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Cast & Sintered - Alnico  Magnets.

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TABLE OF COMMON PERMANENT MAGNET MATERIALS:

MAGNETIC TERMS and DEFINITIONS

Most magnet design engineers are familiar with the widely used Alnico materials: Alnico V, VI, VIII, and IX which require magnetising forces ranging from 3000 to 7000 oersteds, with energy products of the order of 7.5 million gauss-oersteds, as well as the barium ferrites, which require a magnetising force of 12,000 oersteds. These presented some problems in magnetising, initially, but are now used in many production applications. Recent research on the so-called rare earths, which include Cerium, Lanthanum, Lutetium, Platinum, Neodymium and Samarium, has presented the design engineer with many new magnet materials, plus a new set of problems in their utilisation. Typical properties are given below of some of these materials now available in production quantities. They include energy product values of 14 million gauss-oersteds and required magnetising forces of 20,000 to 60,000 oersteds. Theoretical limits are said to be on the order of 45 million gauss-oersteds, which would require magnetising forces of 120,000 oersteds.

The advent of these new materials permits the design engineer to undertake magnetic assembly designs, which were theoretically, and economically impossible several years ago. The engineer must also thoroughly evaluate his design to be certain that once the assembly is fabricated, it can be magnetised. This is especially true of multi-pole structures. In many cases the magnet can be charged prior to installation in an assembly. However, due to ferrous contamination possibilities, physical handling difficulties, and similar drawbacks, may preclude the possibility of such "pre-magnetisation".

The higher magnetising force requirements also necessitate that existing magnetising fixtures be re-designed and, in some cases, the magnetising equipment, in order to achieve the forces required. With materials having coercive forces greater than 10.000 oersteds it is generally not necessary to stabilise a structure to prevent inadvertent change of flux density levels, although treating will be required in some instances, to set or calibrate the level of flux density required. Here the potential high energy of the stored energy treater will bean added asset.

MAGNET TERMS AND DEFINITIONS

In discussing the factors, which must be considered in choosing the type of magnet, charging and stabilising equipment, certain magnetic terms will be referred to. An understanding of these terms will prove helpful. Magnetic terms are generally defined in either of the two types of systems, the SI system, and the cgs (centimetre-gram-second) system. All terms in this publication are defined in the cgs system.

Anisotropic Magnetic Material

Also called oriented material. An anisotropic magnet has a preferred direction of magnetisation. To realise the maximum potentialities from such materials, they must be magnetised along the preferred axis. Orientation is accomplished by means of an applied magnetic field during manufacture of the material. Anisotropic materials do not have a preferred polarity orientation; i.e. either of the poles may be north or south.

  Coercive Force

This is the magnetising force, in oersteds, which must be applied to a magnetic material, in a direction opposing the residual induction, to reduce the induction to zero.  It may be considered the criterion that determines the ability of a magnet to resist demagnetising influences. A material having a high coercive force is more difficult to demagnetise than a material having a low value of coercive force.

Diamagnetic Materials

This describes materials that have permeability slightly less than one. These materials tend to be slightly repelled by a magnetic field.

Ferromagnetic Materials

These materials have characteristics similar to that of iron.  (Having a high degree of permeability. )

Isotropic Magnetic Materials

These materials do not have a preferred axis of magnetisation.

Magnetic Field Intensity

The strength, of a magnetic field, in air, measured at any point in a magnetic circuit,  It can be measured as either oersteds or gauss’s, since, in air, B (gauss’s) numerically equals H (oersteds) in the cgs system. Thus, if we say the field in a magnetic air gap is 3000 oersteds, we are also correct in saying the same field equals 3000 gauss’s. It must be remembered, however, that B and H are two distinctly different physical phenomena, and that the numerical equality exists only in air.

