Effect of magnet weakening in BLDC motors?

[peters]

When the permanent magnets weaken due to overheating, does it increase KV? So it reduces torque and increases top speed? - I guess so, but I need opinion from motor experts.
Magnets weaken gradually or suddenly at high temperature?
Can there be any other change in a motor that is able to increase KV?

(my cromotor seems to have slightly higher top speed after some severe stress in the terrain, and I'm trying to find out the reason. Batteries, controller, wheels and everything else are the same as before.)

 

[Miles]

When the permanent magnets weaken due to overheating, does it increase KV? So it reduces torque and increases top speed? - I guess so, but I need opinion from motor experts.

Yes, that's correct. To be precise, the torque generated for a given current will be less.

Magnets weaken gradually or suddenly at high temperature?

I don't think you can count on any lag between temperature and effect because the strength varies continuously with the temperature. It's just that, above the maximum operating temperature, this effect becomes irreversible.

Can there be any other change in a motor that is able to increase KV?

If the timing is increased, it will have a similar effect.

 

[circuit]

We had this happen on 40kW motor. Magnets weakened, kv increased. One nasty side-effect is a non-uniform weakening, leading to different BEMF between poles and motor shorting itself. So it did spin almost twice faster, however was overheating even with no load.

 

[Offroader]

Hello, I'm looking for some advice.

I have two cromotors, one I never overheated and one I overheated enough to melt the glue holding the magnets, probably something over 150 or even 170c.

When I reglued my magnets and used the motor it feels that it may be weaker. However, it also felt like I may have gotten a higher top speed.

This could all be in my head. I was wondering if there is anyway I could do some kind of test to see if it is actually weaker (compared to my other cromotor) and if I weakened the magnets. I'm looking for some ideas. Thanks.
Maybe there is a way I could find the KV of the motor or something that would tell me if my magnets are weakened.

I use a Adappto Max-E controller.

Thanks

 

[circuit]

In case of higher top speed, this means your magnets have lost part of their strength. It is okay if they are equally magnetized now, but I had an incident when magnets turned out to be of different strength and that caused a lot of no-load power loss. Had to throw it all away.

 

[Offroader]

In case of higher top speed, this means your magnets have lost part of their strength. It is okay if they are equally magnetized now, but I had an incident when magnets turned out to be of different strength and that caused a lot of no-load power loss. Had to throw it all away.

Do you know if I lose any efficiency or better will I have a higher watt hour per mile, or more heat because I weakened my magnets? Assuming magnets are equally demagnetized which I think is the case. The motor runs smooth, I just have the feeling it may be a bit slightly weaker at lower speeds.

 

[circuit]

Since your winding resistance is the same and you are pushing more current to get the same torque as before, you will get lower efficiency.

 

[liveforphysics]

Since your winding resistance is the same and you are pushing more current to get the same torque as before, you will get lower efficiency.

The efficiency can increase or decrease if magnets become weaker. If the motors dominate loss was core losses, weakening the magnets generally improves motor efficiency. If the motors dominate loss was copper losses, it generally would decrease motor efficiency.