electromagnetic chuck question

The chuck runs on DC you have AC coming out the wall. I've had good luck with Walker company for power supplies.

You could make your own too, the rating for the chuck voltage is usually on the data plate of the magnet. It is nice to be able to demagnetize the part by feeding ac into the magnet and then gradually decrease the voltage, or on some power units they reverse the polarity of the dc as they slowly ramp down the voltage. in an emergency, a variac and a bridge diode block will work.

Make sure the chuck is good before you order a power supply. Hook it to a 12 v car battery and see if it magnetizes the chuck and doesn't draw too much current

John

Well ... the one for my Sanford grinder runs (poorly) on AC, and very nicely on DC rectified and filtered from the power line.

The original circuit in the base of the grinder was like this:

DEMAG Switch SPDT +------------------------------------o | OFF /______o-----------+ | resistor selenium \ |

120 VAC o----+-----WWWWWWW----------|>|-----+-----o 3 Mag 20 Ohms rectifier | HOLD 3 Chuck 10 Watts ___ 3 Coil ___ 8 uF 250 VDC Electo 3 | -lytic | Neutral o-----------------------------------+-----o-------------------+

The capacitor was dead, and I did not trust the selenium rectifier so I redesined it -- using a silicon bridge rectifier, and a 2000 uF

250 VDC capacitor. I replaced the SPDT switch with a DPDT switch, so I switched both sides of the coil so the mag chuck was fully disconnected when not turned on.

To hold, you switch to "HOLD".

To demagnetize (after holding) you switch to "DEMAG"

Then you switch to "OFF".

It does not do a complete job of demagnetizing the workpiece, but enough so that it is easy to pull off the chuck. If you want better demagnetizing, keep the switch on DEMAG while you slowly move the workpiece away from the chuck, and only switch it fully off when you are about six inches away.

I knew that my mag chuck could take line voltage and rectified line voltage by looking at the circuit in the base of the grinder. I just re-designed it to be a better system using more modern components. (I kept the wirewound resistor, and (of course) the mag chuck. :-)

I actually just finished yesterday replacing it with a Permanent Magnet chuck, because the manual for the grinder warned (quite reasonably) to *not* use coolant with the electro-magnetic chuck. If you wanted to do that, you would need an isolating transformer. The PM chuck is also just barely smaller than the travel of the wheel in both axes, so it is a better fit than the electromagnetic chuck.

Be warned -- if you use the circuit shown above, *don't* use coolant.

You will need to make sure that the chuck is intended to run on

117 VAC in degauss (DEMAG) mode, and 170 VDC in "HOLD" mode. If not, you may need to change the voltage -- either step it down for a lower voltage one (in which case you have an isolation transformer anyway), or step it up for a higher voltage one. The same applies there, except that you will need to change the voltage ratings on the rectifier diode or bridge, and on the capacitor.

The resistor is there only to keep the surge current to a minimum when the system is plugged in -- or when the switch is set to "HOLD" if you re-wire it so the switch is before the rectifier, which would be a good idea.

Good Luck, DoN.

If I understand your circuit correctly, there is current flowing through the resistor only during the half cycle that the rectifier is conducting. I'm guessing that with the original circuit, half cycle conduction, resistor, and 8uF capacitor, the voltage at the chuck was probably closer to 120VDC than 170VDC. Without knowing the resistance of the mag chuck I'm only guessing but a 20 ohm 10 Watt resistor could drop up to 20V for 50% duty cycle giving a 150V peak and the 8uF capacitor most likely won't hold this voltage very well during the reverse half cycle. Just guessing I'd figure the resistor, diode, and capacitor combination probably supplied close to a

120VDC equivalent.

I did a project one time making a pick and place system that palletized metal parts. I used a McMaster Carr electro magnet and when we switched off the current, the part stayed stuck to the magnet. With a little research, we found out a momentary reverse current would release the parts. I ended up using a relay wired to reverse the power to the magnet, there was a capacitor that charged and caused a momentary reverse current through the magnet. When the relay was on, forward current would flow through the electro-magnet, when off, enough reverse current would flow to charge the capacitor, releasing the part.

