Chuck Mag/Demag ...How does it work?

Ok, so I have a magnetic chuck, for example on a surface grinder. The control for it, which I assume is just a power supply, has a "mag" and
"Demag" function.
What happens in "Demag"? Reverse polarity? AC Pulse? Anyone know?
I would find out myself but the damn thing is dead. Thanks.
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Steve Cothran wrote:

Most magnetic chucks use permanent magnets. The holding power is controlled by rotating the magnet to one of two positions. When holding, the poles of the magnet face pole pieces, and the flux is forced to loop back externally. When releasing, the magnet's ends are each half in one pole piece and half in the other, allowing its flux to take a short internal path. Maybe the knob has come loose from the shaft, or the shaft from the magnet. With freedom to move on its own, the magnet will go to the low-energy -- that is, "demag" -- position.
Some magnetic chucks do use electromagnets. Those have an obvious source of external power. Does yours?
Jerry
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Yeah, its a PCB with some type of micro-p and some big-assed SCR's. There are only two wires going to the chuck, which is about a foot square. From look-intently mode, I can see that the chuck power is derived off the 480V line via the SCRs to the chuck. We have the rescources and wherewithal to buy another but apparently all of Italy goes on vacation this time of year.
The control boad is dead-it has a multi-tap (8 windings and of course potted) power transformer that is bad.. Of course, it is a critical machine, and we're trying to come up with another way of operating the chuck. I have DC power supplies out the yang, but kind of like to have a clue before I do things...Thanks.
We don't have and apparently can't get a schematic.
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Steve Cothran wrote:

Some are simple On/Off and will leave the part and the table partially magnetized. Others use a series of diminishing reversals to overcome the hysterisys loop during demag and leave no residual magnetism in the part. I used one of these ringing demagnetizer from MPS in a machine that employed a magnetometer and it was critical to remove all residual magnetism from the components making up the stages. I believe they have retrofit packages.
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jeff

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Steve Cothran wrote:

The magnet almost certainly is actuated by more-or-less steady DC. (Part of the filtering may come from its self inductance.) Conceivably, a little AC may be applied to facilitate release, but I don't think that likely. The heft of the dead transformer may allow you to estimate the power involved, and the resistance of the electromagnet, along with the power estimate, could hint at the appropriate voltage. Try half of the estimate for a start and see if the holding power seems adequate.
Summation: stare hard to try to psych it out, then poke it and watch how it wiggles.
Jerry
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