What is Maximum Energy Product / BH Max?
The Maximum Energy Product of a neodymium magnet represents the product of the magnetism (B) and a neodymium magnet’s intrinsic coercivity (H). The Maximum Energy Product, also denoted as (BH) max, is a yardstick for determining the strength of a magnetic material.
The value of the BH max indicates the amount of magnetic flux that passes through the neodymium magnet per unit of time or the amount of energy a neodymium magnet can supply to an external magnetic body (circuit). The SI unit of the Maximum Energy Product is kJ/m3. Under the Centimeter-Gram-Second (CGS) unit system, the mega-gaussian-oersted (MGOe) unit is used to represent the (BH) max of a neodymium magnet; 1 MGOe equals 7.958 kJ/m3.
Understanding Maximum Energy Product
The Maximum Energy Product has been used to grade the magnetic performance of a material. This is especially true for rare earth metals – Neodymium Iron Boron magnets and Samarium Cobalt magnets. For instance, a N45 Neodymium Iron Boron magnet has a grade of 45 MGOe BH max, where ‘N’ stands for Neodymium. A Samarium Cobalt magnet typically has a BH max of 16 MGoe to 32 MGOe.
Maximum Energy Product can be explained using the BH max curve. The curve indicates that the Maximum Energy Product is a product of the points on the normal curve of BH max against the field strength of the neodymium magnet. The Maximum Energy Product is obtained at the highest induction level and the smallest volume of the neodymium magnet’s cross-sectional area.
The BH max of a neodymium magnet does not depend on the neodymium magnet’s volume. Two Neodymium magnets of the same grades but different sizes will produce a similar BH max, though their magnetic fluxes may vary. The larger the volume of the neodymium magnet, the more the magnetic flux it yields.
The volume of a neodymium magnet is also related to the BH max of the neodymium magnet. In many cases, the relationship is inversely proportional. Consider a magnetic circuit made up of a neodymium magnet of known volume and having an air gap of known volume, connected by a magnetic core. If the circuit is set up to attain a field strength in the air gap, the total magnetic energy generated in the gap will equal half of the neodymium magnet’s BH max.
From the BH Curve, we can determine the Maximum Energy Product as the area’s largest rectangle that can be inscribed under the Normal Curve. The Maximum Energy Product also exists as the tip of the parabola generated from plotting the data points on the Normal Curve against a neodymium magnet’s field strength, provided that the magnetism is measured in Gauss and the coercivity in Oersteds.
The Maximum Energy Product indicates a neodymium magnet’s energy supply to a magnetic circuit when operating along the demagnetization curve. A neodymium magnet operating at Maximum Energy Product operates at the highest efficiency relative to the energy density of the neodymium magnet. However, the BH max does not guarantee a neodymium magnet’s ability to generate enough magnetic field to perform a particular work. Thus, neodymium magnets are not designed to operate at BH max routinely.
For a neodymium magnet to function at a BH max, other factors such as its physical geometry must be considered. In magnetic design, the geometry relates the length of the neodymium magnet to its diameter. The diameter can be direct (for circular neodymium magnets) or inferred (for neodymium magnets with non-circular poles). Two or more N48 Neodymium magnets with varying sizes will show varying physical geometry and, inevitably, different Maximum Energy Products.
Relationship between Maximum Energy Product and Neodymium Magnet Performance
Predicting magnetic performance based solely on the value of a neodymium magnet’s BH max has limitations. The Maximum Energy Product isn’t a holistic reflection of the neodymium magnet’s performance to magnetic flux or field density. Before deducing performance, certain parameters must be defined, including the physical geometry of the neodymium magnet. Nevertheless, the higher the Maximum Energy Product value, the greater the magnetic field force generated in a particular direction.
The intensity of a magnetic field also contributes to the neodymium magnet’s performance. Magnetic intensity represents the flux density of a neodymium magnet, which is the total density of the magnetic field. Imagine a series of magnetic field lines passing through a neodymium magnet, corresponding to the flow of magnetism over a specified area. The total number of field lines that can penetrate that given area is the flux density of the neodymium magnet.
