Electropermanent magnet - Page 1
Forum section: electronics / projects

author: gaminn
author_rank: Frequent Contributor
timestamp: March 17, 2022, 12:17:52 pm
content: Hi,I came across electropermanent magnetic chuck/plate without power/control unit. This type:https://www.tiekstratechniek.nl/wp-content/uploads/2020/05/SAV-243_70-En.pdf. It is 600 x 300 mm, pole spacing 18 mm. The label on the chuck says current rating 360 V / 30 A. The chuck has 3 wires - 2 wires are coils, third wire is protective earth. My multimeter measures 37 ohms coils resistance.To magnetize/demagnetize the chuck one have to supply short high current dc pulse to the coils. I tried that by using 3 phase full wave rectifier. My line voltage is 230V/50Hz, so 3 phase rectifier givex me approx 550 V and I get around 15 A to the chuck. I turn on the rectifier only for a fraction of second.I'm able to magnetize the chuck (however, I'm not sure whether its holding force corresponds to what it should be 110 N/cm2). The problem is no matter what polarity of DC pulse I send to the chuck it is still magnetized and I'm not able to demagnetize it. Is there any expert who could tell me what I'm doing wrong?

author: BradC
author_rank: Super Contributor
timestamp: March 17, 2022, 01:41:37 pm
content: You are going to have to get a bit "more cleverer" to demagnetise it.From a description of the control module : "All control elements are integrated in the front panel. The control is directly connected to the mains supply (230 V 50 Hz) and the magnet with a multicore cable. A floating relay contact can be used as machine interlock. This contact is closed at energized magnet. The power to the magnet is continuously monitored. The status of the magnet is indicated on the front panel with signal light. (green=energized, red=off, red blinking=demagnetizing). The holding-force control is stepless. The demagnetizing function is microprocessor controlled utilizing electric pulses with alternating polarity."

author: gaminn
author_rank: Frequent Contributor
timestamp: March 17, 2022, 02:02:34 pm
content: Thanks, but I'm quite sure this description is for eletromagnetic chuck, which has to be energized all the time in order to hold work.I have electropermanent magnetic chuck which behaves differently - it needs high current dc pulse, then the power can be removed and the chuck holds its magnetism "infinitely".But you are right that the demagnetisation might be more sophisticated for electropermanent magnet (not just reverse polarity high current dc pulse). I'm obviously not expert in this field.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 17, 2022, 06:12:33 pm
content: https://en.wikipedia.org/wiki/Electropermanent_magnetenjoy.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: March 17, 2022, 06:14:48 pm
content: myGUESSwould be 230v AC connected via alargePTC thermistor. as the thermistor heats up, the AC current trails off, leaving a demagnetized block of metal. at that point you can disconnect power.this is the system used with the degaussing coils on a CRT monitor or TV, and you should be able to salvage such a thermistor from one.cheers,rob :-)

author: BradC
author_rank: Super Contributor
timestamp: March 17, 2022, 11:45:56 pm
content: Quote from: gaminn on March 17, 2022, 02:02:34 pmI looked at the data sheet for the device you supplied. It listed the part number for 2 controllers, I looked up the one I could find posted the description here.

author: gaminn
author_rank: Frequent Contributor
timestamp: March 18, 2022, 07:19:18 am
content: Quote from: BradC on March 17, 2022, 11:45:56 pmYou are right. I found out that my EPM (electroparmenant magnet) isn't probably equipped with permanent magnets, it contains just permanent electromagnets. Then it should be possible to demagnetize it using AC signal.

author: Berni
author_rank: Super Contributor
timestamp: March 18, 2022, 10:21:45 am
content: Yep if it stays on no mater the polarity it is made from all magnetizable electromagnets.The trouble is that demagnetizing is a lot more complicated. Not only do you need an AC signal for that, but you also have to slowly reduce the AC signal amplitude down to zero.The degausing coils in CRT monitors use a PTC resistor to do it. The coil is connected to mains voltage via that resistor. At the beginning the resistor is cold so it passes lots of current, making a nice strong field that can magnetize the screen, but then as the resistor heats up its resistance rises so the current drops and along with it the magnetic field. Eventually the current drops enough for the field to be too weak to magnetize the screen, so it is left demagnetized. At that point the relay disconnects mains from the coil.In principle you could also demagnetize it with a AC field that gets suddenly turned off in exactly the right part of the cycle, but you would need to be pretty precise with it and it would need to be turned for the exact application (since various materials need various field strengths to magnetize)

author: robert.rozee
author_rank: Frequent Contributor
timestamp: March 18, 2022, 12:45:50 pm
content: given this isn't a piece of precision equipment you're working with, you can demagnetize relatively easily, see the attached image from:http://www.ukvac.com/forum/plz-help-me-add-manual-degauss-button-to-circuit_topic328792.htmlcheers,rob :-)

author: gaminn
author_rank: Frequent Contributor
timestamp: March 18, 2022, 08:14:32 pm
content: Thanks for that demagnetizing circuit. I like itHowever, I'm thinking about designing my own circuit to magnetize/demagnetize the plate. Full bridge (MOSFETs or IGBTs) with driver, current measurement circuit and some microcontroller. Then just right know how to be implemented in the microcontroller....

author: gaminn
author_rank: Frequent Contributor
timestamp: March 18, 2022, 08:44:28 pm
content: The original control unit for this EPM can detect low clamping force of the plate (not sure how the plate can demagnetize by itself during its operation) and then e.g. shutdown a machine the plate is mounted on to prevent a damage.Do you have an idea how one can detect low clamping force of the EMP when the only connection of control unit and the EPM are two power wires?I would be able to detect low clamping force e.g. by external hall sensor mounted on the plate. But is it possible without external sensor?