How Heat Impacts a Magnet’s Holding Energy and Methods to Use Temperature Coefficients

Irreversible Loss vs. Reversible Loss

A casual evaluation of the technical data accompanying Neodymium Iron Boron magnet reveals potential heat points.  Upon additional examination of the documentation, one can observe a recommended maximum working temperature, however with a caveat which qualifies the maximum working temperature relative to the geometry of the magnet.  It’s not a hard number and can range depending on the magnetic circuit during which the magnet is working. This maximum working temperature is vital, however, it’s merely the purpose past which the magnet will experience an irreversible loss in net magnetization.  In all actuality, a magnet will lose net magnetization as quickly as it begins to warmth up.  This loss is known as “reversible” as it’s recovered as quickly because the magnet cools again down.  (Conversely, a magnet will acquire net magnetization as quickly as it’s cooled down.)

Reversible Loss’ Impact

At first look, the irreversible loss appears to be the most important situation, however, the reversible loss can cause a negative impact on a magnet’s efficiency as a result of whereas the magnet doesn’t completely demagnetize, it could not generate sufficient subject for the desired software at a specific working temperature.

Temperature Coefficients

To gauge the performance of a magnet whereas uncovered to heat but working under the purpose of irreversible loss, one can estimate the impact with the magnet’s temperature coefficients; α (Br) for the Residual Induction (Br) and α (HcJ) for the Intrinsic Coercive Field Strength (Hci = HcJ).  Most magnet distributors will list these values, however, there may be usually little software data supplied.  (These coefficients are additionally usually supplied as a normal worth, however, they’re truly fairly completely different for various Intrinsic Coercive Area Energy values.)  The Temperature Coefficients signify the p.c change within the related magnetic traits (Br or HcJ) per diploma C.

Use of Temperature Coefficients

The temperature coefficients facilitate the calculation for the lack of Br and HcJ for a magnet.  The coefficients and the ensuing change in Br and HcJ are handled as a linear operate between a specified temperature vary.  This vary is often between zero°C and 100°C.  Exterior this vary the operate is taken into account non-linear and the coefficient technique of calculating magnetic subject degradation can’t be trusted.  (With this stated, coefficient calculations needs to be thought-about first order and verified empirically.)

To What Applications Can Temperature Coefficients Apply?

For work-holding and subject density functions, solely the Residual Induction (Br) Temperature Coefficient will likely be used. The Intrinsic Coercive subject energy (HcJ) Temperature Coefficient is generally used when there may be an exterior demagnetizing subject, however, that’s outdoors the scope of this text.

Temperature Coefficient Limitations

As listed in different Sinda Magnetics’ data base articles, the Residual Induction (Br) is the “closed circuit” maximum induction for the magnet.  This magnetic attribute is quantity unbiased and most magnets don’t function at this level.  The magnet will usually function sooner or later under the Br and so calculate the degradation of the Br doesn’t instantly translate into the Induction Working level (Bd) of the magnet, not to mention a selected subject density or engaging pressure.

Practical Application of the Residual Induction (Br) Temperature Coefficient

The equation for calculating the ensuing Residual Induction (Br) at numerous temperatures is listed on Dura’s Materials pages.  As acknowledged above, calculating the ensuing degradation of the Br is inadequate for specifying the precise impact on a software; nevertheless, it may be used to estimate the impression.

The Residual Induction and the Most Vitality product (BHmax) have been calculated for every temperature listed under in Desk 1 and a 2D FEA simulator was used to resolve the Enticing Force (lb.) for every case.

What turns into obvious is that the Vitality Density (Grade) drops significantly because the magnet is heated up.  The grade 45 basically turns into a grade 38.7. This loss is reversible and the magnet will get better, however, the software could also be in jeopardy as the web subject of the magnet degrades.  What else is obvious is the magnet loses a full two kilos of attracting pressure when going from 20°C to 100°C.

If the engaging pressure dropped then so did the web magnetic subject output of the magnet.  If this magnet was being utilized in a sensor software, the magnet could not produce sufficient subject to function the sensor at an elevated working temperature.

Desk 1: Efficient Br, BHmax, and Enticing Forces on the Listed Operational Temperatures

Correlation

The train means that the degradation of the magnetic subject, and due to this fact the engaging pressure of the magnet, is proportional to 2 instances the degradation of the Residual Induction (Br).  It is a good first order estimate and it may be used, however, needs to be validated empirically.

Key Factors:

• Although a specific grade of the magnet doesn’t expertise an irreversible loss (i.e. permanent demagnetization) when heated up, it’s going to nonetheless lose web magnetic subject.
• For recoverable losses as a result of heating, a magnet will get better when cooled again down.
• Although a magnet didn’t expertise an irrecoverable loss, the recoverable loss could also be such that the magnet fails to ship sufficient subject at an elevated working temperature.
• The Residual Induction Temperature Coefficient α (Br) can be utilized to estimate the degradation of the Br for a specified temperature vary.
• The ensuing p.c lower in engaging pressure or web subject is proportional to 2 instances the p.c degradation of the Residual Induction (Br).
• Choosing the next warmth tolerance grade could not resolve the issue of reversible subject losses as a result of warmth. The next warmth tolerance will imply that the magnet won’t expertise an irreversible loss at greater temperatures.  This doesn’t imply the p.c lack of Br per diploma C will likely be improved to the purpose the magnet will likely be profitable within the software.  The answer could also be within the magnetic geometry design or altering alloys.