# Magnetic Chuck Control Circuit Wanted



## MozamPete

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


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## Bob Korves

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...


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## JimDawson

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.


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## MozamPete

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.


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## markba633csi

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.


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## Bob Korves

markba633csi said:


> 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.


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.


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## Bob Korves

MozamPete said:


> 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.


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?


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## markba633csi

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


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## JimDawson

PWM controller maybe?  Set the base frequency at around 12 kH and none of us would be able to hear it


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## Bob Korves

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?


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## JimDawson

Bob Korves said:


> One of my shortcomings at this is in properly understanding inductive versus resistance loads.



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.  



Bob Korves said:


> 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?



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.


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## MozamPete

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.


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## MozamPete

JimDawson said:


> PWM controller maybe?  Set the base frequency at around 12 kH and none of us would be able to hear it


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


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## Bob Korves

JimDawson said:


> 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.


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...


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## JimDawson

MozamPete said:


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



Hmmmmm, Yeah 



Bob Korves said:


> 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...



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


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## Bob Korves

MozamPete said:


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


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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## Bob Korves

JimDawson said:


> Maybe a DPDT switch to switch between DC and AC?


This switch: McMaster-Carr #7343K28
https://www.mcmaster.com/#7343k28/=1725zzw


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## MozamPete

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


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## JimDawson

Bob Korves said:


> This switch: McMaster-Carr #7343K28
> https://www.mcmaster.com/#7343k28/=1725zzw



More like this one I think,  DPDT On/Off/Mom
https://www.digikey.com/product-detail/en/nkk-switches/S339/360-1915-ND/1006993


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## Bob Korves

JimDawson said:


> More like this one I think,  DPDT On/Off/Mom
> https://www.digikey.com/product-detail/en/nkk-switches/S339/360-1915-ND/1006993


Yes, that one would better isolate the two functions.


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## Bob Korves

MozamPete said:


> 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


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...


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## Bob Korves

MozamPete said:


> 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


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...


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## Bob Korves

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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## Bob Korves

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.


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## JimDawson

Bob Korves said:


> 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.



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


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## markba633csi

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


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## Rick Berk

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.


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## Bob Korves

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.


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## Rick Berk

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.


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## markba633csi

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


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## MozamPete

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.


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## markba633csi

I revised my post, see above
MS


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## markba633csi

Pete what were the specs on the rectifier you fried?
MS


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## Rick Berk

markba633csi said:


> 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


That would be GREAT


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## markba633csi

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


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## markba633csi

ps: I deleted my schematic, we have been discussing other ideas and I'm going to do another version.
MS  4/10/17


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## MozamPete

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.


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## markba633csi

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.


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## JimDawson

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


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## MozamPete

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.


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## markba633csi

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.


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## whitmore

MozamPete said:


> I have an old 8" x 24", 110Vdc magnetic chuck...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)



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>


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## markba633csi

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.


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## whitmore

JimDawson said:


> More like this one I think,  DPDT On/Off/Mom
> https://www.digikey.com/product-detail/en/nkk-switches/S339/360-1915-ND/1006993


Caution:  neither of these is rated for 100VDC switching; that's a tough specification
to meet.   So, turn off the AC into the rectifier, not the DC to the
chuck.


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## markba633csi

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.


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## whitmore

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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## chips&more

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.


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## markba633csi

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


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## chips&more

markba633csi said:


> Hi Chips (Dave): So you haven't had any problems blowing up your bridge rectifier?
> Mark


No, nothing has melted!


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## chips&more

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.


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## markba633csi

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


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## markba633csi

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.
	

		
			
		

		
	



	

		
			
		

		
	
 drawn with gEDA


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## MozamPete

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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## hman

markba633csi said:


> 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.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> View attachment 231472


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.


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## markba633csi

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.


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## Bob Korves

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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## whitmore

Bob Korves said:


> 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 am concerned that a GFI might make it more likely to lose mag power to the chuck, which can be a really bad thing...



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.


----------



## markba633csi

Hi Bob: I wondered where you had gotten off to  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


----------



## Bob Korves

whitmore said:


> 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.


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.


whitmore said:


> 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.


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...


----------



## Bob Korves

markba633csi said:


> Hi Bob: I wondered where you had gotten off to  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


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.


