Magnet operating temperature
Magnet operating temperature
(OP)
OK, here’s my question. How do you calculate the temperature that a magnet can be exposed to without permanent loss? And, how do you calculate how much loss you’ve suffered based on the temperature that your magnet has been exposed to? In particular, I would like to use a magnet to activate a sensor over a given temperature range. This would constitute an open circuit condition.
What I think I understand: A BH curve that is presented by the manufacturer usually consists of the second quadrant information and in some cases the third quadrant. There are usually several curves consisting of various temperatures that make up both the normal and intrinsic portions of the curve. The permeance coefficient will help describe the operating line and operating point of the magnetic circuit and is based on the magnetic circuit described. The temperature coefficient of Br is usually given in percent per degree C. In rare cases, a graph that describes the typical irreversible losses for various working points depending on the temperature is provided. Example: percentage loss for B/µ o = 0, -0.05, -1, -2 etc. with various temperatures. If the operating point falls below the knee on the normal curve, irreversible loss occurs. The amount of recovery will follow a line parallel to the original line.
What I think I understand: A BH curve that is presented by the manufacturer usually consists of the second quadrant information and in some cases the third quadrant. There are usually several curves consisting of various temperatures that make up both the normal and intrinsic portions of the curve. The permeance coefficient will help describe the operating line and operating point of the magnetic circuit and is based on the magnetic circuit described. The temperature coefficient of Br is usually given in percent per degree C. In rare cases, a graph that describes the typical irreversible losses for various working points depending on the temperature is provided. Example: percentage loss for B/µ o = 0, -0.05, -1, -2 etc. with various temperatures. If the operating point falls below the knee on the normal curve, irreversible loss occurs. The amount of recovery will follow a line parallel to the original line.
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RE: Magnet operating temperature
You have a very good understanding of the issues involved. While it can be quite calculated, I've oftentimes found it easier to just bake a bunch of magnets at the maximum expected temperature to find out.
As you mentioned, if the operating point stays above the knee of the normal curve at the maximum temperature, then only reversible loss will occur and you can use the reversible temperature coefficient to calculate the loss while at temperature. In this instance, the magnet will return to full strength after returning to room temperature.
If the operating point is below the knee of the curve at the maximum temperature, then it gets a bit more complicated and it's tough to describe in a text-based forum such as this, but your understanding of it is good. The key thing (as you mentioned) is the recovery will follow a line parallel to the original. If you can measure the percentage difference in the Br (Room Temperature) and B(after drawing the parallel line), you'll have a good estimation of the percentage decrease.
If you can get your hands on one, Parker's book on permanent magnets does a great job illustrating this.
RE: Magnet operating temperature
RE: Magnet operating temperature
It's too bad your colleagues don't realize that while a given magnetic material may be "rated to 150degC", it will produce a different magnetic output at that temperature than it would at room temperature.
If it's not too late, you might want to consider SmCo permanent magnet material. It has a much smaller temperature coefficient than NdFeB. It's likely to have less of an irreversible loss too.
RE: Magnet operating temperature
Mike