Reversing polarity on U-Shaped electromagnet doesn't demag the EM

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
Hi guys,

I have a u-shaped electromagnet that's getting magnetized as I use it, so it's not letting go of metal items when I pull power. So I tried reversing the polarity, however that doesn't help either, it just re-powers it essentially. Any ideas how I can force a demag?
 

kubeek

Joined Sep 20, 2005
5,629
You can either use a different core material, or use some circuit arrangement to achieve demagnetization, coupling it with a big cap to make an LC resonant circuit might work.
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
You can either use a different core material, or use some circuit arrangement to achieve demagnetization, coupling it with a big cap to make an LC resonant circuit might work.
Interesting. I'll try it! How fast do you think it can demag with a cap? And what size cap would you recommend for my electromagnet? I've got aproximately 1 lb of copper total at 24 gauge, roughly 350 wraps on each pole.
 

MaxHeadRoom

Joined Jul 18, 2013
19,202
I noticed the way you showed it in the other thread, the magnet was inverted from the way it is normally for material lift etc. Facing down with the material at the bottom.
If this is the way you are using it what you are seeing is normal.
Max.
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
I noticed the way you showed it in the other thread, the magnet was inverted from the way it is normally for material lift etc. Facing down with the material at the bottom.
If this is the way you are using it what you are seeing is normal.
Max.
Copy that, thanks as always.
 

ian field

Joined Oct 27, 2012
6,539
Hi guys,

I have a u-shaped electromagnet that's getting magnetized as I use it, so it's not letting go of metal items when I pull power. So I tried reversing the polarity, however that doesn't help either, it just re-powers it essentially. Any ideas how I can force a demag?
Describing the degauss circuit from a CRT colour display wouldn't be allowed here because it uses non-transformer derived mains.

But portable colour CTVs used the (UK) 50Hz from the vertical scan output with a low voltage PTC thermistor to produce a decaying AC current flow.

I'd suggest using an isolating mains transformer with anything less than 50V secondary. The thermistor might be tricky, I think a Polyfuse has too sharp knee curve. If you can't find a suitable thermistor, the alternative is make a "degaussing wand" - just a coil of wire you energise with AC, hold it close to the magnetised core and sweep it around to cover it all, then back it away some distance before killing the current.
 

kubeek

Joined Sep 20, 2005
5,629
Since OP talks about polarity, then I guess he means a DC electromagnet and then the old timey degaussing would not work. Also, I had the feeling that a triac dimmer is not a forbidden topic, so why should a degaussing circuit be?
 

MaxHeadRoom

Joined Jul 18, 2013
19,202
As the polarity of the field imposed in the attracted plate would be known, it may require experimenting with the reverse pulse length, as long as the material is consistent.
Max.
 

ian field

Joined Oct 27, 2012
6,539
Since OP talks about polarity, then I guess he means a DC electromagnet and then the old timey degaussing would not work. Also, I had the feeling that a triac dimmer is not a forbidden topic, so why should a degaussing circuit be?
The original degauss circuit was little more than a PTC thermistor in series with the degauss coil across the mains.

Actually there was a little more to it than that; the current required to keep the PTC hot was sufficient through the coil to make the picture shimmer. What they did was put the PTC in series with a load resistor and tap the coil current off via a 68V varistor.

Next came 2 element posistors - a second element was connected directly across the mains to heat the other beyond conduction - some manufacturers opted for a PTC/NTC combo, the NTC served the dual role of inrush surge limiting before the bridge rectifier, and also heating the PTC.

Most degauss posistors in CRT monitors are switched by a relay under control of the front panel, its switched out after a short delay so it can rest cool - usually so a manual degauss can be initiated any time from the front controls.

Pretty much the whole of a degauss circuit is potentially live.
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
The original degauss circuit was little more than a PTC thermistor in series with the degauss coil across the mains.

Actually there was a little more to it than that; the current required to keep the PTC hot was sufficient through the coil to make the picture shimmer. What they did was put the PTC in series with a load resistor and tap the coil current off via a 68V varistor.

Next came 2 element posistors - a second element was connected directly across the mains to heat the other beyond conduction - some manufacturers opted for a PTC/NTC combo, the NTC served the dual role of inrush surge limiting before the bridge rectifier, and also heating the PTC.

Most degauss posistors in CRT monitors are switched by a relay under control of the front panel, its switched out after a short delay so it can rest cool - usually so a manual degauss can be initiated any time from the front controls.

Pretty much the whole of a degauss circuit is potentially live.
Thanks, but I'm afraid this is all way over my head :(
 

shortbus

Joined Sep 30, 2009
7,091
In a electro-magnet grinding chuck, the "demag" works by reversing the polarity back and forth. But it also lowers the voltage to the coils with each polarity reversal. If you just keep the same amount of voltage and reverse the polarity, you just end up with the same amount of magnetism with a reverse polarity.

A good discussion of what I'm saying - http://www.practicalmachinist.com/vb/general/magnetic-chuck-197941/

A patent that describe's how to do it - http://www.google.com/patents/US2445459
 

alfacliff

Joined Dec 13, 2013
2,458
can you chane he core material? iron thats too soft will magnetise too fast to demagnetise, you wont see the short time it demagnetises before it remagnetises.
 

ian field

Joined Oct 27, 2012
6,539
Try powering the magnet from AC instead of DC and that should do the trick.
The core will retain some magnetisation if the AC cycle is anything other than zero at the instant its switched off - and slight residual from the last half cycle if you break at zero crossing.
 

MaxHeadRoom

Joined Jul 18, 2013
19,202
The main problem is the material resting on the inverted magnet rather than the conventional lift magnet where the material can just drop off when de-magged.
Max.
 

erjs2000

Joined May 19, 2012
2
Hi guys,

I have a u-shaped electromagnet that's getting magnetized as I use it, so it's not letting go of metal items when I pull power. So I tried reversing the polarity, however that doesn't help either, it just re-powers it essentially. Any ideas how I can force a demag?
You seem to have made a version of Edward Leedskalnin's "perpetual motion holder". If the coils are wired in series, end of one coil to start of the other, it won't let go when the power is removed if the metal bar it is holding is attached to both poles.

Check it out. http://www.leedskalnin.com/
 
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