----------



## markba633csi

The PTC (positive temperature coefficient) device is the same little gizmo that they used to control the degaussing coils around your color tv picture tube.  It tapers the current down as it heats up. John H. and I (and others) were thinking it would give a better demag effect than just a shot of AC because if your shot happened to stop right on the peak of the AC sinewave- well you get the idea. Of course if you have a variac in the circuit you could just flip the switch to demag and turn the voltage down.  Same effect.
The MOV (metal oxide varistor) is a voltage clamp like a double zener diode.  Protection for the bridge rectifier and toggle sw.  from inductive kick from the chuck coil.  Small 0.1uf cap also. Omit them if you dare LOL  Someone else (Pete?) zapped a bridge already.  But his setup was somewhat unusual.
Extra filtering caps- optional as far as I'm concerned.  Another part to dry out and fail someday. If you can get good results without it...Large high voltage electrolytics are always the weak link- and expensive. Use the smallest uf value you can.
Mark


----------



## markba633csi

Be careful out there everyone- don't do the 60 cycle shuffle unless you have a partner.  
MS


----------



## Bob Korves

I was studying this while you were writing, Mark.  OK, I understand using the variac and dialing it down to demag, bypassing the rectifier.  I like that idea, and have the stuff to do it.

I thought that a fairly large cap was needed to smooth the DC ripple created by the rectifier.  It sounds like I may be missing something here...  My electronic knowledge leaves a lot to be desired.


----------



## Rick Berk

YES,  I'm still here and learning, think I have located a variac, will know the details by Saturday.


----------



## whitmore

Bob Korves said:


> I thought that a fairly large cap was needed to smooth the DC ripple created by the rectifier.


A smoothing of the DC  ripple is also accomplished by use of an inductor, like... the
hulking electromagnet.   It is so effective at smoothing current, in fact, that
the switch and Variac windings are likely to get arc discharge damage, if the MOV is
omitted from the circuit.   A large capacitor would smooth low-frequency ripple,
but a small one prevents RF switching hash (so the Variac adjustment doesn't
put static onto every AM radio in the vicinity).   An early schematic has small
capacitors across every diode in the bridge rectifier (a sure sign that someone
is a ham, IMHO).


----------



## markba633csi

No, never keyed up, actually.  A little shortwave listening only.  One can only have so many hobbies LOL
I think whitmore and I are on the same frequency.   I didn't know if a big cap would be needed on the dc output so I didn't show one.  
If you do use a big filter cap be sure to hang it across the bridge rectifier output not the chuck terminals. You don't want to put AC on it.  AC is ok for the little cap and the MOV. 
MS


----------



## whitmore

markba633csi said:


> I didn't know if a big cap would be needed on the dc output so I didn't show one.
> If you do use a big filter cap...



The magnet, as inductor, takes the rectified AC and makes its RMS value
(about 120V) average across the load.   A big filter cap, takes the rectified AC
and potentially makes the peak (170V) appear, on average, across the load.  That
exceeds the expected applied DC voltage... by too much.   The filter capacitor
is not particularly useful, unless the AC ripple on the inductor causes
hum (in some way that disturbs the cut).   If that were to be an issue,
a stepdown transformer, or variac with a stop, would fix it, at some cost.

The switching of a 170VDC power source is hard to accomplish: most
switches cannot handle DC voltages that high.  The DigiKey offerings top out
at 125 VDC for toggle switches (but 277VAC if switching alternating current).


----------



## MozamPete

whitmore said:


> The magnet, as inductor, takes the rectified AC and makes its RMS value
> (about 120V) average across the load.   A big filter cap, takes the rectified AC
> and potentially makes the peak (170V) appear, on average, across the load.  That
> exceeds the expected applied DC voltage... by too much.



The inductance of the magnetic chuck will tend to smooth out and average the current flowing, not the voltage.  Without a capacitor the voltage waveform will still be the same raw rectified AC voltage and you will still be exposing the coils to the 170V peak, just only for a short period 120 times a second (instead of continually if you had smoothing capacitor).

This has been my main problem with trying to develop a transformerless design - and exasperated by the fact I'm starting with a 230V ac supply over here so have a potential 325V peak.


