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Magnetic Chuck Control Circuit Wanted

Discussion in 'ELECTRICAL ISSUES - POWER YOUR MACHINES & SHOP' started by MozamPete, Apr 4, 2017.

  1. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    I was playing last night with mine and managed to fry my rectifier bridge when switching it off. Not a major problem as I have replacements on hand, but think I need to do some more research on inductive loads and improve the snubber circuit to protect the electronics.
     
  2. markba633csi

    markba633csi United States Active Member Active Member

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    I revised my post, see above
    MS
     
  3. markba633csi

    markba633csi United States Active Member Active Member

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    Pete what were the specs on the rectifier you fried?
    MS
     
  4. Rick Berk

    Rick Berk United States Active User Active Member

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    That would be GREAT
     
  5. markba633csi

    markba633csi United States Active Member Active Member

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    Ok give me a couple hours, I'll open my gimp program and whip up something.
    MS
     
  6. markba633csi

    markba633csi United States Active Member Active Member

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    ps: I deleted my schematic, we have been discussing other ideas and I'm going to do another version.
    MS 4/10/17
     
    Last edited: Apr 13, 2017
  7. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    Mark,
    I think you would need some sort of snubber circuit downstream of the switch or you will be getting arcing and voltage spikes when switching. An RC circuit or a Voltage Dependant Resister to provide an alternative path for the back EMF driven current on switching.
    I would also add a large smoothing capacitor on the D.C. side - any AC ripple into the chuck is going to cause some magnetic hysteresis which will lead to additional heating. Not a consern for a short demag cycle, but may impact tolerances if using it on a surface grinder and the part is there for a few minutes. One of my goals is for the D.C. Output to be as smooth as possible for this reason.

    My rectifier bridge was a KBPC5010. Good for 1000v and 50A so I thought it was overkilll and I have about 20 of them in a draw so they were on hand. And they were only a dollar each from memory. May of just been a bad one, or may of got over 1000v switching spike.

    I was thinking of actually measuring the inductance of my chuck as in reality I'm not sure how big an inductor it actually is - I'm just assuming very big.
     
  8. markba633csi

    markba633csi United States Active Member Active Member

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    Pete: I agree about the need for a snubber. When you mentioned hysteresis, are you concerned about the part heating up or the chuck (or both)? Hadn't considered that.
    The large cap would have to be in the circuit for DC mode only of course. It may have an effect on the waveform in AC mode though- here's where we start getting into second and third-order effects. Good old low efficiency linear. It would be good to know how big a kick the chuck makes when you cut the juice. I'm thinking a voltage clamping device would be better than an RC network.
    Rick: If you need help picking out a variac, I saw several on Ebay last night for around 30-40$ incl. shipping. I can send you some links while Pete and me fine-tune the circuit. You would be building this thing in a box of some kind so you would want a "bare" variac not one that's already in an enclosure- saves money and allows you to see that the windings are ok. Variacs often get abused/overloaded/shorted and develop a burned spot on the wire-
    I also saw a few iso transformers in the same price range.
    Mark S.
     
  9. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Mark, wouldn't those caps in your drawing introduce ripple into the DC?
     
  10. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    Mark, my added complication is im trying to use an scr on the ac side for the voltage control, and I'm chopping 230v ac to start with. To get down to an average 110Vdc D.C. I'm left with a very peaky waveform. Then when I include a capacitator to smooth it out it starts impacting the firing circuit.
    Think I may need to step down with a transformer to closer to the voltage I need so I can tweak it with the scr and get a larger firing time - or change to a variac.

    One of the disadvantages I read about electromag chucks was the heating and that causing expansion and difficulties in holding tolerances - have no idea how much of a problem it is in the real world, but thought any significant ac ripple on the dc may agrivate the problem.
     
  11. markba633csi

    markba633csi United States Active Member Active Member

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    Jim: those caps are pretty small and wouldn't have a large impact on the waveform. Actually as Pete mentioned, it might be better to have a voltage
    suppressor (MOV) right across the chuck terminals- you could dispense with the 4 diode caps most likely. They won't protect the switch anyway.
    Pete was also concerned about AC residue in the DC output causing heating- my feeling is the chuck is going to heat up anyway even if you fed it pure DC, so
    I thought the small amount of AC harmonics wouldn't make much difference. I've never used an electromagnetic chuck so I'm kind of making educated guesses here
    about the proper way to power it. Main thing would be to make sure the rectifier and switch don't get clobbered by the chuck voltage spikes.
    Pete: I hear you about the SCR situation, that's a whole 'nuther level of complexity. Rick (I think) wants the minimum parts count that's reliable.
    Rick: There should be a suppressor on the chuck terminals. Metal Oxide Varistor. I have a couple, I could send you one. They are about the size of a quarter with two wires coming out. It will limit the voltage spike to about 180 volts. Part number V130L20. Amazon may have them too.
    Mark S.
     
  12. whitmore

    whitmore United States Active Member Active Member

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    I like the motor-control idea (also consider transformer-capable dimmers,
    which also use a triac with inductive-switching capability) for current control.

