Magnetic Chuck Control Circuit Wanted

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

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.
 
Hi Bob: I wondered where you had gotten off to :D My circuit is just a jumping off point, it certainly could be customised. I'm not sure a GFI is really needed or even desirable based on what you said. Maybe not even a fuse either.
The transformer could be followed by a variac if adjustable mag force was desired. I used this little exercise to check out some freeware schematic editors and found one I like. Hopefully the circuit will be of use to somebody so it's a win/win.
Mark
 
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.
Click to expand...
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.
Click to expand...
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...
 
markba633csi said:
Hi Bob: I wondered where you had gotten off to :D My circuit is just a jumping off point, it certainly could be customised. I'm not sure a GFI is really needed or even desirable based on what you said. Maybe not even a fuse either.
The transformer could be followed by a variac if adjustable mag force was desired. I used this little exercise to check out some freeware schematic editors and found one I like. Hopefully the circuit will be of use to somebody so it's a win/win.
Mark
Click to expand...
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.
 
You must log in or register to reply here.
Back
Top