----------



## markba633csi

Here's the proper switch


----------



## whitmore

MozamPete said:


> The inductance of the magnetic chuck will tend to smooth out and average the current flowing, not the voltage.  Without a capacitor the voltage waveform will still be the same raw rectified AC voltage and you will still be exposing the coils to the 170V peak, just only for a short period 120 times a second (instead of continually if you had smoothing capacitor).



Yes, the current (which determines resistive heating) is limited by the 120V average
if there's no big filter capacitor.   Resistive heating, and overcurrent causing
saturation effects, are things to avoid.   Overvoltage, on the other hand... that's
no problem for any reasonable wiring insulation: all the wiring in motors and
such gets kilovolts applied for safety testing.    The peak DC voltage doesn't
threaten the electromagnet.   The average voltage, though, does.

A variant on a light dimmer (the kind of light dimmer that runs transformer-operated
lights) can effectively lower average voltage, by switching the AC input.   Another way
to feed less voltage from a too-high-voltage input, is to use a ballast (like,
the old-fashioned magnetic fluorescent ballasts) before the rectifier.
One might build a decent 100V/1A current source by connecting AC through
old-style magnetic fluorescent ballasts (one or more in parallel).

Dimmers are, alas, likely to fail in full-brightness mode, so fuses are not
optional if the dimmer is the only voltage limit.


----------



## Bob Korves

I already have, on hand, two variacs.

The first one shows either:
Input: 120V 50/60hz -- Output:  0-120v -- Amps 2.25  -- V.A. 270
or
Input: 120V 60hz -- Output 0-132V -- Amps 2.25 --  V.A. 297

The second one:
Input 120V 50/60hz
Output: 0-140V  KVA =1 (big mutha')

I also have a 450V, 470 μF  electrolytic cap, 105C

I also have a drum switch and a more than capable rectifier bridge, and a bleed resistor...

Whitmore and Mark, tell me where I might go next with this start...


----------



## markba633csi

Bob what are the specs again on your mag chuck?  (Input voltage and amps or watts)
MS


----------



## whitmore

Bob Korves said:


> I already have, on hand, two variacs.
> 
> The first one ...
> Input: 120V 60hz -- Output 0-132V -- Amps 2.25 --  V.A. 297
> 
> The second one:
> Input 120V 50/60hz
> Output: 0-140V  KVA =1 (big mutha')
> 
> I also have a 450V, 470 μF  electrolytic cap, 105C
> ...a drum switch and a more than capable rectifier bridge...



Well, MozamPete says the 8x24 incher (192 square inches) takes 130W,
so your 5x11 (55 square inches) ought to be comfortable with 38W.  
To start, get a fuse (1/2A, maybe less) and run the Variac output
through the fuse into the rectifier.   Check that the chuck terminals (there
should be two, and a ground) don't have a short to ground, then
connect the rectifier output to (1) a DC-voltmeter and (2) through a
DC-ammeter, into the chuck.   MOV at the chuck, if you have one.

Start at something safe (20V?) on the variac, and note current and voltage readings,
preferably on graph paper... take small steps (2 to 5V) and plot a few V-versus-I  points,
as you turn up the Variac.

If the chuck starts to buzz, it's probably magnetically saturating (and further
current increase is counterproductive).   Note the variac reading, and DC voltage.

If you get all the way to 120 on the Variac and the fuse hasn't blown, shut down.

Now, look at the graph: if it's just a straight line, no problem.   If it starts
straight, and curves UP to higher current at some voltage, note the onset
of that curve and subtract 10 percent.  The upcurve happens when the magnet
starts to saturate, you do NOT want that to happen in operation.  It's
equivalent to overloading a motor, which hums, heats, emits smoke...
The ripple current after the rectifier will upswing sharply at saturation.

Speaking of which, even if the fuse doesn't blow, if you smell something
from the magnetic chuck, shut down.   Obviously.

The idea here, is just to get a current and voltage operating point.
It would be great if you could put an AC ammeter in series with the DC 
ammeter: the AC ammeter will show a strong indication when magnetic 
saturation occurs, and the DC ammeter will tell you the operating current to stay well under.
The AC ammeter will also jump every time you adjust the Variac.