    The demagnetization is best accomplished with an AC excitation that diminishes
    with time; a PTC thermistor, such as is used for motor starting in refrigerators,
    is an elegant way to accomplish this. These are easily available as repair parts,
    with several seconds time delay when given ~ 1A .

    <http://www.ebay.com/itm/3PIN-Terminals-Refrigerator-PTC-Starter-Relay-12-Ohm-Resistance/172347087382>

    The old (Klixon) data sheets indicate about 1:5 cold/hot resistance ratio for these items
    <https://web-beta.archive.org/web/20070221224341/http://www.sensata.com:80/files/8ea.pdf>
     
  13. markba633csi

    markba633csi United States Active Member Active Member

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    Good idea Whit- I'm going to have to edit the schematic for sure now. Not sure if a variac would even be needed if you did that instead... just throw the switch and let it auto-demag.
    You following this Rick? I would hold off buying any parts just yet, we are still pushing ideas around.
    Mark S.
     
    Last edited: Apr 9, 2017
  14. whitmore

    whitmore United States Active Member Active Member

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  15. markba633csi

    markba633csi United States Active Member Active Member

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    Actually I was going to insert the PTC in one of the ac lines from the bridge to the switch. So, switch up: pulsating 120 Hz DC out. Switch down: AC diminishing (demag) out. Yes you would need a pretty beefy DPDT switch. PTC elements have a finite lifespan too.
    Variac optional if one wanted a means of adjusting the mag force. Can't see a need for it tho. Seems like either full on or off would be fine.
    Mark S.
     
    Last edited: Apr 9, 2017
  16. whitmore

    whitmore United States Active Member Active Member

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    Maybe a multitap transformer can be gleaned from an old stereo; my antique Pioneer has +/-44VDC derived from a power transformer, with low-voltage windings that could be connected in series.
    Line isolation is good practice, but a GFI module could be the other safe option.

    I'm thinking the high price of electronics for these chucks is a figment of the
    market, not a realistic cost for the modern bill of materials.
     
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  17. chips&more

    chips&more United States Active User Active Member

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    I built a power supply for my mag chuck a few years ago now. I kept it simple. Full on for the mag, not variable and the AC was on a momentary switch. Have not needed an adjustable force of magnetism. And the little shot of straight AC is all the chuck needs to let go. I was concerned at first about my design and application. So I was feeling the chuck for heating and put a scope on the switch looking for spikes. For my set-up everything is happy and works great…Dave.

    PS: my power supply has an isolation transformer.
     
    Last edited: Apr 10, 2017
  18. markba633csi

    markba633csi United States Active Member Active Member

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    Hi Chips (Dave): So you haven't had any problems blowing up your bridge rectifier?
    Mark
     
  19. chips&more

    chips&more United States Active User Active Member

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    No, nothing has melted!
     
  20. chips&more

    chips&more United States Active User Active Member

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    Sorry, I do not have any pics of the power supply I made. And it’s been too long to recall exactly what components I used. But here is a pic of the switches and face plate that I made. The holes were already in the machine base so I had to work around them…Dave.
    I do remember at first having a current limiting resistor in the circuit. But that did not give me ALL the magnetism that I could achieve. So I took it out. I just made sure I have a carefully selected circuit breaker.
    magchuck.JPG
     
    Last edited: Apr 10, 2017
  21. markba633csi

    markba633csi United States Active Member Active Member

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    Dave: I'm going to do another schematic; I'll try to re-create what you did and maybe add some protection for the rectifier as well. Can't hurt.
    I hesitate to recommend going without an iso xfmr, although I probably would personally and just use a GFI like whitmore mentioned. Maybe I'll do two versions.
    Look for it sometime this week... I have taxes to do and other stuff- I'll try to squeeze it in
    MS
     
  22. markba633csi

    markba633csi United States Active Member Active Member

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    Here's the schematic I promised- Couple notes:
    You could probably use a GFI instead of the isolation transformer, unless the mag chuck has too much leakage current to ground.
    If you have a GFI laying around I would certainly give it a try; save some cost and weight.
    The bridge rectifier will need a heatsink of some kind. Experiment.
    John Herrmann was going to order some PTC devices for the demag function (gives a tapering-off AC to the chuck) from Alibaba- I asked him to keep us posted. Dave (chips&more) above reports that he's just using a momentary shot of AC and it works for him , so consider the PTC device optional. The toggle switch could have one side momentary if you wanted.
    The MOV varistor and the cap protect the switch and the bridge rectifier, Part # would be something like V130L20 or similar. Check Ebay and Amazon for those.
    Mark S. MAGsup2xa.jpeg drawn with gEDA
     
    Last edited: Apr 16, 2017
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  23. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    Mark,
    You will still be getting a 170v peak on the output though (120 x sqrt(2)).
    Should be OK but not ideal for a 110Vdc device. Adding a variac would solve this.

    Note: you can't normally just replace the transformer with a variac to achieve this as most (all?) variac are autotransformers so don't provide the electrical isolation.
     
