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  1. #1
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    Default Electromagnetic chuck controller

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    I have purchased a rotary magnetic chuck that was part of a surface grinder. Unfortunately when the seller scrapped the rest of the machine, he also scrapped the controller, leaving just two bare wires.

    When I got the chuck home I tested it with 12V DC and it does indeed work and has a 50 ohm resistance. It has stamped on the side 110V 240W which with ohms law gives us the 50 ohms I measured.

    I need to be able to power the chuck. I can purchase a controller from Taiwan but at $400 I think I can build something much cheaper, especially as I do not expect to be using this a great deal. So if someone can go give Ray a poke as I am sure this sort of thing is right up his alley.

    I think I need a 110V DC variable power supply that has the ability to also inject AC power into the coil to demagnetise everything when the job is finished.

    I am suspecting something like a variac 0-110V device, a bridge rectifier plus a capacitor to smooth out the ripples. Then some sort of switch to bypass the bridge rectifier and inject AC for demag mode.

    I already have 110V AC on hand as the optical comparator I got has a whopper of a 240-110V transformer in it.

    20151217_163732.jpg 20151217_163726.jpg 20151217_162257.jpg20151217_162212.jpg 20151217_154601.jpg 20151217_131629.jpg







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  2. #2
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    Hi .RC,

    I think you've already got the right idea, just a 110V DC supply @ 2A (or more) is what you need, I'm not so sure about doing demag with 50Hz AC, the one on my SG is fairly low frequency, maybe 1Hz or less.

    You could just use a drill speed controller type circuit, ( phase angle fired triac) and rectify the output from that rather than waste an expensive variac.

    The thing that will bring you undone is the inductive kickback from the mag chuck coils, you will need to put a decent sized diode across the chuck coil, to absorb the flyback voltage spike.

    If you are then reversing the current to demagnetize the chuck, the reversing switch needs to be between the flyback diode and the chuck, other wise the diode will short out your DC supply.

    So the demag procedure would be power off, reverse the connections, power on, wait 1 second, power off, reverse, power on... and so on.


    I hope that all makes some kind of sense...

    Ray

    PS .. I had a quick look on line, and seems like lots of demag circuits just feed the AC straight through?

  3. #3
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    Thanks Ray. Something like this could be adequate then M012 Power Control 110 - 240 V/AC, 1200 VA

    I wonder do they vary voltage or output an adjustable width square wave (is that what they call pulse width modulation)? I am not sure how electromagnets will work if it just outputs a square wave of varying width turning off every 1/50th of a second. It will not be a pure DC form.

    I did some reading over on PM forum and it mentioned there the problem of a voltage spike when turning off and decent diodes were recommended.
    Gold, the colour of choice for the discerning person.

  4. #4
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    Quote Originally Posted by .RC. View Post
    Thanks Ray. Something like this could be adequate then M012 Power Control 110 - 240 V/AC, 1200 VA

    I wonder do they vary voltage or output an adjustable width square wave (is that what they call pulse width modulation)? I am not sure how electromagnets will work if it just outputs a square wave of varying width turning off every 1/50th of a second. It will not be a pure DC form.

    I did some reading over on PM forum and it mentioned there the problem of a voltage spike when turning off and decent diodes were recommended.
    That's a chopper type speed controller, ( phase angle fired triac) should work fine, remember that the output waveform is still AC, that is a chopped sine wave, so, you still need to put a rectifier on the output.

    Back to the demag question, rethinking it, I guess that just connecting the 50 Hz chopped waveform to the coils would probably work, the parts will buzz at 50hz and vibrate their way off the chuck. That should be a bit of excitement for anyone nearby with a pacemaker.

    Ray

  5. #5
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    Thanks Ray. I already have one of the Kemo units here. I use it as a speed controller on my Biax. I find it very good.

    I see now the phase angle fired system is different from pulse width modulation https://en.wikipedia.org/wiki/Phase-fired_controllers https://en.wikipedia.org/wiki/Pulse-width_modulation

    I am currently cleaning up the rotary table portion. Unfortunately the brushes broke when I was disassembling it, but that is not a big deal, they are just bog standard type, 6mmX9mmx25mm carbon.
    Gold, the colour of choice for the discerning person.

  6. #6
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    A couple of pics of the clean up process which is mostly done.

    The table top seems to be very parallel with the ways underneath it.

    20151219_190247.jpg20151219_112705.jpg
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  7. #7
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    I have cleaned up the hydraulic motor. I was expecting a gear motor or at the outside a geroller/gerotor style motor.