----------



## Bob Korves

markba633csi said:


> Bob what are the specs again on your mag chuck?  (Input voltage and amps or watts)
> MS


No tags.  Resistance is 86 ohms across the the plug on the electrical cable.  Everything is properly soldered in the chuck, so that number should be usable.  I am sure it was meant to be fed 120VDC.  I am guessing(!) that I would not want to run more than 120 watts (1 amp) to the chuck at 120VDC.  The variacs will give me some wiggle room there...


----------



## Bob Korves

whitmore said:


> Well, MozamPete says the 8x24 incher (192 square inches) takes 130W,
> so your 5x11 (55 square inches) ought to be comfortable with 38W.
> To start, get a fuse (1/2A, maybe less) and run the Variac output
> through the fuse into the rectifier.   Check that the chuck terminals (there
> should be two, and a ground) don't have a short to ground, then
> connect the rectifier output to (1) a DC-voltmeter and (2) through a
> DC-ammeter, into the chuck.   MOV at the chuck, if you have one.
> 
> Start at something safe (20V?) on the variac, and note current and voltage readings,
> preferably on graph paper... take small steps (2 to 5V) and plot a few V-versus-I  points,
> as you turn up the Variac.
> 
> If the chuck starts to buzz, it's probably magnetically saturating (and further
> current increase is counterproductive).   Note the variac reading, and DC voltage.
> 
> If you get all the way to 120 on the Variac and the fuse hasn't blown, shut down.
> 
> Now, look at the graph: if it's just a straight line, no problem.   If it starts
> straight, and curves UP to higher current at some voltage, note the onset
> of that curve and subtract 10 percent.  The upcurve happens when the magnet
> starts to saturate, you do NOT want that to happen in operation.  It's
> equivalent to overloading a motor, which hums, heats, emits smoke...
> The ripple current after the rectifier will upswing sharply at saturation.
> 
> Speaking of which, even if the fuse doesn't blow, if you smell something
> from the magnetic chuck, shut down.   Obviously.
> 
> The idea here, is just to get a current and voltage operating point.
> It would be great if you could put an AC ammeter in series with the DC
> ammeter: the AC ammeter will show a strong indication when magnetic
> saturation occurs, and the DC ammeter will tell you the operating current to stay well under.
> The AC ammeter will also jump every time you adjust the Variac.


Thanks, whitmore!  That sounds like a very good way to test the chuck for how much power it can accommodate before it saturates.  I will do that and report back, may take a while to find enough components to make a reasonably safe and worthwhile test run.  The chuck body has no connection to the power wires, already tested...


----------



## markba633csi

Sounds good.  You don't HAVE to have an isolated supply to run your chuck, but it is safer from a shock hazard standpoint.  Code would require it. But in your own
house you can decide.  Just make sure the chuck is well grounded whichever way you go. 
Cheers,
MS


----------



## Bob Korves

markba633csi said:


> Sounds good.  You don't HAVE to have an isolated supply to run your chuck, but it is safer from a shock hazard standpoint.  Code would require it. But in your own
> house you can decide.  Just make sure the chuck is well grounded whichever way you go.
> Cheers,
> MS


I plan to change the cord to a three wire one, and ground to the chuck casing.  The existing cord is at least 50 years old and is petrified anyway, but interestingly has no cracks or other issues, just very stiff.  To hell with code, I do it to standards that are considered safe by those in the know...  It is my tail that is on the line!  Thanks!


----------



## Bob Korves

whitmore said:


> Well, MozamPete says the 8x24 incher (192 square inches) takes 130W,
> so your 5x11 (55 square inches) ought to be comfortable with 38W.


6x18" chucks typically draw about 100-120 watts, maximum.


----------



## MozamPete

Bob Korves said:


> No tags.  Resistance is 86 ohms across the the plug on the electrical cable.  Everything is properly soldered in the chuck, so that number should be usable.  I am sure it was meant to be fed 120VDC.  I am guessing(!) that I would not want to run more than 120 watts (1 amp) to the chuck at 120VDC.  The variacs will give me some wiggle room there...



