Loss of magnetisation with temperature.

[WheezyRider]

I've read on the web about the dangers of over heating hub motors as it could cause permanent loss of magnetisation of the magnets and this can start from temperatures as low as 80 Deg C.

This worried me a bit, it would be quite easy for a hub motor to get hotter than that. However, seasoned e-bike users here say they have never had a problem with overheating or loss of magnetisation.

I did a bit of digging and while it is true that certain grades of NdBFe magnets can start losing their magnetisation at only 80 Deg C, those used in e-bike hubs are likely to be made of grades with better temperature resistance.

Eeze say on their site:

"We use N35UH Neodymium magnets that retains magnetic properties up to 180°C"

From this chart you can see that N35UH can be operated up to 180 Deg C:

https://www.magnetsales.co.uk/wp-content/uploads/2016/08/Sintered-NdfeB-Grades.pdf

There are also some grades that are even fine up to 220 Deg C.

It would be really interesting to see what magnetic grade different motor manufacturers use for their magnets.

 

[Sturmey]

It should be possible to measure the 'loss' of magnetism on a motor and then to assess if its a problem or not. That's because a loss of magnetism will weaken the magnetic field and this will reduce the torque and efficiency proportionally. However, field weakening actually makes the motor spin faster when unloaded (increases the rpm) and this can be measured. So anyone with an unrestricted hub who has done the top speed test (with the wheel off the ground) with a fully charged battery, if they repeat the same test under the same conditions, and if the motor speed has increased, this would indicate that some field weakening has taken place. (I think).

 

[vfr400]

if it were a problem, we'd have loads of people coming on here and saying, "Why is my motor so weak?" That's never happened in the 10 years I've been on this forum., and many of us have run motors a long way above what the designers expected, like my 33 mph Cyclamatic Power Plus.

 

[Andy-Mat]

Apparently, the magnet has to get very hot to completely "destroy" the magnetism, and some links even claim it needs to get to red heat to do that.
I have to believe them as I have never had the opportunity to observe such things, though they do get taught in theory classes...
The temperatures reached in normal running, should never get anywhere near that, (I would guess less than 250°C at the very worst, and most probably do not exceed even 100°C) and although the field strength will be slightly reduced when warm, it should return to normal as it cools.
The end effect being unnoticeable in normal usage.
If it wasn't unnoticable, then manufacturers would make the casings finned, and possibly of aluminium, to increase cooling. But they don't, so its not seen as being needed!
And its not only hub motors that are affected. Mid motors are built using the same technology, and will show the exact same effects, maybe even more so, as the mid motors tend to be buried, deep in the casings, where the slip stream while riding cannot cool as effectively, as for instance a standard hub motor will be.....I would therefore expect mid motors to generally run hotter because of that....careful designs could help to cool better of course!
But, its also technically (but impractical as far as I am aware) possible to build motors with no permanent magnets, but using extra static field windings , to simulate the magnets normally used, but more complicated and I believe therefore probably less efficient, making them very unattractive for e-bikes.
I have never seen or heard of one being used in practice!
The whole point of magnets is that they do not need to be powered separately to produce the needed fields, so they are easily placed on the revolving part of the motor, negating the need for slip-rings, which are subject to wear and dirt (carbon from the brushes), inside such a motor.
The normal brushed DC motors, previously used on certain e-bikes, also have brushes, but with a commutator, not slip-rings.
Do not mix up the two styles!
I hope this eases the understanding of the effects of heat within BLDC motors on the magnets.

 

[WheezyRider]

if it were a problem, we'd have loads of people coming on here and saying, "Why is my motor so weak?" That's never happened in the 10 years I've been on this forum., and many of us have run motors a long way above what the designers expected, like my 33 mph Cyclamatic Power Plus.

Yes, any reputable motor manufacturer will use magnets made of grades capable of withstanding high temperatures. So it looks like it's not something we need to worry too much about, despite what some people say on the web.

 

[Woosh]

most motors are made with high grade magnets, with added Samarium. In any case, the loss is only partial.
However, I have repaired a couple of motors in the past where the heat melted the glue and detached a couple of magnets.