Destroying motor by demagnetization

[gianstraccio]

Have you ever had such a experience?

In consideration that demagnetization of a permanent
magnet is a perfectly possibile physical operation.
In what condition, do you think, it could occur in
motors currently used for electric bikes?

G.

 

[dazzassj6]

how can you demagnetise an magnet when the very composition of it is magnetic. Think thats impossible.

 

[Miles]

If you heat them above the temperature that they are rated for....

http://www.physlink.com/Education/AskExperts/ae472.cfm

http://www.engconcepts.net/Magnet_Ratings.htm

 

[Sputnik]

I've heard that exposing them to a large enough opposing magnetic field as well can cause them to lose their magnetism. I don't know how likely that is though.

Sputnik

 

[gianstraccio]

I've heard that exposing them to a large enough opposing magnetic field as well can cause them to lose their magnetism. I don't know how likely that is though.

Sputnik

Correct !!

 

[dogman dan]

I melted a motor this summer. After glueing the magnets back, the motor ran real hot. The second time it ran even hotter, and the third time it ran so hot it only lasted about a mile. Snowcrow says the magnets were getting ruined by heat. My experience seems to back that up. The magnets seem pretty strong still, but the motor did run hotter each time I reglued the melted off magnets.

 

[Link]

Hit the curie point of the magnets, a REALLY powerful field in opposition to theirs, or enough physical impact and they'll lose their properties. Luckily, I've never had this happen and I hope I never do.

 

[MidniteTweeker]

Permanent magnets can be de-magged by subjecting them to an opposing field, but it seems unlikely this would happen in normal use. When permanent magnets are initially magnetized, great effort is made to fill or reduce any air gaps so that the magnetix flux is easily conducted through the magnetization fixture. The idea being that every air gap acts like a series resistor that opposes magnetization. The magnetizers I've worked with used capacitive discharge through through electromagnets which had pole pieces shaped to concentrate the lines of flux to precisely match the outline of the target magnet.

So conversely, it seems like de-magging them once they were installed in an application where air gaps were present would be quite hard to do.

Freshly mag'd magnets can lose a bit of strength, so we would reverse the current flow at a much lower level to slighly knock them down and stablize them at a level slightly below peak.

I don't know what temp would be required to demag, but my guess is it's hotter than what your wire insulation is rated for.

I'm wondering if there's something else going on. That is, did you replace the magnets with the proper poles in the proper places?

MT

 

[knoxie]

Yeah

Heat will do it for sure, I ran a Puma too hard for too long it got very hot and has about half the power it once had really notice it on hill climbs, the big danger is cracking the magnets esp on motors like the Kolmorgen, once the rotor magnet cracks you may as well bin the motor. Best to fit a stat to your motor if you intend on running high power.

Knoxie

 

[gogo]

Freshly mag'd magnets can lose a bit of strength, so we would reverse the current flow at a much lower level to slighly knock them down and stablize them at a level slightly below peak.

^Thats n interesting bit of information. I work at a hydrostatic transmission factory where I zap the magnets for the pilot control device. We demagnetize them just a bit until they behave predictably.

I'd be happy to try re-magnetizing in our setup for anyone with suspect magnets.

 

[Cackalacka]

Yeah, its been a decade and a half since freshman Emag, but from what I remember: in addition to heat and the presence of an overwhelming magnetic field, if you strike a magnet with a hammer or something you may disturb it enough to have it lose its properties.

 

[Miles]

if you strike a magnet with a hammer or something you may disturb it enough to have it lose its properties.

If it was Nd-Fe-B, it would certainly lose its singularity....

 

[Doctorbass]

I believe too that it's possible to decrease the magnetixation of the magnets.. but it would need a VERY large opposite magnetic field!!!.. like supraconductor magnet to build a mag field like that in the tesla range (10 000gauss)

Heat could affect the magnet more than opposite mag field i believe.

We are a couples of member here that use their X5 motor up to 12 times their nominal 750W rated power...

Mine reach often current like 105A at 90V drop.. that'S 9kW.. and the worst conditions happen when i drive with hard stop and go.. the motor inner steel reach always like 130 degree C and sometime peak to the dangerous range of 160-170!..

And these times i can see that the power decrease.. but when it cold down, to ambient temp, it recover.. so i would believe that its because of the motor winding resistance that increase... not the magnet loosing their force.

I really think wou will roast the winding BEFORE to demagnetize these magnet!