The chuck operating power should pretty much just be based on the DC resistance of the winding - in steady state operation on dc the inductance of the winding has no effect. So

I = V/R   and
power = V²/R

At 120Vdc and 86 ohms take would imply

Current = 1.4 Amps
Power = 167 Watts


For comparison my 8" x 24" chuck name plate is  110Vdc, 1.3A, 144W and measuring the dc resistance of the chuck I get 92 Ohms.


----------



## Bob Korves

MozamPete said:


> The chuck operating power should pretty much just be based on the DC resistance of the winding - in steady state operation on dc the inductance of the winding has no effect. So
> 
> I = V/R   and
> power = V²/R
> 
> At 120Vdc and 86 ohms take would imply
> 
> Current = 1.4 Amps
> Power = 167 Watts
> 
> 
> For comparison my 8" x 24" chuck name plate is  110Vdc, 1.3A, 144W and measuring the dc resistance of the chuck I get 92 Ohms.


Yes, and that sounds like too much power to me.  Perhaps the voltage needs to be lower.  The variac can do that easily.  I like whitmore's method of testing for saturation, and I will do that.


----------



## skydownbr

Hello to all.


Enviado do meu iPad usando Tapatalk


----------



## hman

Hello to you, and welcome!  Do you have a surface grinder?


----------



## markba633csi

Hello Skydown
MS


----------



## awright

whitmore said:


> Well, MozamPete says the 8x24 incher (192 square inches) takes 130W,
> so your 5x11 (55 square inches) ought to be comfortable with 38W.
> To start, get a fuse (1/2A, maybe less) and run the Variac output
> through the fuse into the rectifier.   Check that the chuck terminals (there
> should be two, and a ground) don't have a short to ground, then
> connect the rectifier output to (1) a DC-voltmeter and (2) through a
> DC-ammeter, into the chuck.   MOV at the chuck, if you have one.
> 
> Start at something safe (20V?) on the variac, and note current and voltage readings,
> preferably on graph paper... take small steps (2 to 5V) and plot a few V-versus-I  points,
> as you turn up the Variac.
> 
> If the chuck starts to buzz, it's probably magnetically saturating (and further
> current increase is counterproductive).   Note the variac reading, and DC voltage.
> 
> If you get all the way to 120 on the Variac and the fuse hasn't blown, shut down.
> 
> Now, look at the graph: if it's just a straight line, no problem.   If it starts
> straight, and curves UP to higher current at some voltage, note the onset
> of that curve and subtract 10 percent.  The upcurve happens when the magnet
> starts to saturate, you do NOT want that to happen in operation.  It's
> equivalent to overloading a motor, which hums, heats, emits smoke...
> The ripple current after the rectifier will upswing sharply at saturation.
> 
> Speaking of which, even if the fuse doesn't blow, if you smell something
> from the magnetic chuck, shut down.   Obviously.
> 
> The idea here, is just to get a current and voltage operating point.
> It would be great if you could put an AC ammeter in series with the DC
> ammeter: the AC ammeter will show a strong indication when magnetic
> saturation occurs, and the DC ammeter will tell you the operating current to stay well under.
> The AC ammeter will also jump every time you adjust the Variac.


----------



## awright

Whitmore, I'm not sure that you will get the desired indication of saturation in your graph if you are relying only on a DC ammeter.  Granted, at saturation the incremental inductance disappears (or is reduced immediately or gradually, depending upon the hysteresis curve of the magnetic core), but because the DC current is determined only by the applied voltage and the DC resistance of the wire in the coil, not by the inductance, I would think the DC I/V curve would be immune to magnetic saturation effects.  There would, however, be an indication on an AC ammeter looking at the ripple current if you are using an unfiltered, simple rectifier power supply.  Having a sensitive tong type AC ammeter on the coil wire would give a useful indication of saturation, assuming an unfiltered supply.  AC tong tools are relatively cheap these days.

You do mention the desirability of having an AC ammeter in the circuit and the fact that it will indicate saturation, but I think an AC ammeter, not the DC I/V plot, should used as the primary indicator of magnetic saturation.

