Temperature and Neodymium Magnets

[el_walto]

I'm trying to find the max safe operating temperature of my MAC motor. In searching I found this interesting article. A little too technical for me to fully grasp.

http://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets

If i understand correctly. The larger(thicker?) the magnet is, the higher temperature it can withstand without damage. If it is wider it can withstand less temperature. Also depends on the type of magnet material.

It's starting to look like the 80c max temperature, as many people state looks reasonable, but once your magnets accidentally get to 140c once, you may as well make that 140c as your new max temperature.

I guess the only true way to figure this out would be to test some of the magnets, as hub motor manufacturers don't seem to provide this information.

 

[Kingfish]

I use this chart: Neodymium Magnet Physical Properties

If thickness is held contant, generally the stronger the magnet is, the lower its' safe thermal limit. In motor design, I try to balance the Flux Density with the expected heat range; the density is determined by the proximity with opposing forces + the magnet thickness + characteristics of the magnetic circuit e.g. back iron/flux ring + the current flowing through the coils + the resistance of said coils + additional heating due to eddy currents and other design-induced issues. Together these factors determine the strength and quality of the magnetic material, thus the rating and heat-limitations.

Through study I have determined that the magnets in my 9C hubs are not that high strength – however this could be translated into meaning that they have high thermal tolerance (at least – I’d sure like to believe that).

The best method for you and your motor is to query the supplier or manufacturer for the magnet spec/datasheet – or if nothing else ask for the maximum operating temperature of the motor.

Good hunting, KF

 

[dogman dan]

I got a heinzmann motor hot enough to demagnetize some of the magnets. Upon disassembly, they wouldn't stick to the fridge anymore. Smoke pouring out of the motor gave me a clue I might have gotten it a tad hot.

But I've smoked some 9c motors pretty good too, and still reused the rotors with other stators. We took a temp of 450F with an infared thermometer as flames came out the vent holes on one of them, and those magnets are still in use today on one of my dirt bike 2812 motors.

So I'd say the magnets on 9c motors are likely to take more than your windings can.

 

[el_walto]

I got a heinzmann motor hot enough to demagnetize some of the magnets. Upon disassemble, they wouldn't stick to the fridge anymore. Smoke pouring out of the motor gave me a clue I might have gotten it a tad hot.

But I've smoked some 9c motors pretty good too, and still reused the rotors with other stators. We took a temp of 450F with an infared thermometer as flames came out the vent holes on one of them, and those magnets are still in use today on one of my dirt bike 2812 motors.

So I'd say the magnets on 9c motors are likely to take more than your windings can.

If this is true, then why not set temp limit on CA to 140c instead of 80c? I guess my question would be, What is the efficiency of the 2812 motor you cooked today, vs the day you bought it. Maybe it has lost %20 of its capability? Maybe the no load speed is still %100?

I keep finding magnets online that are rated to 80c. But there are so many factors I don't think we'll ever get a really clear answer.

 

[dogman dan]

Good questions I don't have answers to. I only know that hub A, which was not overheated as much seems to perform about equal to hub B, who's rotor went though the 450F at the death race. The stator from that hub is toast, but I reused the rotor (which has the magnets) in one of my 2812's.

Both seem to be able to put out about the same power, based on how far I get up a hillI can't climb to the top of.

All I really know, is the 450F didn't completely ruin it. But it sure ruined a winding, so at least with the 9c type motors, the magnets aren't really the weak point, IMO.

 

[neptronix]

100c is a nice general limit for all motors.