Doc

 

[fechter]

With NdFeB rare earth magnets, it is virtually impossible to demagetize them by pumping too much current through the motor. They are easily demagnetized by excessive temperature though.

Ceramic magnets are a different animal and I have personally demagnetized a couple of motors by running excessive current. Ceramic magnets are good for much higher temperatures.

 

[MidniteTweeker]

Is there a magnet material typically used in hub motors?

MT

 

[Doctorbass]

Is there a magnet material typically used in hub motors?

MT

Yes, the NdFeB rare earth magnets

 

[kZs0lt]

Heat will do it for sure, I ran a Puma too hard for too long it got very hot and has about half the power it once had really notice it on hill climbs,

They are easily demagnetized by excessive temperature though.

Is there a method to tell if a motor has weaken magnets due to hard riding and excessive temperatures? Could measuring back EMF give a hint on that? I can imagine back EMF dropping, but maybe it's very slight, and can notice it only on hill climbs(more Amps)? Any other measuring suggestions?

 

[Miles]

Yes, the back EMF constant/torque constant will change.

It will run faster if the magnets are weakened.

Measure the present no load speed and compare with the published spec. ?

 

[dogman dan]

Heat definitely will do a rare earth magnet in. Just to be sure, I did it again to a heinzmann! :lol: :lol: :lol: After I pulled the stinking smoking thing apart, the magnets wouldn't even stick to the fridge anymore.

 

[kZs0lt]

Heat definitely will do a rare earth magnet in. Just to be sure, I did it again to a heinzmann! :lol: :lol: :lol: After I pulled the stinking smoking thing apart, the magnets wouldn't even stick to the fridge anymore.

How did you succeeded killing the magnets before frying the winding?

I really think wou will roast the winding BEFORE to demagnetize these magnet!

Is high ambient temperature to blame?

 

[Doctorbass]

Guys there is something here that need to be clarified...

on wich temperature the demagnetisation happened?


I really measured 164 degree celsiud on the winding frame of the stator of my motor!.. and the external hub motro casing that have the magnet was at 120 degree C !! the rain just boiled on it!

And... it still have the same torque and acceleration.. no weak part...

could it be because it's a X5 5305? These kind of nearly bulletproof motor?

Doc

 

[dogman dan]

One fried motor was ridden for an hour uphill, nonstop, every day for a month in 104 F weather. It lasted 400 miles. That was a WE brushed. It melted the epoxy and when I reglued the magnets in the wrong orientation it began to run REALLY HOT! That was when the magnets cooked, about the time the brushes smoked. I think the brushes go up before the winding so it saves the winding on brushed motors.

The second one was a 24v heinzmann that I overvolted. It lasted for one hour, and also melted down the brushes, It's magnets were so cooked they would no longer stick to the fridge. That motor was used, so no telling what it went through before I got it. It was 50 F the day I smoked it. I don't know how hot it got, but wires melted the insulation off. I didn't have a thermometer with me.

 

[Doctorbass]

Here is THE ARTICLE WE ALL NEED TOREAD ABOUT THTA GREAT SUBJECT!



http://www.ctc-control.com/customer/elearning/servotut/younkin/tempEffectsOnTimeK.pdf


Doc

 

[dogman dan]

Math way way over my head, but to the extent I understood the text, it totally confirms what I felt the motor doing as I was climbing those hills. As the motor got hotter and hotter, it got weaker and weaker, and the weaker it got, the faster it got hot. It was sort of an exponential thing where you were hot, but ok, but as soon as you got too hot, the motor noticeably slowed down, and noticeably got hotter faster.

If your motor is suddenly sluggish on a hot day, STOP. Continuing at that point, may smoke it. My WE brushed hub typically runs at 140 F measured on the case cover, while the brusless WE runs at about 90F, so this is a much bigger problem with a brushed motor. The hot windings have more resistance, I knew that, but this article says the magnets weaken as they overheat too, and that causes more heating as well. Verrrrry interesting article.

 

[kbarrett]

Yeah, its been a decade and a half since freshman Emag, but from what I remember: in addition to heat and the presence of an overwhelming magnetic field, if you strike a magnet with a hammer or something you may disturb it enough to have it lose its properties.

You can also use a hammer strike to magnetize ... it's an old trick.

Point a two foot metal rod at magnetic north, and hit the south end repeatedly with a big hammer. After several minutes of this, a lot of the atoms in the rod will re-arrange themselves in N-S order, and the rod will now pick up paperclips.