However,  I think the thermal dissipation of the coil would be the primary limiting factor on permissible applied DC voltage, probably long before saturation in a properly designed chuck.  This is fairly difficult to evaluate non-destructively unless you have temperature sensor imbedded within the coil windings, something that you can only accomplish during winding of the coil.  There would be an upward bend in the DC I/V curve due to heating of the wire, but thermal damage might occur before you saw the effect.  One might get a useful indication of maximum permissible current by inserting a temperature sensor as deeply into the windings as possible (probably being only at the surface, depending upon the tidiness of the windings and the presence of any potting resin), and VERY GRADUALLY increasing the voltage/current while watching the temperature and feeling the surface of the chuck.   By gradually, I mean over many hours, as the temperature will be maximum in the middle of the windings and  it will take time for the surface to indicate possibly limiting temperature in the core.

Be aware that I am not an electrical engineer (or even a magnetic or thermal engineer - or even a professional machinist).


----------



## awright

Recognizing that all power dissipation in the core is thermal, I think earlier discussions in this thread of scaling permissible power input from the dimensions of similar chucks with known ratings (and preferably, identical vintage since insulating materials have evolved) is a reasonable way to estimate permissible power dissipation in a chuck of unknown rating.  Whether the scaling should be on surface area or volume is beyond me.  Starting with a known standard of nearly identical dimensions would be ideal.


----------



## whitmore

awright said:


> Whitmore, I'm not sure that you will get the desired indication of saturation in your graph if you are relying only on a DC ammeter. ... the desirability of having an AC ammeter in the circuit and the fact that it will indicate saturation, but I think an AC ammeter, not the DC I/V plot, should used as the primary indicator of magnetic saturation.


My concern was that AC ammeters are sometimes RMS, sometimes peak-and-correction, sometimes rectify-and-correction; it's usually unclear what exactly is being 
measured (a transformer-coupled oscilloscope would be ideal, though).   
Hopefully, all DC meters average over a few tenths of a second.

If the magnet saturates, the current rise time gets faster, so the peak current
happens sooner than when the full magnet inductance was present.
Because inductive current is phase lagged, that means more power transferred
to the load, and should cause a deviation on the I/V curve.   I think.   

Maybe a true-power meter on the AC input is a cleaner choice?


----------



## benmychree

Pardon, if this is not germane to the discussion, but I have a Brown & Sharpe Micromaster Surface grinder that came with a Neutrofier chuck control;  when it degaussing feature is in use, one can see the grinding particles on the chuck poles lean one way, and the other way in a diminishing strength as the cycle does its thing; this suggests to me that the cycle is using direct current and diminishing the voltage as each reversal cycle is gone through, until there is no residual magnetism present in the chuck or work piece, at least not a discernible amount.  From the noise it makes, it would seem that an electric motor drives a switch that does the current reversal.   I also had a Chinese control on a 12 X 36 Thompson grinder; it was solid state and was tiny compared to the Neutrofier .  It seemed to work the same way, by reversing DC input in a diminishing series of reversals.  Both worked excellently.


----------



## hman

That sure sounds like what a PTC would do in an AC circuit - reversals of diminishing strength - except that when using AC, the rate of reversal would be too fast to readily discern by watching the particles. 

I'm still waiting for the PTCs I ordered (slow boat from China), but as soon as I have them, I'll announce it here and offer them to anybody for just the cost of postage.


----------



## Bob Korves

benmychree said:


> Pardon, if this is not germane to the discussion, but I have a Brown & Sharpe Micromaster Surface grinder that came with a Neutrofier chuck control;  when it degaussing feature is in use, one can see the grinding particles on the chuck poles lean one way, and the other way in a diminishing strength as the cycle does its thing; this suggests to me that the cycle is using direct current and diminishing the voltage as each reversal cycle is gone through, until there is no residual magnetism present in the chuck or work piece, at least not a discernible amount.  From the noise it makes, it would seem that an electric motor drives a switch that does the current reversal.   I also had a Chinese control on a 12 X 36 Thompson grinder; it was solid state and was tiny compared to the Neutrofier .  It seemed to work the same way, by reversing DC input in a diminishing series of reversals.  Both worked excellently.


Hi, John, welcome to the thread!  John (benmychree) is who I got the chuck we are talking about from, and a B & S surface grinder to go along with it. 
Edit:  John, I will see you Saturday morning at the mill.


----------



## Dabbler

I hope this isn't too late for your projects, but here is the circuit from my walker chuck:


----------



## markba633csi

Thanks Dabbler, I'm sure there will be some interesting discussion and analysis around that schematic- on first glance it looks like a light dimmer circuit driving a bridge plus a few refinements...cool
Mark S.


