How to choose the right magnet ?

WHICH MATERIAL ?

Several characteristics have to be taken into consideration when deciding which permanent magnetic material to use. These are: -

• Flux requirement for the particular application.
• Maximum operating temperature.
• Cost.
• Availability.
• Degree of corrosion likely to be encountered.
• Magnetic stability required.
• Size and/or weight limitations.

Although there are many different types of permanent magnet material the following information considers only the four major types in use today. These are Alnico, Ferrite, Samarium Cobalt (SmCo) and Neodymium-Iron Boron (NdFeB). The individual grades within each type will be considered separately.

Most of the materials supplied are classed as Anisotropic, that is a preferred magnetic axis is determined during manufacturer and so the magnets can only be magnetised in that predetermined direction. Isotropic material may be magnetized in any direction, but are generally lower in performance than the Anisotropic grades.

COMPARISON OF MAGNETIC PERFORMANCES

The most convenient method of comparing the magnetic performance of different types and grades of permanent magnet is to consider their maximum energy product (BHmax). This is the point where the magnet will provide most energy for the minimum volume, so:-

MGO in CGS units, kJ/m3 in SI units

It is frequently necessary to know what flux density will be on the pole face of a magnet. Often this is erroneously thought to be the Remanence (Br), but this figure is purely the induction in a closed circuit conditions and bears little relationship to the actual flux density under normal working conditions.

The following table shows typical pole face flux densities for the four grades when working at approximately their BHmax points.

TEMPERATURE EFFECTS

These are in two distinct categories, reversible and irreversible. The reversible changes with temperature, are dependant upon material composition and are unaffected by the shape, size or the working point on the demagnetisation curve. These types of losses disappear completely without need for remagnetisation when the magnet is returned to its initial temperature, whereas irreversible losses do not come into operation until a certain temperature has been exceeded. These types of losses can also be limited by operating at as high a working point as possible. There are unrecoverable losses caused when excessive temperatures are reached and metallurgical changes occur within magnet.

REVERSIBLE EFFECT OF TEMPERATURE (20?C - 150 ?C)

MAXIMUM WORKING TEMPERATURES (BEFORE IRREVERSIBLE LOSSES COMMENCE)

The maximum working temperature is dependant on the working point of the magnet in the circuit. The higher the working point the higher the temperature the magnet can operate

N.B. Irreversible losses may be restored by remagnetising the magnet

UNRECOVERABLE LOSSES (CURIE TEMPERATURE)

Each material has a maximum temperature where metallurgical changes occur within the magnet structure and where the individual magnetic domains breakdown. Once these losses occur they cannot be reversed by remagnetising.

EFFECTS OF SUB-ZERO TEMPERATURES

The effects of low temperatures are different for each material group and are heavily related to the magnet shape and therefore the its working point on its demagnetization curve.

MAGNETIC STABILITY

The effect of temperature has the largest effect on magnet stability, but exposure to high external fields can influence certain types of magnets with the following degree of effect: -

TIME

The effect of time on magnets is negligible and averages a loss of less than 1 x 10-5 per annum at 20 oC. In a 100,000 hour period (11.4 years), these losses range from essentially zero for SmCo to less than 3% for Alcomax III at low permeance coefficients.

SHOCK & VIBRATION

Shock and vibration was always an issue with our earliest magnets but for modern magnet materials this is no longer a problem, apart from the most closely calibrated devices.However, magnet materials may be brittle and subject to fracture from mechanical impact (SmCo being the most brittle).

RADIATION