What (besides heat) can demagnetize a motor?
 

I have again demagnetized (or nearly so) my Plettenberg Big Maxximum. First time around was with my MGM16024, with less than 30 min. run time. I sent in for warranty repair, but they blamed the controller. This time around I only got about 15 min. on the MMM. I never ran more than 5 or 10 min at a time as I was still trying to make adjustments and had to stop and do that. Temps never had time to get very hot, 130 - 135 F, maybe 150 after the motor started acting up (after getting demagnetized).

Plettenberg said to set controller for 4 pole motor and the part number say P4 (4 pole), but when I ran it on the MGM set to 4 pole, I got a reading of 75,000 rpm. This would not be possible with a 2300kv motor on 6s. If I set the MGM to 6s, i get 50,000 rpm, which is right for 2300kv on 6s. I wonder if something may be mismatched; windings or rotor. Is it possible to have 4 pole windings and 6 pole rotor, (or vice versa), or is the poles all dependent on the rotor.

I can't help but think something is not right, and of course they (plettenberg) are going to swear up and down that all is right and good with their motor. I have only manage to demagnetize one other motor, a Nemesis blue 8XL, but that was nearly 30 min. continuous running and it was near 200 when it went, and was in a different vehicle. Then FD replaced it no problem.

I have run at least 4 different motors in the same rig on the MGM, and 2 motors with the MMM. All worked well and ran cool. I even ran a 7XL on 6s and it ran much cooler than it ever did on 4s, and it was every bit as powerful as the Big Maxx (I have to agree with Serum on that one).

I am just trying to decide if I should just by a new rotor for this thing or if I should rattle Plettenbergs cage some more or just write this (now very expensive after repair) motor off as loss. So any input on what else could have happened will give me some ammo to fire at Plettenberg. Thanks guys.

Well, I was always of the impression that they were 6 pole motors, until someone contacted me about their BM that was being repaired- they are indeed 4 pole motors.

As for de-maging the rotor, only heat couod do that, or a poweful electromagnet wrapped around the motor for a long time.

And it is the rotor/magnet that has the poles 2-4-6 etc, the windings just determine the kv rating.

Why would the MGM show 75,000 rpm for it when set to 4 pole, unless they have the wrong windings in it (to high of kv)?

Beats me- maybe its an older design rotor with 6 poles, or maybe the MGM is nuts, or both?

Bruce,

Demag happens when the magnetic force applied to the magnet exceeds the coercive strength of the magnet. In other words, if the strength of an external magnetic field (like the field winding of an electric motor) exceeds a certain level, the magnet will be demagnetized (or remagnetized in a different orientation.) So any field of sufficient strength will demag a magnet.

It just so happens that the field strength at which demag happens is also affected by temperature. Look at this picture:

http://i37.photobucket.com/albums/e9...t/BH_curve.jpg

This is for an N44SH magnet (fairly common magnet type.)

If the field strength exceeds the coercive force of the magnet (at any temperature) the magnet will fail. It's just MUCH more sensitive to demag at high temperature (see the sharp downward part of the diagonal curve (at 140C)? That's the demag point. And the higher the temp, the lower the power required to demag.) On the chart, B is the field strength of the magnet itself, and H is the applied field trying to demag the magnet (field from windings in a motor...)

Curie temperature is the temperature where the magnet will fail, just by being at that temperature (think about the atoms moving enough that they start jumping around -- and out of alignment with each other.)

SO -- you can still demag a magnet if the applied field is very very strong, even at low temperatures (for this type of magnet, you would have to apply a field so strong that it crosses the knee on the -B side of the graph, which isn't shown.) And at high temperatures, it takes much less field strength to demag a magnet. This is why BOTH the temperature grade of the magnet, AND the field strength of the magnet are important. The higher the field strength of the magnet, the more resistant it is to demag at lower than max operating temperatures with very strong fields. The higher the temperature grade of the magnet, the more resistant it is to demag at elevated temperatures. IF you operate above the "knee" in the diagonal line on the graph, the demag is temporary, below the "knee" is a permanent demag.


That said, this is just a tutorial on how demag can happen. I don't know if your motor is demagged or not...

wow thanks for that, i never really understood what was happening, but now i do. thanks :)

Thanks Patrick!

That answers my question as to how it could have demagnetized without ever getting very hot. Though I don't think it has completely demagnetized yet, it is noticibly weaker in power and everything heats up fast. Certainly not worth running it to risk destroying something else (my new MMM).

That is one good detailed info but still in layman's term so everyone can understand...

Thanks Patrick

I agree, so I copied the post to a thread in the FAQ section, and stickied it: http://www.rc-monster.com/forum/showthread.php?t=14381