----------



## Dabbler

exactly what it is, but a magnetic chuck needs almost exactly that...


----------



## whitmore

Dabbler, it's good to see a schematic!  

The use of TWO fuses is very odd.   And R12-R13 just
looks like a waste of energy, but might be there to save the switch contacts
from burn scars.   If those components are left in place, the AC for
release will be less effective (an MOV instead of the resistors and diode
would possibly be suitable).  The 3P3T switch does disconnect the DC,
which is a tough job.

R3A sets a fixed light-dimmer setting as is sensible.   Triac device variation would
be the reason not to use a fixed value, and "R3" is probably the manufacturer's
tweak, removed during final assembly.


----------



## Dabbler

R3 is used to set the low threshold of the magnets.  each magnetic chuck, even if they try to make them identical will have different characteristics in the low end.  It just sets a minimum bias by forcing the phase shift to be nonzero.  A good starting point is less than 10% of the pot, or about 33K.  Mine is 50K.

F1 is the normal fuse you'd expect.  F2 is there to prevent catastrophic failure if one of the diodes or the triac calls it quits.  better to blow a fuse than all the components.

R12 and R13 is there to bleed the massive energy stored in the magnet when the switch is turned off.  It is set up as a voltage divider to lessen the stress on the bleeder diode D5.  With this circuit, the energy will bleed off quickly, in about 1/10th of a second on my chuck.  If an operator is very aggressive it stops the extreme strain on the circuit if the stored energy is fed back into the circuit due to a fast transition from 'magnetized' and 'demagnetize'.  I think R12 helps smooth the waveform during the inrush...

A quick note about the release:  First it is reverse DC that causes the demagnetization, and second you need a much weaker field to demagnetize the part lest you remagnetize it with the opposite poles...  You can all ways give it a quick 'nudge' to lower the force to remove it from the chuck...

I hope this makes the circuit a little clearer.  This unit has been in operation since 1990 without failure, so I assume it is a good circuit.


----------



## markba633csi

I don't quite follow what they're doing with the triac gate circuit switching- is the triac in full on mode for release? Then what power level do they use for mag on?
 Why not just leave the triac setting the same for release as when mag on?  Wish I could reach through my laptop and put a scope probe on a couple points 
M
ps I see the two fuses would not make Bob K. happy! Also no isolation
Is the 500k pot an operator adjustment or an internal trimpot?


----------



## Dabbler

The Triac is phase switched - the RC (R3 + C2) circuit causes a phase delay in triggering Q1 using Q2 diac for edge transition.  By doing so, they change the available voltage potential (because of the Triac turning on later in the phase)  Because of the inductance of the magnet, the limiting the voltage will limit how much energy the magnet will be using to hold down the part.

On release, the triac is still energized in the same way as it was in holding mode.  The extra resistance in the demagnetize circuit is to lower the magnetic force a little to use less force in demagnetizing the part.  You use less energy to demagnetize.  If you used the same energy, it would just magnetize the part with opposite poles.

The pot is an operator control to change the strength of the mag chuck when holding small parts that you don't want distorted.  That is the beauty of an electromagnetic chuck.  You can choose the holding power.


That is about all the knowledge I have on the subject.  Please, if there's an electrical engineer on board, maybe you could explain further/better/more accurately...


----------



## markba633csi

I guess I should have been more specific in my question: I understand how the phase angle control of the triac is accomplished with the diac and r/c time constant. What I don't quite get is the physical switching as pertains to the demag mode. The triac is full on, but then the pickoff from the bridge is giving reverse polarity half-wave (or half amplitude) DC to the chuck at reduced current I think is what they're doing, and D5 is actually forward biased.  Personally I would have used tapering AC in some way for the demag,  but apparently this works well too.
Mark S.


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## Dabbler

I get what you mean now - sorry to have missed it. Yes it is half-wave DC.  I think you have the right of it - trying to reduce the power in the chuck during the release... There is an important omission on the diagram - the release position is momentary, spring return to 'off'.  The 'on' position will stay on.  From my current limited experience with the chuck, about a second on 'release' is more than sufficient to release a part.


