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[See update at the bottom]

We build brushless DC motors of about 1 kW power at 48 volts. In order to drive the motor, we need to know the rotor position and, to do that, we fit a disc-shaped magnet on the top of the shaft, and read the rotor position using the Infineon TLE5012B angular sensor (datasheet: https://www.infineon.com/dgdl/Infineon-TLE5012B_Exxxx-DS-v02_01-EN.pdf?fileId=db3a304334fac4c601350f31c43c433f).

The overall setup is as described in this image, taken from the sensor user manual:

setup

The magnet is a disc 10 mm wide and 4 mm thick:

enter image description here

Looking at the image above, it is not that the north is on one face, and the south on the other face: both north and south lie on the same face, with the north "up" (for example) and the south diametrally opposed.

When the motor+control board are assembled, we run a self-learning procedure which determines precisely the orientation of the magnet in respect to the rotor. It is very important that the board knows the precise offset (in degrees) of the north of the magnet, because once per turn the sensor generates an index pulse, and that pulse tells the board the rotor angle. The requested precision of this offset is about 5 degrees: if the offset is wrong by more than that, the motor sinks a lot of current and turns badly especially at higher speeds.

This system normally works well but, in about 10% of the cases, after some time (days or weeks) the motor starts to turn badly. Re-running the self learning procedure solves the issue. It seems that the magnet is not the same as before, a permanent change happens, correctable only by changing the stored offset (through the self learning procedure which, essentially, searches for the zero position which is now different than before).

Initially we thought that the magnet could mechanically move, so we started to fix it with some drops of glue. But this is not the issue, it seems.

So here is my question: is it possible that the north and south of a magnet like that change their position, especially thinking that the magnetic disc is mounted on the rotor of a brushless DC motor, where there are strong magnets? A move of one or two degrees does no harm; if that happens, we are facing a movement of 5 or more degrees.

---UPDATE---: What I mean with "change position of the poles" is that, after some unknown event (maybe temperature, or anything), the offset ("calibration") stored in the control board is no more valid. A new self-learning procedure corrects the problem. This means probably, that the poles have changed their position. The offset stored by the board is no more valid for that magnet. If both poles become weaker, together, probably there is no problem (the sensor auto regulates its sensitivity). But if the loss of magnetism is uneven among the two poles (who knows?), the "geometry" of the magnet changes and, to the sensor, it appears as the two poles have moved. May it be?

---UPDATE 2---: I have to clarify why I think there is a permanent change in the magnetic disc.

There is a "zero position" of the rotor. This "zero" is when the rotos is oriented in a precise position: at zero degrees (out of 360). Now suppose the sensor disc magnet is positioned perfectly onto the rotor such that, when the rotor is in its mechanical zero, the disc is perfectly aligned with the sensor, so the sensor emits a INDEX pulse. If I don't fit the disc in the correct position, the sensor will emit an index pulse anyway, but out of phase with the mechanical zero position of the rotor. For example, the index will come, every turn, at 5 degrees instead of 0 degrees.

By running the self-learning procedure, the control board is able to detect this misalignment or offset (5 degrees). This offset is then stored in non-volatile memory. From that moment, I can turn off the system, power it back again, and everything works because the absolute position of the rotor is corrected by the stored offset, so the electronic board knows the real position of the rotor and can energyze the coils correctly.

Now suppose I move the disc 10 degrees clockwise and power up the system: the control board will always get the wrong position of the rotor (always 10 degrees), so it will energyze the wrong coils (or not the "wrong coils", the correct ones, but too early or too late).

At this point, I run again the self-learning procedure, and the control board notices that the offset is not 5 degrees as before: now it is 15 degrees. The new offset is stored in memory, and everything works well again.

This is what is happening to us: we calibrate the system and send it to customers. Sometimes (10%), the customers calls us telling the motor fails. A new calibration solves. Only a couple of motors have been returned for inspection, at the moment, and we are investigating. Now, we understand that the offset has changed, but we have not been able to replicate the problem. We are sure that the position of the disc does not change, because we mark the position and also fix it with glue. It really seems that the poles of the magnetic disc change their position: and not only slightly, because an error in the offset is tolerable up to a few degrees (say, 3 or 4).

Here comes the question: is it possible that a magnetic disc moves its N and S poles in a permanent way, due to the continuous presence of external magnetic fields, perhaps in conjunction with high temperature events? It happens only in a few cases, and there are different situations. Sometimes the motor can heat, perhaps only once during installation, or maybe only once, or repeatedly, the motor is over stressed with big currents and hence anormally strong magnetic fields. But the result, it seems, is the same: the offset must be recalculated.