RogerN

The chuck is marked 115VDC 1.8 AMP. so a variac and full wave bridge with cap should work. I'll need to make a panel up with switches and relays to switch in AC power. although with the switch after the diode and cap no relay would be needed. I kind of like the power switch to be the first thing in the circut, but the above way does simplify things.

How do the "other" guys do it. The factory B & S control panel has what looks like a 1watt pot to contol the power.

see ebay Item number: 130270640953

would like to see a schematic for one of these. Thank You, Randy

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Hey guys,

Any I've seen used a 20 Amp DPDT Center Off non-shorting spring-loaded-one-side, with a full wave bridge and an in-line cap. Switch markings as ON-OFF-RELEASE, with the RELEASE being the spring-loaded side of the DPDT. Not sure why they would require 20 Amp, other than to be physically more robust and therefore a bit longer lasting.

I do believe that I have seen, but never used, some VERY old models that were just AC, some of them without even a switch. Jes plugger in! If they are AC, they won't really want to run DC, nor vice-versa I would think.

Take care.

Brian Lawson, Bothwell, Ontario

20A may have been the AC rating. Or a physically larger switch may have been necessary to get enough separation in the contacts to control arcing. With AC you get 120 chances per second to quench any arcing that occurs on opening the switch. Not so with DC. Switching DC that's supplying a large inductive load is very hard duty for a switch.

Yes -- but very little current once the capacitor is charged. Its primary function is to keep the initial charge surge from frying the selenium rectifier, which were rather more sensitive to excess current even for short times than the Silicon ones. I kept it once I saw how much of a spark plugging in the grinder caused with the full wave bridge and the 2000 uF capacitor. I figured that would also shorted the life of the switch if I didn't keep the resistor to lower the surge current.

I think that it was closer to the 170 VAC. The chuck never felt warm, even after a long time holding a workpiece (unless I was grinding too heavily, which was uncommon.)

O.K. Time to check the resistance of the chuck. Back in a few minutes ... how about 36K? I don't think that there will be much ripple, or drop in the resistor once the cap is charged.

That would be 4.7 mA at 170VDC -- or a bit less than 10mV drop across the resistor. If it were drawing enough to drop half of the voltage across the resistor, we would need a 360W resistor or so. :-)

But for the short cap charging period, even with the cap bumped from 8 uF to 2000 uF, the high current is short enough so the average power in the resistor is well within specs.

O.K. With a selected capacitor value. Too little would not demagnetize it enough, too much would re-magnetize it in reverse, before the stock could start to fall. :-)

Enjoy, DoN.

Wow, I would have never guessed the chuck resistance to be that high. Another poster said his chuck was rated for 115VDC at 1.8A. His chuck is over 200W and your is 0.8W. I was guessing your chuck to average maybe 0.7A or so. If I'm calculating correctly, you have less than 1 mW power dissipation in the 10W resistor, they could have used a 1/2W resistor and still been 500X oversized.

The design you made sounds more like a DC power supply. The half wave rectifier and 8uF capacitor sounded like something that was suppose to deliver less than the peak voltage. Thinking about it a little bit here, are you sure that wasn't 0.36K Ohms?

RogerN

Except during the time of charging the cap. I guess that if you turned it on and off frequently, you might overheat a 2W carbon resistor.

I'm sure -- but what I am *not* sure of is whether there are problems in the existing winding. It may have been intended to draw more in the past. I've got to try re-flowing the solder on the connector and measure again. It is still apart form my looking inside it when trying to verify the observed resistance.

Enjoy, DoN.

Got the chuck today, had to take the little box apart. hooked up as follows:

120v (hot) to fuse than switch SPST then full wave bridge 120v (n) to full wave bridge

positive from bridge to DPDT switch top contact negitive from bridge to DPDT switch top contact

common (center) contacts DPDT switch to chuck

DPDT switch top contact left to bottom contact right (AKA diagonal jumper)

DPDT switch top contact right to pushbutton NO, other NO contact on push button to a 2200 ohm resistor 1/2 watt ( red, red, red) the to bottom left contact on the DPDT switch.

DPDT switch is magnatize in one position and demag in the other when the push button is pushed, not sure if that will do much, if anything, with that resistor in there.

If this confused the hell out of you I can draw it and scan the dwg and post it somewhere.

Thank You, Randy

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