Magnetic Chuck Control Circuit Wanted

Bob Korves said:
One of my shortcomings at this is in properly understanding inductive versus resistance loads.
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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.

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?
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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.
 
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
 
JimDawson said:
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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Are you implying we are all old?
12kH should be hearable to 'good' ears. May just anoy the grand kids.
 
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.
Click to expand...
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...
 
MozamPete said:
Are you implying we are all old?
12kH should be hearable to 'good' ears. May just anoy the grand kids.
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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...
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That would work fine I think. Maybe a DPDT switch to switch between DC and AC?
 
MozamPete said:
Are you implying we are all old?
12kH should be hearable to 'good' ears. May just anoy the grand kids.
Click to expand...
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. :(
 
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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