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    \$\begingroup\$ Why not? The magnetic pole has changed on the earth... \$\endgroup\$
    – Solar Mike
    Jan 20 at 7:38
  • \$\begingroup\$ It shouldn't AFAIK. Maybe there some stray field from the motor winding which over time remagnetized the disc? does the magnet heat up for some reason? it would make it more susceptible to changes (especially for cheaper magnets). \$\endgroup\$ Jan 20 at 8:07
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    \$\begingroup\$ Check the Curie point of your magnet, some materials can demagnetize as low as 80°C in my experience. If it's an EC motor can't you simply use the commutation signals for position? depending on the speed it could also even have hall sensors directly on the windings \$\endgroup\$ Jan 20 at 8:17
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    \$\begingroup\$ This website (sorry, in french) reports that neodinium magnet type N demagnetize "somewhat" already at a temperature of 80 °C supermagnete.be/fre/faq/… \$\endgroup\$
    – Antonio51
    Jan 20 at 8:55
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    \$\begingroup\$ yes. . it's a thermal problem that affects phase .. or "3" offset error that shifts phase. How consistent are the magnets? Bmin vs T and Remanent Flux Density tolerance \$\endgroup\$ Jan 20 at 9:24

2 Answers 2

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If you use neodynium magnet, maximum usable temperature is 80°C, although Curie point is ~300 °C !

EDIT: Some characteristics and specs in french, sorry.

Remark : somewhere in this page ...

https://www.supermagnete.fr/faq/Les-aimants-les-plus-forts-du-monde

A strong external magnetic field, such as a large neodymium supermagnet, could weaken the magnetization of an AlNiCo magnet, remove it or even reverse the north and south poles.

enter image description here

Use SH or higher

enter image description here

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  • \$\begingroup\$ It's a thermal calibration sensitivity issue I believe and not a temp extreme limitation. \$\endgroup\$ Jan 20 at 9:13
  • \$\begingroup\$ It shows only (sorry, in french) that max Temperature usable for "neodyme" is around 80 °C ... loose already "some" magnetization above, except for some until 150°C. \$\endgroup\$
    – Antonio51
    Jan 20 at 9:17
  • \$\begingroup\$ Thank you, I am investigating on that. It turned out that the magnet has "N" marking, and the producer admits that over 80°C a irreversible loss happen. Rarely, the motor can reach that temperature. But the producer says the position of the poles will not change even in case of permanent loss, while I am not so sure. \$\endgroup\$ Jan 20 at 15:47
  • \$\begingroup\$ I add some information about "magnetic" specifications for neodyme \$\endgroup\$
    – Antonio51
    Jan 20 at 18:19
  • \$\begingroup\$ @linuxfansaysReinstateMonica From this site supermagnete.fr/faq/Les-aimants-les-plus-forts-du-monde , a remark : <<<A strong external magnetic field, such as a large neodymium supermagnet, could weaken the magnetization of an AlNiCo magnet, remove it or even reverse the north and south poles.>>> So I think also for neodyme magnet. \$\endgroup\$
    – Antonio51
    Jan 20 at 20:46
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Your results hint that the magnet temperature weakens the field and affecting the calibration and/or chip temperature is exceeding approaching or exceeding 135'C perhaps causing phase-error and necessitating self-calibration.

There are two types of flux loss with high temperature: 1) reversible 2) non-reversible.

enter image description here enter image description here

This reminds when I discovered infrequent, random write errors in Toshiba 8" HDD's only for long seeks in one direction and the problem was weakening of magnet with temperature rise, requiring a diode thermal sensor and predetermined thermal calibration of overshoot. (circa mid-80's)

  • It took 1 day to find the symptoms and several months to fix the problem in production. The weakened magnet affected the servo loop gain and thus damping factor and overshoot which could cause a write error if it went offtrack, precisely when the expected sector was synchronous to it going ontrack in one direction of a long seek and the overshoot was around 20% offtrack around 70'C !

Can you report the temperature rise of the magnet and chip and predict phase errors?

Can you do the following?

  1. inspection tests on non-reversible thermal losses of these (N) and alternate high temp magnets (SH) with statistical analysis
  2. Sensitivity Analysis and Design Validation Tests on phase errors with gap changes, center offset and irreversible magnetic flux loss
  3. Consider better cooling for your motor and magnet with possible thermal insulation of attachment.?
  4. Contact Shinetsu tech support for better magnets on non-reversible magnet loss (relative loss might be lower on weaker magnets)

enter image description here

ref

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  • \$\begingroup\$ I am not sure this is clear: the change is permanent, a new search for zero position of the magnet is needed. Things go like this: search zero and store in the board. The motor works (not continuously) for days. Then something happens, and the motor stops working. A new search for the zero position is needed, because now that zero position is different than before. Then everything starts to work well again. \$\endgroup\$ Jan 20 at 15:43

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