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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 have an old 8" x 24", 110Vdc magnetic chuck I picked up cheap at an auction site. Just the chuck as it didn't come with a controller. It is really a bit big for me - weighs about 130lbs - so it just got stuck away in a corner.

    I finally got around to having a play with it last night and rigged up a variable dc supply and powered it up and it appears to work OK - ramped the voltage up to 110Vdc and the current was in line with the name plate and it held a work piece nice and solid.

    Now if I'm actually going to use this I want to build a proper power supply/controller with a de-mag function (with the setup last night the work piece stayed stuck for about 20 second after the power was turned off), suitable snubber circuits to minimise any voltage spikes during switching, maybe variable hold power, etc.

    So, does anyone have a suitable circuit diagram for such a controller? I am an electrical engineer by training so could develop something that worked from scratch (although it's been a long time since I did any circuit design), but there is no need to reinvent the wheel if someone else already has a tried and proven design that works.

    The new controllers available on ebay seem a little expensive as I only have the equivalent of US$50 invested in this so far so spending another $300 plus on a controller isn't going to happen - if necessary I would live with the setup I rigged up last night for the minimal time I would probably use the chuck.

    Edit: Chuck is a Walker Pilana model 210 manufactured in the Czech Republic
    110Vdc, 1.3A, 144W, 200mm x 600mm, 60kg
     
    Last edited: Apr 4, 2017
  2. Bob Korves

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

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    I have an 5x11" electromagnetic chuck that I also want to get going, not so much for use on my surface grinder (I have a 6x18" permanent magnet chuck on it), but for holding difficult work on the milling machine, for holding scraping victims, and whatever else it might be useful for. The chuck came with my 6x18 surface grinder, so I have no additional investment in it.

    I am certainly not the brightest kid in the class when it comes to electronics, but I have had some help with what might work well. First, an isolation transformer, then a variac, then a rectifier circuit with an electrolytic capacitor (the one I bought is 470 mf) and bleed resistor, and this switch, McMaster-Carr #7343K28, to switch from dc for holding work to momentary ac for demag. My chuck should use about 80 watts at 115vdc.

    Since you are an electrical engineer, maybe you can help ME! And, yes, store bought mag chuck rectifiers are stupidly expensive, new or used...
     
  3. JimDawson

    JimDawson Global Moderator Staff Member Director

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    I'll be watching this thread, I need to build one also. :) I don't expect to get to it for a couple of months, right now I can't even find the grinder, it's buried in the shop somewhere.:rolleyes:
     
  4. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    Bob that was the basic idea of last nights setup, a SCR speed controller feeding into a four terminal rectifier and just adjusting the output to get 110Vdc.
    For the de-mag circuit I see some people use AC but the commercial controller appears to use an alternating polarity dc, and a ramping down of the voltage. I'm not really familiar with the advantages so will have to do some more research to determine what functions to include in a simple controller. But then I should be able to design something usable to provide the basic functions required.
     
  5. markba633csi

    markba633csi United States Active Member Active Member

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    Don't even need the rectifier if the controller is putting out pulsating DC already.. but you might need some kind of phase reverser relay/timer for the demag function.
    Mark S.
     
  6. Bob Korves

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

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    The rectifier is making dc from the 115v ac source current. The rectifier makes unfiltered dc and then a capacitor smooths the current to something near smooth dc.
     
  7. Bob Korves

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

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    OK, you are saying SCR, and I was talking variac. One of my shortcomings at this is in properly understanding inductive versus resistance loads. Which we are really creating here, and what would be the best choice for controlling the eventual dc voltage and current downstream?
     
  8. markba633csi

    markba633csi United States Active Member Active Member

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    No doubt a pure linear approach would be the smoothest and quietest, both electrically and acoustically, but with the attendant weight and cost.
    Phase-angle control is hummy and generates lots of emi (electromagnetic interference) but with big weight and cost savings. SCR controllers can drive inductive loads so I figured a magnetic chuck would be no problem.
    MS
     
  9. JimDawson

    JimDawson Global Moderator Staff Member Director

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    PWM controller maybe? Set the base frequency at around 12 kH and none of us would be able to hear it :grin:
     
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  10. Bob Korves

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

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    Well, my pragmatic approach is to use a variac, because I have two on hand that are each rated to easily handle the voltage and current. Will they work OK in this application?
     
  11. JimDawson

    JimDawson Global Moderator Staff Member Director

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    The coil in the magnet would present a pure resistive load to pure DC once the inrush was complete.....but it would look like (is) an inductor to pulsed DC.

    A variac would work fine, but you would want to limit the AC output voltage to 77 volts. 110 (VDC) * .707 = 77 (VAC) volts. If you crank it up to 110 AC output, the DC output voltage would be 155 VDC.
     
  12. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    If you have a variac that would would work fine for reducing the voltage to the correct operating level. SCR controllers are just cheaper - less than $5 - and was what I had on hand. We are only talking a very lite load here (140w) so the interference/harmonics generated are small. And feeding into an inductive load will also naturally tend to smooth the current on the dc side.
    One possible advantage of the SCR (or PWM) controller is I can probably rig a circuit to change the firing angle and use the same controller to do the ramping down of the voltage during the de-mag cycle - to do the equivalent with a variac would require to motorise it.
    So realistically using a variac could set the 110v normal operating voltage but some other controller would probably still be necessary to do a ramped de-mag if you went that way.
    Well that's my initial thoughts anyway.
    I have found a Walker Smart B O&M manual which while it doesn't give circuits or details of how it does it, does give a good description of the functions it provides and its de-mag philosophy as something to try and imitate.
    IMG_3530.PNG
     
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  13. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    Are you implying we are all old?
    12kH should be hearable to 'good' ears. May just anoy the grand kids.
     
