allows more conductive windings for the same inductance. A lower operating temperature is obtained by circulating the magnetic fluid 1 and continuously displacing hot magnetic fluid from the core zone.
The correct functioning of this invention requires that the magnetic field 1 possess magnetic and fluid properties. Any singular, or any combination of substances having these properties will suflice. A composition of 80% finely powdered ferrite and 20% oil, has a relative permeability of 10 to 20 and is useful in many high frequency applications. It is to be understood, however, that this invention is not to be so limited to the combination of ferrite and oil. Other substances could be employed and include, any finely powdered material having magnetic and fluid characteristics, and any powdered magnetic material suspended in a gas or dispersed in a liquid.
That it also be understood that the particular conduit 2 discussed is not critical to the operation of this invention and the invention is not to be so limited. More flow paths through the core zone forming a plurality of closed magnetic paths may be employed. A plurality of cooling loops, each containing a circulating device and a heat exchanger may be employed. The conductive windings 12 may be inside the conduit 2, or external, or both; and must be at least one in number.
The conduit 2 may be of the form of any volume containing a circulating device. The conductive windings would be enclosed within the volume, and would be geometrically positioned with one another so as to have a common magnetic path. Heat exchange would occur on the exterior surfaces of the volume.
Another form of the conduit to be included in this invention is a single closed loop, similar to a doughnut. The closed loop would provide both the closed path for the closed magnetic field and the cooling loop. Conductive windings, circulating means, and heat exchanger means would be included along the closed loop.
The correct functioning of this invention requires relative motion between the conductive windings 12 and the magnetic fluid 1. In the embodiment discussed the conductive windings 12 were stationary and the magnetic fluid 1 moved in relation thereto. An arrangement wherein the conductive windings 12 move and the magnetic fluid 1 is stationary is to be included in this invention.
This invention is not peculiar to transformers and inductors, but includes any induction device having operating limiting characteristics such as described.
Although this invention has been described with re- 4 spect to particular embodiments thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention as defined by the dependent claims,
I claim as my invention: 1. In a high frequency induction device in which heat is gene-rated due to the induction action therein, the combination comprising:
a conduit means formed by a first section and a second section and a third section all connected in parallel;
magnetic fluid contained within said conduit means, the magnetic fluid in said first and second sections forming a relatively low reluctance closed magnetic path;
at least one conductive winding wound in an inductive relation about at least one of said sections included in said magnetic path, said at least one winding adapted to be energized by a high frequency current, the induction action between the current in said at least one Winding and said magnetic fiuid generating heat in the magnetic fluid forming said magnetic path;
flow forcing means connected in said third section for circulating said magnetic fluid around a heat removing flow path formed by said third section in series with the parallel combination of said first and said second section to continuously remove and replace the heated magnetic fluid by forcing said magnetic fluid to flow from said third section through said first and second sections in the same direction and then return to said third section;
heat exchange means connected in said third sec' tion for removing the heat from the heated magnetic fluid therein as said magnetic fluid circulates around said heat removing flow path.
2. The combination as specified in claim 1 wherein two conductive windings are wound in an inductive relation about at least one of said sections included in said magnetic path.
References Cited by the Examiner UNITED STATES PATENTS 2,440,556 4/1948 Paluev 33657 2,607,542 8/1952 Spellman l9221.5 2,651,258 9/1953 Pierce l92-2l.5 X
FOREIGN PATENTS 394,858 12/1908 France.
636,127 4/1960 Great Britain.
LEWIS H. MYERS, Primary Examiner.
T, J. KOZMA, Assistant Examiner.