How to demagnetize a ferrite magnet?

[John in CR]

I need to demagnetize some ring shaped ferrite magnets cannibalized from speakers to use as ferrite cores for large toroidal chokes to tame my low inductance motors, so they stop killing controllers. If I can make this work it will be the holy grail for us performance chasers. I finally found a toroidal coil calculator, and if Arlo1's estimates are correct, I need less than 100uH, which according to the calculator will take less than 10 turns around a large ferrite ring. That means nice fat wires are possible for reasonably sized coils.

I realize the only way to do it is to increase the temperature to the Currie Point of 460°C (860°F). I tried on a stovetop already since a red hot burn is hotter than that, but it wouldn't heat evenly enough and the magnet shattered. Anyone have any practical ideas for a garage builder? The magnets are doughnut shaped with a rectangular cross section around the ring, so it has 2 flat faces.

The only thing I've come up with is to put a steel plate on each face held in place by the magnetism to distribute the heat evenly, and then build a charcoal furnace like I've done to get steel hot enough to bend, and put it in. Once the steel plates release, then I'm done.

Any better ideas? Do you think the preceding approach will work? I have a finite supply of cheapie speakers to cannibalize, and I need 3 unless I can wrap all 3 phases on one ferrite core, so I can't do too much trial and error, and being rainy season I have a fairly small furnace window in the mornings.

John

 

[Harold in CR]

Using an AC(alternating current)field

The method uses a magnetic field generated by a current flowing through a solenoid to change the ordering of the magnetic dipole. An AC current is used to produce magnetic field of changing direction near the magnet. The(electro-)magnetic field caused the magnetic dipoles to switch direction many times. Each time the field point to one direction some magnetic dipoles will try to align along that direction. If the field is big enough, many will align along that direction. When the field is reduced and reversed not all these magnetic poles will reverse. Hence, by repeating this process many time (that's why an AC current is needed) and also by reducing the magnitude of the current as we go, to reduce the strength of the (electro-)magnetic field, the magnetism of the magnet can be reduced to almost zero or perhaps for most practical purpose zero. In an electromagnet, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.

Have you noticed that sometimes when you have your computer monitor placed in some location of your house you find that the display is discolorated? Ever pressed some buttons on the monitor and select the degaussing function to remove the color distortion on your computer monitor? What do you think is happening when you do that?

Conversely, as you have learned, you can use the field generated by the current to magnetized a magnet. The magnetism may or may not stay after you remove the external magnetic field due to the current depending on the property of the material. For example the magnetic material used in our hard-disk for storing information will keep its magnetism (data) after the current is removed. On the other hand the little "horse-shoe" electromagnet in the recording head that is used to produce the magnetic field for recording becomes a strong magnet in the present of the current but weaken drastically when the current is removed.

 

[bigmoose]

Also try heating and cooling it slower going up to the Curie temperature. Slow the dT/dt.

 

[Njay]

If you're talking about the kind of speaker magnets I think, they are 2 parts, one ferrite and another iron. Maybe it shattered due to different heat expanding coefficients of both materials - iron expanded faster and shattered ferrite.

 

[John in CR]

If you're talking about the kind of speaker magnets I think, they are 2 parts, one ferrite and another iron. Maybe it shattered due to different heat expanding coefficients of both materials - iron expanded faster and shattered ferrite.

The iron plates come right out. There's glue in between the plates and the mag. The back plate includes the pole piece, but there's an air gap between it and the mag, which is where the moving voice coil resides on the former. Different expansion coefficients won't matter, and I'm actually leaning toward leaving the top plate and back plate in place during the cooking process to slow the temperature increase and help spread it evenly.

The one I shattered was just the raw magnet I put on a cast iron skillet. It heated too fast and unevenly with the top face exposed to air. The next attempt will be inside a concrete fiber box warmed slowly to max oven temp and then I'll transfer that box to the freshly lit fire built within heat retaining walls to get it the rest of the way up to temp with the growth of the fire. If that doesn't work, I'll have to come up with something even more insulated to build the temperature more gradually.

 

[TylerDurden]

I would try binding a few in opposition and repeatedly heat them in the oven. The repelling poles might help demagnetization at lower temperature than curie.

I might also try winding one with one leg of an appliance cord attached to a toaster or similar. Using the appliance might create enough ac field to degauss (or poorly regauss) the ring.

If you have any friends at a TV station, they might have bulk-degausser for metal tape that has enough mojo.

 

[John in CR]

I would try binding a few in opposition and repeatedly heat them in the oven. The repelling poles might help demagnetization at lower temperature than curie.

I might also try winding one with one leg of an appliance cord attached to a toaster or similar. Using the appliance might create enough ac field to degauss (or poorly regauss) the ring.