The intensity varies with the magnetic circuit type. Generally, a neodymium magnet with an open circuit shows a lower level of intensity than a neodymium magnet having a closed circuit. The rate of decrease depends on the magnetic length and the surface area of the neodymium magnet. The flux density of Neodymium magnets, having a straight line demagnetization curve, elevates when the neodymium magnet comes in contact with a circuit.
Another factor that determines the performance of a metal is the pull strength. The pull strength is the force required to separate a neodymium magnet from a flat steel surface when in direct contact with the steel object. Neodymium magnets with high pull strength/force can take on more weight than their low-strength counterparts. The pull force is described in Newton, kilogram, or pounds.
When combined with a neodymium magnet’s flux density and pull strength, the Maximum Energy Product suggests the neodymium magnet’s performance.
Maximum Energy Product and Grades of Neodymium Magnet
Neodymium magnets are the strongest permanent magnets known. They are less apt to be demagnetized and are often graded using the prefix ‘N,’ representing Neodymium.
The higher the grade, the stronger the Neodymium magnet. The mega-gaussian-oersted (MGOe) of Neodymium magnets ranges from 30 MGOe to 55 MGOe. An N42 neodymium magnet will not be as strong as an N52 neodymium magnet. However, the performance of the N42 neodymium magnet can be elevated by altering its pole size or, in some neodymium magnets with thin poles, the temperature of the neodymium magnet.
Samarium Cobalt magnets have MGOe ranging between 16 MGOe and 32 MGOe. The common Samarium Cobalt magnets are graded 16, 18, 20, 22,…, 32. As with Neodymium magnets, higher-ranked neodymium magnets are stronger than lower-ranked neodymium magnets under controlled environmental circumstances.
What happens if a neodymium magnet has a letter after its grade, N42H? Since temperature can affect a neodymium magnet’s performance, neodymium magnets are often graded to indicate the temperature at which they can perform.
A Neodymium material graded as N42 shows that the neodymium magnet can operate at temperatures below 80oC. An N46M neodymium magnet can be used at a maximum operating temperature of 100oC. Other known temperature grades are:
- “H,” for up to 120C
- “SH” up to 150C
- “UH” up to 180C
- “EH” up to 200C, and
- “TH” up to 220C
Should neodymium magnet grades be a priority when purchasing neodymium magnets? It depends. High-grade neodymium magnets cost more than low-grade ones, which is a determining factor for the kind of purchase made. Besides, the letters behind each grade inflate the price of the neodymium magnet. So, an N36H neodymium magnet will possibly cost more than an N36 neodymium magnet. And in some cases, a low-grade neodymium magnet with a higher letter will cost more than a high-grade neodymium magnet, such that an N32SH neodymium magnet may cost more than an N40 neodymium magnet.
Should you purchase the highest strength material every time? Perhaps not. The strongest neodymium magnet isn’t always the best neodymium magnet for an application. You may need to use a neodymium magnet that can produce a specific magnetic field at a specific distance. The right neodymium magnet for a magnetic project may be a Neodymium 34, so using a Neodymium 42H will likely cause a malfunction in operation.
Another consideration for choosing neodymium magnets is the possibility of combining neodymium magnets. In some situations, a neodymium magnet can be combined with another neodymium magnet to induce a focused magnetic field. The strength of such neodymium magnets is of minor importance. A high-grade neodymium magnet with a thin diameter may underperform compared to a low-grade neodymium magnet with a thick diameter.
Selecting neodymium magnets for different applications requires extensive knowledge about the Maximum Energy Product and other factors that affect a neodymium magnet’s effectiveness. A technician can offer insights to help you determine the suitable neodymium magnets for your work.
Appendix of Terms
- Magnetic induction (B): This is the flux per unit area relative to the direction of a magnetic path. It is the field induced by a magnetic field strength H. The magnetic induction is a vector quantity.
- Area of air gap: This is the cross-sectional area within which a magnetic interaction occurs.
- Neodymium magnets: These are rare earth magnets due to the presence of the rare element Neodymium.
- Samarium Cobalt magnets: Like the Neodymium magnets, they are rare earth magnets because of the Sm and Co in their composition. The two series are SmCo5 and Sm2Co17.
- Oersted: This is the unit of the magnetic field strength, H, based on the electromagnetic measurement system. It equals one gilbert per centimeter of flux strength.
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