PERMANENT-MAGNET MATERIALS AND CHARACTERISTICS.

PERMANENT-MAGNET MATERIALS AND CHARACTERISTICS.

The sustained success of the permanent-magnet industry in developing improved magnet characteristics can be realised from the graph shown below;

The latest addition being neodymium-iron-boron which has been pioneered by Sumitomo as 'Neomax', General Motors as 'Magnequench', Crucible ('Crumax'), and IG Technologies ('NelGT'). At room temperature NdFeB has the highest energy product of all commercially available magnets. The high remanence and coercivity permit marked reductions in motor frame-size for the same output compared with motors using ferrite (ceramic) magnets. However, ceramic magnets are considerably cheaper.
Both ceramic and NdFeB magnets are sensitive to temperature and special care must be taken in design for working temperatures above 100°C. For very high temperature applications Alnico or rare-earth cobalt magnets must be used, for example 2-17 cobalt-samarium which is useable up to 200°C or even 250°C.
NdFeB is produced either by a mill-and-sinter process (Neomax) or by a melt-spin casting process similar to that used for amorphous alloys (Magnequench). Powder from crushed ribbon is bonded or sintered to form the MQI or MQII grades produced by Magnequench Division of GM. The MQI bonded magnets can be formed in a wide variety of shapes. They are not 100 per cent dense and coatings may be used to prevent corrosion. With MQII and other sintered materials a dichromate coating may be used, or electroplating.
For lowest cost, ferrite or ceramic magnets are the universal choice. This class of magnet materials has been steadily improved and is now available with remanence of 0.38 T and almost straight demagnetization characteristic throughout the second quadrant. The temperature characteristics of ferrite magnets can be tailored to the application requirements so that maximum performance is obtained at the normal operating temperature, which may be as high as 100°C.
A brief summary of magnet properties is given in the table below. More detail can be obtained from suppliers' data sheets, as the examples show. Specialist data and measurements are often made by permanent-magnet research and development bodies; for example, in the UK, the Magnet Centre at Sunderland Polytechnic, and in the USA, the University of Dayton, Ohio. Activity in magnet research is also well reported in IEEE and specialist conference proceedings.

B-H LOOP AND DEMAGNETIZATION CHARACTERISTICS: PERMANENT-MAGNET MATERIAL.

The starting-point for understanding magnet characteristics is the B-H loop or 'hysteresis loop'. In figure shown, the x-axis measures the magnetizing force or, 'field intensity' H in the material. The y-axis is the magnetic flux-density' B in the material. An magnetized sample has B = 0 and H = 0 and therefore starts out at the origin. If it is subjected to a magnetic field, as for example in a magnetizing fixture (an electromagnet with specially shaped pole pieces to focus flux into the magnet), then B and H in the magnet will follow the curve OA as the external ampere-turns are increased. If the external ampere-turns are switched off, the magnet relaxes along AB. Its operating point (H, B) will depend on the shape of the magnet and the permeance of the surrounding 'magnetic circuit'. If the magnet is surrounded by a highly permeable magnetic circuit, that is, if it is 'keepered', then its poles are effectively shorted together so that H—0 and the flux-density is then the value at point B, the remanence B_r. The remanence is the maximum flux-density that can be retained by the magnet at a specified temperature after being magnetized to saturation.

External ampere-turns applied in the opposite direction cause the magnet's operating point to follow the curve from B through the second quadrant to C, and again if they are switched off at C the magnet relaxes along CD. It is now magnetized in the opposite direction and the maximum flux-density it can retain when 'keepered' is —B_r. To bring the flux-density to zero from the original positive remanence the external ampere-turns must provide within the magnet a negative magnetizing force —H_c, called the coercivity. Likewise, to return the flux-density to zero from the negative remanence point D, the field +H_c must be applied. The entire loop is usually symmetrical and can be measured using special instruments such as the Hysteresis graph made by Walker Scientific Instruments.

If negative external ampere-turns are applied, starting from point B, and switched off at R, the operating point of the magnet 'recoils' along RS. If the magnet is still 'keepered' the operating point ends up at point S. Now if the external ampere-turns are re-applied in the negative direction between S and R, the operating point returns along SR. The line RS is actually a very thin 'minor hysteresis loop' but for practical purposes it can be taken as a straight line whose slope is equal to the recoil permeability. This is usually quoted as a relative permeability, so that the actual slope of RS is µ_recµ₀ H/m. Operation along RS is stable provided that the operating point does not go beyond the boundary of the original hysteresis loop.

