Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-05-11
2001-06-12
Hall, Carl E. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S602100, C219S121690, C219S121710
Reexamination Certificate
active
06243940
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to methods for avoiding DC current magnetization in magnetic cores, and in particular to use of lasers in the manufacture of magnetic cores so as to break DC magnetizing paths in the cores.
2. Background Description
In the design of magnetic devices, both the alternating and direct current affect the characteristics of the device. When discussing magnetic materials, they are referred to as being “hard” or “soft”. Hard materials retain their magnetism after a magnetic field has been applied; e.g. steel. These materials' molecular particles line up their magnetic poles to create magnetic lines of flux, and they stay lined up thereby creating a permanent magnet. Soft materials will magnetize, but will not retain the magnetic properties: their molecular particles will scramble when the magnetic field is removed, leaving the material in its original, non-magnetized state.
Transformers need to be made with soft materials: the magnetic particles must be able to switch direction quickly to concentrate the winding field. A magnetized core will inhibit the transformer action. In practical transformer circuits, it is sometimes necessary to allow a direct current (DC) to flow through a winding. This DC component magnetizes the core and adversely affects the properties of the transformer. To counteract the effects of the direct current, the core must be “gapped”, i.e. physically broken so that it does not provide a magnetic path in a complete, continuous circle. This gap breaks the magnetic flux inside the core so as to keep the core material from aligning with the direct current, freeing the material to align with the alternating current, i.e. the signal which is to be operated on by the transformer. The amount of gap is dependent upon the operating characteristics required by the transformer. Among other parameters, the inductance of the core is indirectly proportional to the width of the gap. Direct current capacity (i.e. the ability to handle direct current in the windings without saturating the core) is directly proportional to the width of the gap. High frequency circuits usually need lower inductance, so gapped cores find many uses, such as in switch mode power supplies which have higher direct current. These parameters must be addressed in the final circuit design.
In the prior art, the core gapping process would physically break the core (e.g. as shown at junction
21
in
FIG. 2A
) into pieces
22
as shown in FIG.
2
A and re-assemble the pieces
22
with gapping paper or other non-magnetic material
23
as shown in
FIG. 2B
, using an adhesive. Cores gapped using this method require fixturing to hold the core in position while the adhesive cures. Cores may also be sliced (with a saw blade) on one side, and filled with an adhesive material. These methods are slow and time consuming. Further, when a manufacturing process involves multiple cores it is necessary to assure that the pieces match when reassembled. Also, the core material is brittle and may crack or chip at undesired locations, producing extra gaps and rendering the core useless. The core surface must be maintained to allow the winding of wire over the core without interference. When gapping cores less than 0.5 inches in diameter, this process becomes difficult due to the small size of the core, which makes it difficult to hold the core in position during the curing phase.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an efficient and inexpensive method of core gapping, particularly for small ferrite cores.
The present invention is a method for gapping a magnetic core, particularly applicable for small toroid cores less than 0.5 inches in diameter, by coating the core with a stabilizing material, fracturing the core with a laser beam, opening the fracture to set a desired inductance, and sealing the core gap. The core is opened by inserting a wedge into the core internal diameter and using the wedge to spread apart the fracture, meanwhile monitoring the inductance of the core until a desired inductance is reached. A conductor through the center of the core is sufficient to measure the inductance. The core gap is then sealed by submersion in a low viscosity adhesive (e.g. epoxy) bath in a vacuum, then returning the submerged core to atmospheric pressure, thereby impregnating the fracture with epoxy. The core is then removed from the epoxy bath, cleaned of excess epoxy and cured in an oven. Optionally, before curing, the inductance is checked and adjusted.
REFERENCES:
patent: 3507039 (1970-04-01), Craige
patent: 4267427 (1981-05-01), Nomura et al.
patent: 42 34 342 A1 (1994-04-01), None
Hewitt Richard P.
Rund Larry D.
Sigafoos Stuart O.
Christofferson Clyde R
Hall Carl E.
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