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  24. hman

    hman Active User H-M Supporter-Premium

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    I have a lot of 50 on order, but they'll be on a "slow boat from China." Took a chance they's be the correct type of device. The price was right (about 20¢ each)! They're the kind of PTC that used to be used for automatic degaussing coils for CRT type color TVs, so they should be OK for 110 volts and several amps.

    Once I've received them, I'll give 'em a test with a benchtop magnetizer/demag I cobbled up from a motor stator. Assuming they don't fry themselves, I'll post again and offer them - for just the cost of postage.
     
  25. markba633csi

    markba633csi United States Active Member Active Member

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    Yes Pete you are correct about peak volts; I'm assuming the chuck wants average dc power so it may be ok as is, but if not you would be looking at either isolation and a variac, or a step down transformer, or a big dropping resistor. I don't own a mag chuck myself so I can't test. I guess the simple thing to do would be to power the chuck with just a bridge rectifier and see how hot the chuck gets, before you go building the whole circuit. If the average current is about 1 amp you should be golden.
    Dave (chips&more) doesn't remember what he did on his circuit- filtering or not, etc. Maybe if we pester him enough he'll open his up and redraw it? He did use isolation he said.
    MS
    ps I assume the chucks are built to use the simplest possible supplies- ? Just rectified line current
    but I don't know for sure.
     
  26. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    Don't know what happened, but somehow I stopped getting messages for posts to this thread, maybe somewhere around post #30. Still shows that I am following the thread. I missed a lot of good stuff, guys!

    As I understand things, many pro grinder hands like electromagnetic chucks over permanent magnet chucks specifically because they can control the power of the the magnetic flux to get only what they need. Thin work wants to warp on the chuck if clamped too hard. Any parts are more likely to be pulled down to positions that are not parallel with the machine axes when clamped too hard. Experienced grinder hands know how much magnetic force is needed to hold down the work, and do not use more than enough to just hold parts down when chasing tenths -- as I understand things. I am certainly not experienced enough to be able to even know what is necessary to hold any particular part on a given chuck against a given cut, and at this point just want a good, safe hold on the part. If I cannot move the part by hand, I am happy. But that might change. I also plan to use my chuck for scraping in parts, bench work, and perhaps on the milling machine, which can both involve thin parts as well.

    I must confess to not knowing what a PTC device is, or a MOV varistor, and what they do for us. Also note that my old school mag chuck has a two prong 115V wall plug on the petrified two wire cable that I have not changed out yet. It plugged into an old selenium rectifier with a two wire receptacle, this is a commercial rig. Were they tempting fire, or being smart? I definitely do not show continuity between the mag chuck exterior and the two power wires. My current plan definitely is to use a variac for the input AC and a 450V 470 mf cap to smooth the current between rectifier and chuck, at least until I learn otherwise. I am trainable, however.

    I am concerned that a GFI might make it more likely to lose mag power to the chuck, which can be a really bad thing, throwing parts off a grinder is a bad idea to be carefully avoided. Beyond that, the part moving may result in a major crash, exploding the wheel and damaging the grinder, not to mention the health of the operator. This is pretty serious business, not to be taken lightly.

    Educate me, please...
     
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  27. whitmore

    whitmore United States Active Member Active Member

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    The insulated windings in the chuck are isolation, just like a transformer, so the 'need' for
    an isolation transformer in addition is a kind of belt-and-suspenders thing. For
    real use, a GFI will protect against problems in an unknown-condition
    piece of used equipment, but as you say, it would also drop the chuck
    power. If it were to sound an alarm, or drop out a motor-run interlock
    relay at the same time, it'd stop the machine as fast as the chuck demagnetizes...
    and most big equipment has provision for this kind of interlock.

    A metal-case mag chuck with a two-wire AC power cord is pretty definitely out
    of compliance with modern safety requirements. Selenium rectifier? Fifty
    to seventy years out of date.
     
  28. markba633csi

    markba633csi United States Active Member Active Member

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    Hi Bob: I wondered where you had gotten off to :D My circuit is just a jumping off point, it certainly could be customised. I'm not sure a GFI is really needed or even desirable based on what you said. Maybe not even a fuse either.
    The transformer could be followed by a variac if adjustable mag force was desired. I used this little exercise to check out some freeware schematic editors and found one I like. Hopefully the circuit will be of use to somebody so it's a win/win.
    Mark
     
  29. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    In my home shop, I would be happy with a red warning light that showed a ground fault but did not kill the circuit, and I am not likely to see a motor-run interlock in my home shop.
    Yes, I figured that was obsolete and unacceptable wiring. It was all mainstream commercial equipment, however, not cobbled together. Still has the Rockford brand labeling. I have removed the selenium rectifier and the drum switch from the case so I might use the case for whatever I build. My understanding is that selenium rectifiers go bad over time, and mine has had well more than enough time. Scrap metal...
     
  30. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    Mark, your circuit is very close to what I originally had in mind. I just don't understand the PTC device or the MOV varistor and small capacitor. Oh, and I would add a variac, and I have two candidates on hand. And a smoothing capacitor.
     

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