If you have any friends at a TV station, they might have bulk-degausser for metal tape that has enough mojo.

No friends that have access to a degausser, but I like the other 2 as easy to try first before attempting full currie temp, so thanks for those TD.

John

 

[amberwolf]

It's not terribly powerful, but every CRT TV and monitor has a degausser built into it, as a rectangle of wire around the inside face of the unit, right around the outer edge of the face of the CRT.

 

[texaspyro]

CRT degaussers tend to not be very powerful...

Also ferrite tends to not make very strong inductors and has large core losses and poor saturation performance... and it tends to crack and shatter under meachinical stress. It is probably not the best thing to use for motor inductors.

 

[John in CR]

...Also ferrite tends to not make very strong inductors and has large core losses and poor saturation performance... and it tends to crack and shatter under meachinical stress. It is probably not the best thing to use for motor inductors.

Texaspyro,
What would you suggest as a better core material at our PWM frequencies? The calculator I found for toroidal coils listed only ferrite and powdered iron as the core types, with the ferrite cores being far superior. I do have magnetite that I could mix with epoxy to mold some toroidal cores. I figure that could be better than powdered iron, but not as good as ferrite. Even if I could source some transformer steel, I wouldn't even know which way to orient the laminations for a toroidal core.

 

[Solcar]

Have you considered what would happen if they were wound while still magnetized? i would think that the magnetic domains are aligned differently in wound toroid use than when emitting magnetic fields out from the flat faces as in speaker motors. Just so long as the magnetizing influence of the coils reorients the domains, which I think is the case since the coils force the magnetic domains into a circular loop rather than all being oriented in one direction, the cores should be able to store energy from the current passing through the copper windings (and have useful effect).

 

[parabellum]

Do you really need to demagnetize it for your purpose?

 

[John in CR]

If leaving them as magnets works, that would be even better. Operating it may even demagnetize them right on the bike, since they'll see a high alternating current. As long as I don't increase the risk of blowing my controller, I'm game.

 

[fechter]

I wouldn't worry about demagnetizing them first. If the core goes to saturation with the AC from the motor, it will demagnetize itself if needed. Saturation could be an issue, but they are large pieces, so with very heavy wire and few turns you may avoid it. It would be nice if there was a way to test the saturation current or calculate it.

 

[John in CR]

I wouldn't worry about demagnetizing them first. If the core goes to saturation with the AC from the motor, it will demagnetize itself if needed. Saturation could be an issue, but they are large pieces, so with very heavy wire and few turns you may avoid it. It would be nice if there was a way to test the saturation current or calculate it.

Sounds perfect. I have just 3 larger ones that I was willing to sacrifice, so I couldn't use those if I was going to try getting to currie temp. The other cheapies that I have 7 of that were on the chopping block only have about half the magnet bulk.

If I prove it works, but hit saturation too early, I do have larger speaker drivers I'd be willing to sacrifice to save controllers, so in the long run there's no way I'll run out of magnet. Actually now that I thing about it, I have some paper cone 12's that got bug and water damage, so I can go straight to some nice size mags right off the bat.

I take it that saturation will show up as hot magnets, and also that with toroidals I don't have to worry about interference between coils.

Thanks everyone. Multiple heads works great, and hopefully I'll have some good news to report for the other acceleration junkies. 8)

John

 

[dnmun]

i agree no need to demagnetize to use in a choke but how will this help your controller? will it cause higher voltage spikes that the caps can't handle?

 

[texaspyro]

I'd look at using laminated iron core inductors for this application. They have their issues, but I suspect they will hold up better... after all the motor is build that way.

 

[John in CR]

I'd look at using laminated iron core inductors for this application. They have their issues, but I suspect they will hold up better... after all the motor is build that way.

If I'm looking at running 100A+ battery side, that's 250A+ through the phase wires. It seems like toroidal is the only reasonable answer because it requires fewer turns by about half. I also like that the flux created is so much more confined. What am I missing?

The motor is wound with almost 8ga worth of magnet wire, and each phase is 2 turns around 17 teeth, so the inductance is quite low. I'm shooting for the controller to see an additional 100uH, so that's 50uH on each coil, which the calculator I found showed just 3-6 turns with a 2.5" diameter ferrite core (the largest option in the calculator) depending on the material. While my ferrite material is unknown, I'm using a 4.5" diameter core and the calculator shows larger cores need fewer turns for the same inductance, so I plan to try 4 or 5 turns first.

When I see this as an example of a low inductance toroidal coil on Wikipedia, and I've seen similar wit many electronic devices, it makes the approach seem more forgiving.