Magnetising Force

This describes the magnetomotive force (force, which tends to produce a magnetic field) per unit length. Symbolised as H, the unit is called the oersted. For example: Alnico V magnet material requires a magnetising force of 3000 oersteds for saturation. In reference to magnet chargers, it is the magnetising force developed by the charging fixture, either in the air gap between charging poles, in air surrounding a magnetising conductor, or in the cavity of a solenoid-charging fixture.

Magnetic Induction

Symbolised as B (gauss’s) and is the magnetic flux per unit of a magnetic section perpendicular to the direction of flux. An interesting point to note is that if a magnet material (Alnico V for example) is placed in a magnetic field (assume 3000 oersteds in this case) the flux density in the magnet will instantaneously rise to a value B (15000 gauss’s for Alnico V), far in excess of the magnetic field in air. The relationship of B and H in air therefore no longer holds, since B is measured in a magnet material having permeability greater than the permeability of air.

Paramagnetic Materials

These are materials that have “a permeability” only slightly greater than one, usually between 1.000 and 1.001. When ferromagnetic substances are heated to a temperature above their Curie point they become paramagnetic, until their temperature is reduced to below the critical point. Such materials are said as being “feebly attracted by a magnetic field”.

Reluctance

Generally speaking, reluctance is a measure of the ability or inability of a material to transmit or carry a magnetic field. Air may be considered a high reluctance path and soft iron a path of low reluctance.

Residual Induction

The magnetic induction remaining in a permanent magnet after the magnetising force is removed. It is measured in gauss’s.

Retrace-ability

Generally refers to the ability of a magnet assembly to have a predictable flux density at any given temperature within certain limits. For example: Assume a magnet assembly to have a flux density of 2000 gauss’s at 250C after magnetic stabilisation. The structure is then subjected to temperature extremes of 0 deg. C to 1000 degree    C. After the temperature cycling, the assembly may have a measured flux density of 1980 gauss’s at 250C. However, any further temperature cycling, providing the limits of 0 C and 100 C are not exceeded, will not affect the flux density at 25 degree C, i.e., 1980 gauss’s. The flux density' at any temperature between the cycling extremes, will also be retraceable. For Alnico V the flux density usually drops 0.02% per degree C rise above a specified temperature, and conversely exhibits a 0.02% per degree C increase, with lowering temperature.

Saturation

Describes the condition of a magnet when it is as fully magnetised as possible. To realise the full stability potential of a magnet it should always be saturated during charging, even though some demagnetisation may be necessary for the proper operation in the final magnetic assembly.

Stabilisation

Reducing the residual induction in a magnet to a level where it will not be affected by any demagnetising forces that may be encountered during normal operation of the finished magnetic assembly. Often called artificial aging. This is generally accomplished by subjecting the magnet assembly to an alternating magnetic field of adjustable intensity until the required flux density is reached in the magnetic air gap. Temperature stabilisation is accomplished by subjecting the magnet assembly to high and low temperature cycles, simulating the maximum temperature extremes that will be encountered by the assembly in its operating environment. Temperature cycling does not prevent changes in flux density with change of temperature, but it does allow operation of the structure with good retrace-ability characteristics.

CONVERSION FACTORS

Symbol CGS Unit SI Unit Conversion of CGS to SI

Induction (B) Gauss weber/meter2 x 10~-4

Induction (B) Gauss Tesla (T) . x 10~-4

Magnetic Field Intensity (H) Oersted ampere-turn/meter x79.58

Permeability (U) Gauss/oersted Henry/meter x 12.56 x 10 ~-6

Induction flux (Q) Maxwell (line) Weber x 10~-8

Magnetomotive force (MMF) Gilbert Ampere-turn x 79.55 x 10~-2

Reluctance (R) Gilbert/Maxwell Amp-tum/Weber x 79.55 x 10~-3

Permeance (P) Maxwell/Gilbert Weber/amp-turn x 12.56 x 10~-9

'Note: Induction is designated as Tesla in the internationally established SI system.  

TABLE OF COMMON PERMANENT MAGNET MATERIALS:

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