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  14. Bob Korves

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

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    No pulsed DC, there is a capacitor to smooth the DC. I would probably be using AC for demag, while turning down the voltage. I guess... Any advice is welcome... My plan was to rig up something easy using mostly what I have on hand, and the project is not worth a big investment in time or money, just trying to mostly use what I have on hand to make a multi app mag chuck...
     
  15. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Hmmmmm, Yeah ;)

    That would work fine I think. Maybe a DPDT switch to switch between DC and AC?
     
  16. Bob Korves

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

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    WHAT DID YOU SAY??? 66 years old, and have the hearing aids to prove it. Playing rock and roll and blues in bands, shooting all kinds of guns, working in big truck and heavy equipment shops (2 stroke Detroit diesels getting tuned up without their intake silencers at high idle were about the worst), Arcair cutters, BFH's, old open cockpit taildragger airplanes with straight pipes, and air hammers. To name just a few... It is a lot quieter these days. :(
     
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  17. Bob Korves

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

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  18. MozamPete

    MozamPete South Africa Active User H-M Supporter-Premium

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    BOB, THAT SHOULD WORK FINE ;)
    I'm thinking of just switching between different control modes/driver circuits for a single SCR/PWM voltage controller:
    1. Fixed 110Vdc output for normal/full power
    2. The normal pot for variable power (not sure when this is normally used, I'm guessing for part hold during setup of the workpiece or for reduced heating if the workpiece geometry is such the enough hold can be obtained at reduced voltage)
    3. Off
    4. De-mag ramping/alternating polarity
     
  19. JimDawson

    JimDawson Global Moderator Staff Member Director

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  20. Bob Korves

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

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  21. Bob Korves

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

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    I also have a drum switch I could use to manually reverse the DC polarity, back and forth, while I ramp the input voltage down manually with the variac. That was my first idea. Apparently there is some disagreement on whether AC or DC is better (or perhaps more expedient) for demag. I wish I understood demag better...
     
  22. Bob Korves

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

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    Peter, I must admit to not understanding well the circuitry for SCR and PWM controllers, so I would need a complete circuit diagram and an understanding of it before I would go in that direction. Another thing I am trying to achieve here is a variable magnetism force to better accommodate both heavy and solid as well as thin and flexible parts for both machining and hand scraping, and perhaps other currently unanticipated work holding chores as well.

    Edit: This is intended to use a chuck and some other stuff I have on hand to achieve something that works for occasional odd jobs. This will not be 40 hour a week tooling...
     
  23. Bob Korves

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

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    The biggest advantage electromagnetic chucks have over fixed magnet chucks is the ability to control the strength of the magnetic force generated. There are also disadvantages, not the least of which is the possibility of throwing a part off a machine due to losing electric current flow to the chuck. In my mind, any controller should keep both of those tenets firmly in mind.
     
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  24. Bob Korves

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

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    This is a great discussion and learning opportunity for me, and hopefully for others as well... Thanks, all, it will be interesting to see what ideas we can come up with.
     
  25. JimDawson

    JimDawson Global Moderator Staff Member Director

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    I hadn't thought about that :eek: Good point. I'll be designing mine with a UPS in the circuit.
     
  26. markba633csi

    markba633csi United States Active Member Active Member

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    Good thread, I can't even tell if we are off topic or not! LOL
     
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  27. Rick Berk

    Rick Berk United States Active User Active Member

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    I also have a mag chuck with no power supply, I was given 10 years ago, I have a monster power supply that I can use but building a dedicated one would be great. Looking forward to a simple diagram to build that I can understand with no circuit board. Thanks.
     
  28. Bob Korves

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

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    I have two factory built controllers that came with my surface grinder. One is a very old 460v unit with two thyratron vacuum tubes, the other is not quite so old and has a multi plate selenium rectifier and a drum switch. I think those are pretty worthless for my needs. I am not above scavenging anything useful from them, but about all I see that might be useful to me is the drum switch.
     
  29. Rick Berk

    Rick Berk United States Active User Active Member

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    Mine is a LW CHUCK Co TOLEDO, OHIO. it is a 6X18 and at least 100 lbs, it is marked 110VDC at 1.0 amps. I researched the internet and found nothing but found other controller in the 800-1000 dollar range, wonder why I have never needed it that bad. I would like to make my input 120VAC since it is available in the garage everywhere, no 460 here or any 3 phase.
     
  30. markba633csi

    markba633csi United States Active Member Active Member

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    Rick just look for an isolation transformer at about 300 watts and a bridge rectifier rated at about 400 volts at 10 amps. A couple small high voltage capacitors to protect the bridge from spikes. I can post a diagram if you need one.
    Shouldn't cost more than a couple hundred, maybe less. The chuck should have a good solid ground.
    If you need to adjust the magnetic force you would need to add a variac to the circuit (more cost).
    Mark S.
    ps a GFI would be a good idea too
     
    Last edited: Apr 8, 2017
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