TEMPERATURE EFFECTS: REVERSIBLE LOSSES: PERMANENT-MAGNET MATERIALS

High-temperature effects: 

Exposure to sufficiently high temperatures for long enough periods produces metallurgical changes which may impair the ability of the permanent magnet

material to be magnetized and may even render it non-magnetic. There is also a temperature, called the Curie temperature, at which all magnetization is reduced to zero. After a magnet has been raised above the Curie temperature it can be re-magnetized to its prior condition provided that no metallurgical changes have taken place. The temperature at which significant metallurgical changes begin is lower than the Curie temperature in the case of the rare-earth/cobalt magnets, NdFeB, and Alnico; but in ceramic ferrite magnets it is the other way round. Therefore ceramic magnets can be safely demagnetized by heating them just above the Curie point for a short time. This is useful if it is required to demagnetize them for handling or finishing purposes. Table shows these temperatures for some of the important magnets used in motors

Reversible losses: 

The B-H loop changes shape with temperature. Over a limited range the changes are reversible and approximately linear, so that temperature coefficients for the remanence and coercivity can be used. Sometimes a coefficient is also quoted for the flux density at the maximum-energy point. The table shown, gives some typical data. Ceramic magnets have a positive coefficient of H_c , whereas the high-energy magnets lose coercivity as temperature increases.

In ceramic magnets the knee in the demagnetization curve moves down towards the third quadrant, and the permeance coefficient at the knee decreases. Thus ceramic magnets become better able to resist demagnetization as the temperature increases up to about 120°C. The greatest risk of demagnetization is at low temperatures when the remanent flux density is high and the coercivity is low; in a motor, this results in the highest short-circuit current when the magnet is least able to resist the demagnetizing ampere-turns. In high-energy magnets the knee moves the other way, often starting in the third quadrant at room temperature and making its way well into the second quadrant at 150°C. Grades with a high resistance to temperature are more expensive, yet these are often the ones that should be used in motors, particularly if high temperatures are possible (as they usually are under fault conditions).
All the magnets lose remanence as temperature increases. For a working temperature of 50°C above an ambient of 20°C, for instance, a ceramic magnet will have lost about 10 per cent. This is spontaneously recovered as the temperature falls back to ambient.

TEMPERATURE EFFECTS: IRREVERSIBLE LOSSES: PERMANENT-MAGNET MATERIALS

Domain relaxation: 

Immediately after magnetization there is a very slow relaxation, starting with the least stable domains returning to a state of lower potential energy. The relaxation rate depends on the operating point and is worse below (BH)_max , i.e. at low permeance coefficients.

In modern high-coercivity magnets at normal temperatures this process is usually negligible, particularly if the magnets have been stabilized (by temperature cycling and/or a.c. flux reduction) immediately after magnetization. Elevated temperatures during subsequent operation may, however, cause an increased relaxation rate. This can be prevented by temperature-cycling in the final assembly over a temperature range slightly wider than the worst-case operating range. Subsequent relaxation is reduced to negligible levels by this means. The 'natural' stability of different magnet materials at 24°C, is shown below,

Operating point effect: 

Temperature alters the B-H loop. If this causes the operating point to 'fall off' the lower end of a recoil line, there will be an irreversible flux loss. This is illustrated in image below.

Operation is initially at point a on the load line 0a, which is assumed to remain fixed. The remanent flux-density corresponding to point a is at point A. When the temperature is raised from T₁ to T₂ the operating point moves from a to b, and the corresponding remanent flux-density moves from A to B'.

Note that because the knee of the curve has risen above point b, the effective remanent flux-density is at B and is less than that at B', which is what it would have been if the magnet had been working at a high permeance coefficient.

If the temperature is now reduced to T₁ the operating point can recover only to a', which lies on the recoil line through A'. There has been a reversible recovery of remanence from B' to A', but not to A. The magnet has thus suffered an irreversible loss that can be recovered only by re-magnetization at the lower temperature. If the whole cycle of changes is repeated it stabilizes with the remanence at A' at the lower temperature T₁.
Manufacturers' data for irreversible loss should be interpreted carefully to distinguish between the long-term stability and the effects just described. Since the irreversible loss is dependent on the conditions of the application, in particular the permeance coefficient, irreversible loss is usually quoted at a fixed permeance coefficient. If the magnet is used at a higher permeance coefficient, the irreversible loss over the same temperature range will be lower.