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

 

[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...

 

[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.

 

[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.

 

[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.

 

[Bob Korves]

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.

 

[Bob Korves]

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?

 

[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

 

[JimDawson]

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

 

[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?