Application Guide

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

Magnets are used within particle beam deflection applications and it is recommended that magnets with a higher Hci are used in such environments. Tests have shown that SmCo displays significant losses when exposed to high levels of radiation (109 to 1010 rads) and NdFeB losses 50 % at 4 x 106 rads and 100% at 7 x107 rads. At low levels of radiation, losses are in line with that of temperature looses. Note that some magnet materials contain Cobalt, which can retain radiation after exposure.

SHAPE

Magnet shape does have an effect on performance and therefore its stability. The shape of the magnet determines its working point along the demagnetization curve. Higher up the curve the magnet operates, the closer it is to its optimum working point and is therefore less susceptible to external influences. Magnets used in a closed magnetic circuit or with a longer length, perform better and are magnetically more stable.

It is possible to help magnetic stability in performance, by exposing the magnet in advance to any possible detrimental influences by thermal cycling and controlled demagnetizing fields (ageing) techniques.

Another cause of loss of performance is the total breakdown of composition due to environmental effects causing structural breakdown, such as corrosion or in the case of NdFeB, exposure to Hydrogen.

CORROSION RESISTANCE WHEN UNPROTECTED

There are many protective coatings available and commonly used to help prevent corrosion in magnets. Alnico uses powder coating and electroplating when required and NdFeB magnets often have Nickel, Zinc, Lacquer, Epoxy or Parylene as a protective coating.

COMPARITIVE COSTS

Shape, tolerances and quantity will influence the prices of individual magnets but the most significant effect is the cost of the basic raw material.

Tooling is sometime necessary for new sizes and for volume production and fixtures are sometime required for close tolerance machining.

MAGNETIC RULES

• Flux lines always follow the paths of least resistance or in magnetic terms, path of greatest permanence (lowest reluctance).
   
• Flux lines repel each other if their direction of flow is the same and can never cross.
   
• Flux lines will always follow the shortest path through any medium
   
• Flux lines will normally always move in curved paths
   
• Flux lines always leave and enter the surfaces of ferromagnetic materials at right angles
   
• All Ferromagnetic materials have limited ability to carry flux. When they have reached their limit (saturated) then they behave as if they were not there, like an air gap or similar.
   
• Flux lines will always travel from the nearest north pole to the nearest south pole, in a path that forms a closed loop.
   

SOME USEFUL DESIGN ADVICE

1. Stronger is not necessarily better

There are many factors to consider in magnetic design, Flux strength is only one of them.

2. Using a steel pole piece can often improve a magnets performance.

Sometimes it is useful to use a steel pole piece to help divert the flux to a more useful part of the magnetic circuit.

3. Always be aware of the working temperature of your application

Temperature is the biggest threat to magnetic stability, so always consider it as part of your design and your material/grade choice.

4. For holding/attracting applications, two poles are better than one.

But remember the flux will not travel as far, so keep the air gap between the magnet and what you want to attract as small as possible.

5. It is difficult to focus flux lines when using Rare Earth Magnets.

The use of steel poles in this instance is generally not effective.

6. The best test for a magnetic device is to replicate the application.

As there is not a simple single test which tells you all about a magnet, then it is often best to recreate the work the magnet will see in its application and include this somewhere in your testing procedure.

TYPICAL MAGNETIC USES & APPLICATIONS

Permanent magnets are used in a wide variety application in all industries but they can all be classified into one of the sections below:-

• Conversion of Electrical Energy to Physical Motion
  Actuators, speakers, motors, meters and other instrumentation.

• Conversion of Physical Motion into Electrical Energy
  Generators, microphones and sensors.

• Producing Mechanical Energy
  Holding, lifting, attracting, repelling, conveying, driving & separating

• Controlling Fields
  Annealing, plasma control, sputtering, focusing & NMR.

• Mechanical to Heat
  Eddie Current and Hysteresis drives