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## markba633csi

I had proposed a circuit a few dozen postings back about one possible approach and here we have a circuit that's quite different.  Fascinating, as Mr. Spock would say.
More than one way to skin a chuck.  I may do yet another schematic with all the best features...
MS


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## MozamPete

My mag chuck has one figure on the name plate stamped '66' but I can no longer make out what the value is for. Now I'm thinking it may be a de-mag voltage rating. 110Vdc operating, 66Vdc reverse for demag. Would sort of tie up with what this circuit is doing using only a half wave rectifier for the demag function.

It is pretty much similar to the curcuit I was trying to develop - triac feeding a rectifier with some reduced voltage for demag. I was trying to changing the triac triggering for the reduced voltage but the full to half rectifier is a nice approach.


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## Dabbler

It seemed quiet elegant.  If you'd like I can take the cover off and post a picture...


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## markba633csi

Sure why not? I'd be curious to see if there's a pcb or if they wired it point to point.  Also the heatsinking.
Mark S.


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## Dabbler

Here are some pictures, at least you can get the idea...


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## markba633csi

Do I see a burned resistor over on the right near the orange caps?
MS


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## Dabbler

Good catch! It is the resistor that is used in the voltage divider circuit...  The unit is working fine, but I should check that it isn't 'open' and near its specified resistance....  hmmm... 

-- Thanks for noticing!


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## markba633csi

Look closer I think it's the 150 ohm 1/2W resistor in the triac circuit, R7-  the color code (or what's left of it) looks like brown green brown = 150
M
PS maybe it's not burned and it's just a shadow on the picture-


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## Dabbler

I'll still check it out before turning the machine on!  Frankly there's problems with all 18 Bijur flow control devices.  I may have to replace almost the entire lube system - the chuck is 2nd priority at the moment...


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## hman

OK ... I finally received my order of PTCs from China (thru Alibaba).  Tried one in my homemade magnetizer/demag, but initially with unsatisfactory results.  So I measured the "cold" resistance of the PTC - about 20Ω.  The motor stator I'm using has about 3Ω resistance, so the issue was one of impedance matching.  I wired four of the PTCs in parallel and tried again.  This did the trick.  Holding down the switch, I can feel the "buzz" start at a good power level, then dissipate in about ½ to 1 second.  It does a good job of demagnetizing things like screwdrivers.  If I want to magnetize, I just blip the switch very briefly.

I think these devices will work well to control the demag circuit of a magnetic chuck.  Just wire a set of paralleled PTCs in series between the demag switch and the coils.  I have quite a few of these left, and will be more than happy to send some to anybody who asks (until I run out).  Just measure the DC resistance of the coil and decide how many of the 20Ω PTCs it would take to approximately equal that resistance.  PM me with your mailing address and how many you'll need.

PS - Don't try to test your circuit immediately after soldering to the PTCs - let them cool for about ½ hour or so, or the residual soldering heat will have them in a high resistance state.  I'll let you guess how I discovered that little tidbit of info


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## markba633csi

Very nice John- so the parallel ptc group you have now is about 5 ohm cold.  If you had to order them again I'm guessing you would order a lower resistance value.
Come to think of it, did anyone post the cold resistance of their chuck?  Anyhow, it looks like the chuck and ptc ohms should be about equal. Good to know.
Mark S.
ps it might be good to point out that resistances divide in parallel = 4 X 20 ohm devices in parallel =
5 ohms.  2 X 20 ohm devices = 10 ohm and so on...


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## hman

markba633csi said:


> Very nice John- so the parallel ptc group you have now is about 5 ohm cold.  If you had to order them again I'm guessing you would order a lower resistance value.


That's quite an "if"   Would if I could
-but-
It took me a good deal of searching to even find _any_ suitable PTCs.  Nearly all of them have a very sharp cutoff, and are intended as thermal switches.  Finally went to Alibaba, where nearly all of the vendors wanted to sell in quantities like 5000 units.  I thought I was very fortunate to find the one I did ... minimum order quantity 50.  

Having used 4 for my mag/demag, I still have lots left to give away.


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## markba633csi

I like Alibaba (and Aliexpress) too, some good bargains there if you don't mind a bit of a wait


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