    Instead what I have is an axial piston type motor. It works like this, inside the housing is a series of pistons running parallel with the output shaft. These are fed pressurised oil by a rotating valve in the top of the motor. These pistons act upon a swash plate, which is a plate at an angle to the output shaft. So the pistons push against the plate on an angle forcing it to rotate. Other hydraulic motors operating on the same principle have the swash plate running perpendicular with the output shaft and have the pistons on an angle, the motor will look like a boomerang.

    I have seen car air conditioning compressors use the same principle, and no doubt someone has made a working engine on the same principle.

    There is some corrosion on the swash plate no doubt from contaminated hydraulic oil and the big angular contact bearing on the end has some minor corrosion on the outer race. As it is a 7212 bearing so not real cheap, I will leave it as is for the time being.

    bare housing

    20151223_094339.jpg

    housing with pistons

    20151223_095112.jpg

    swash plate/output shaft assembly

    20151223_095622.jpg 20151223_095613.jpg
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  8. #8
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    I have picked up another tiny electromagnetic chuck. Not sure if this is commercially made or not. it has no markings at all on it.

    I have powered it with 5V from a computer PSU and it draws 3 amps and works very well for larger items.

    I was thinking of using it for TC grinder duty.

    20160129_152438.jpg 20160129_090504.jpg

    My big chuck has stalled trying to find a brush supplier, I need 6mm X 9mm X 25mm brushes. Waste of time going to local shops, they stock nothing other then small brushes. They could get them made though. I can do that myself if I can find a suitable larger brush to cut down.
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  9. #9
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    Quote Originally Posted by .RC. View Post
    ... I can do that myself if I can find a suitable larger brush to cut down.
    You might look for starter motor brushes. You may even be able to get two the size you need out of one of those. ANother advantage is that they have decent sized copper connections.
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  10. #10
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    Quote Originally Posted by .RC. View Post
    Thanks Ray. Something like this could be adequate then M012 Power Control 110 - 240 V/AC, 1200 VA


    I did some reading over on PM forum and it mentioned there the problem of a voltage spike when turning off and decent diodes were recommended.
    A simple way of reducing or even eliminating the inductive spike kick back voltage when feeding DC into an inductor is to place a capacitor across it. The capacitor reduces the speed at which the voltage drops when the supply is removed. This capacitor often will have a resistor in series with it to adsorb some of the energy.
    Best Regards:
    Baron J.

  11. #11
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    Quote Originally Posted by BaronJ View Post
    A simple way of reducing or even eliminating the inductive spike kick back voltage when feeding DC into an inductor is to place a capacitor across it. The capacitor reduces the speed at which the voltage drops when the supply is removed. This capacitor often will have a resistor in series with it to adsorb some of the energy.
    Yes, exactly right, we use them by the hundred, they are called snubbers. More for AC spike suppression than for DC, DC spike suppression can be MOV's Varistors, or sometimes just a flywheel diode.

    Ray

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    Well I ended up buying a US made controller that was on US ebay for a very reasonable price. It has not arrived yet, also it does not have variable controller on it but I might be able to get around that. It does have demag.

    I have drilled and tapped three holes on each side into the main housing where the drive chain runs. I am going to put some bolt studs in there so when I want to lock up the table I will be able to. Been looking into alternative ways to drive the table. Hydraulics is just horrendously expensive. Wondering about a 12V motor and worm gearbox. I need about 70rpm. Was looking at those scooter electric motors and make a worm drive for it. The motors seem to be very common and hence cheap.

    Another job for the horizontal borer. Just spin the table for each side.

    20160223_175441.jpg
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  13. #13
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    Quote Originally Posted by .RC. View Post
    Another job for the horizontal borer. Just spin the table for each side.

    20160223_175441.jpg
    Wonderful the jobs that become simple when you have one of those beasts in the shed, isn't it?

    I've got a big steering quadrant for a rudder to bore out as soon as it's extracted from the boat. New rudder stock is bigger than the current one. A lot of ways this could be done, but the simplest is on the HBM.

    PDW

  14. #14
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    The controller turned up today. It has been modified in that someone has installed a small rheostat and cut a track on the circuit board and soldered in some wires. Not yet sure if it does anything as turning the knob makes no difference to the output voltage which is only 50V, although the electromagnet really sticks anything to it with it plugged in. better then by permanent 6X12 magnetic chucks I have.

    There is nothing to these electromagnetic controllers, so not sure why they retail for such big $.

    20160226_170834.jpg 20160226_144532.jpg 20160226_144439.jpg
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  15. #15
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    Hi RC,

    Check the open circuit voltage, and then re-measure with the chuck connected, If you have a DC clamp meter measure the amps the supply delivers.

    You are right about being simple, it's just a glorified DC supply.

    Ray

    PS. I'm guessing the relay clicks on and off reversing the current when you select demag?

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