Magnets & Temperature

How hot is too hot? - By: Cory Boehne (Miami Boardshop / Armor-Dilloz)

Modern Magnet Tech

The types of magnets used in modern small electric vehicle motors rely on the optimization of three important material properties - coercivity (resistance to losing magnetization) - maximum energy product (how much magnetic energy can be stored in the material) - and the maximum operating temperature (this has two important points- the first is where the magnet loses strength temporarily and the second is the point at which irreversible damage occurs to the magnet)

Naturally, a trade-off is made between these properties, and consumer products are not using the highest possible grade magnets - but, rather, the commercially available products that are in common use and are a good balance between cost and performance, as well as saving weight. While samarium cobalt (SmCo) magnets would be great for high temperatures, the weight of these magnets and their size makes them unsuitable - so, manufacturers use sintered Neodymium Iron Boron (NdFeB) magnets, which are smaller, stronger and lighter than SmCo, as well as being much tougher and more temperature tolerant than bonded NdFeB magnets - but have lower temperature limits than SmCo.

Ratings and Marketing Names

These are rated in mega-gauss-oersteds (MGOe), which is a unit of measurement for the maximum energy product and is usually between 35 and 52. This is usually how you will see magnets sold, such as N42, or N50 grade. Look at the table below, and you’ll see a suffix chart - this will be shown after Nxx, such as N42UH, or N52M, which denotes the temperatures they are intended to operate at (this is the first point at which magnetic strength starts to decline)

Conclusions

The commonly accepted knowledge is that consumer grade magnets are best in an operating range that is below 80C (175 degrees F or lower), which means that it’s likely permanent damage starts to occur at temperatures around 200 degrees, but certainly by 210 degrees there is degradation happening.

The best products, should, at least in theory, use SH grade, which should be resistant to 150 degrees - however, real world motors are regularly showing signficant degradation without ever reaching the temperatures where damage would be expected.

However, there is a very good reason for this! Very few motors have temperature sensors that are directly in contact with the magnets, so the temperatures you are seeing in readings from the motor are often the temperature of an area that is well away from the highest temperature areas in the motor.

Accordingly, the best recommendation is to mitigate heat as much as possible, as it will cetainly increase performance and will likely preserve the life of your motor as well.