magnetic chuck control

It sounds like just a bridge rectifier, which can be had for well under $5. Cost to replicate with box, switch, line cord and suitable connector would probably be under $20.

Also a capacitor to minimize ripple, and a resistor to limit turn-on surge. A DPDTCO switch does the job of selecting between AC and CD output, as well as turing it off.

This is exactly what is in the base of the small surface grinder which I have -- and it had an even simpler circuit before I "improved" it. It had not a bridge, but a single Selenium rectifier, and a SPDTCO switch. The original capacitor was marked 8 uF, and actually *measured* something like 0.001 uF or less. I replaced it with about a 200 uF cap, which is total overkill, but is what I had on hand in the proper voltage range.

Enjoy, DoN.

Since the spec is 115 VAC in, 115 VDC out, there can't be much of a ripple-reduction capacitor, if any. A cap charges to the peak voltage of 163 volts each half-cycle, decays according to the cap size and load current but the avg DC voltage will always be > 115VDC because (neglecting diode drops) the avg DC voltage is 115 with no cap at all. A substantially inductive load like a magnetic chuck wouldn't have much current ripple anyway. There's also no turn on surge; dI/dt is limited by the inductance of the load and steady state current is determined by the load's resistance.

Glad you got the Selenium rectifier out of there - when it fails, a poisonous gas is released! Rather nasty in the small shop. The 8uf cap was an electrolytic type and dryed out - becoming an insulator almost.

Mart> >> >>

Very, very dangerous these things.....losing hold power on a magnetic chuck when in use can be very exciting indeed. I am afraid that if I really needed to use one of these things, the circuit would be anything but simple. I would absolutely make certain there would be enough hold time to get the wheel off the work in an emergency. Steve

Just curious, there's also a spring loaded "reverse" switch to de magnetise on these chucks. Are they just switching the + and - leads in there?

Karl

It's probably more elaborate than that. Electro-matic makes these chuck controllers.

See patent numbers:

My guess is that it applies AC thru a PTC thermistor to apply a diminishing AC current.

i have a neutrofier control and if any one is interested, I can take a lot of pictures.

I have a Netrofier control on one of my bigger surface grinders. You can adjust the dc voltage to the magnet to regulate the amount of holding power to the piece. When you are ready to take the piece off you release it with the control applying ac voltage to the chuck in diminishing pulses of about one second duration. The process takes about 10 seconds. I think I have the manual on the unit in my files. Blanchard grinders use them a lot.

Walker also makes magnetic holding equipment. They are nice units. I have replaced several of the old tube type controls with them and they work flawlessly.

John

The mechanical Neutrofier I worked on has a big rotary switch that sequences a number of polarity reversals and voltage reductions of the DC magnet supply. As far as I know the magnet never sees AC.

Thanks, I'll try to remember that if the Neutrofier becomes uneconomical to repair. The last time they called me the problem was in the commutator under the chuck, not the controller.

If the price is right and it happens to match my customer's chuck, they might be interested. My recollection is that the magnet is 240VDC at around 3A. It's on a 30" or 36" Blanchard grinder. Is it a mechanical unit or one of the more modern replacements?

Ned, you can email me at ichudov AT algebra DOT com if you have any questions.

Igor

...

that's the trouble with asking a EE a question. You don't understand the answer

Karl

If I remember right they use a magnetic amplifier type system to regulate the AC voltage going to the magnet. Its been a number of years ago ant to my recollection the transformer was bad on the one I was fixing. I just rewound the transformer and put it back in service.

I learned winding transformers when I was a kid building high power ham transmitters. The old Dumont TV transformers were great for getting

John

I took some photos of the device. Can anyone tell from these whether the switch feeds in AC or reverses the polarity to demagnatize? Don, it is a little more elaborate than you speculated, it has a choke coil also. The cap is .001MFD

Eng> Karl Townsend wrote:

engineman fired this volley in news:ceb8d5a7-88fa- snipped-for-privacy@k2g2000pro.googlegroups.com:

You can't "reverse the polarity to demagnatize" an electromagnet. If current is flowing, it's magnetized. If it happens to be wound for AC, it will be constantly reversing polarity at 60Hz, anyway. If it's wound for DC, reversing the direction of current flow merely reverses the magnetic poles.

OTOH, as suggested, if the chuck were wound for AC, and the power supply were set up to supply a rapidly diminishing AC voltage to the magnet, it would eventually settle down to nearly zero flux.

This is necessary, because suddenly breaking the current can leave the poles magnetized even after current is stopped.

Your power supply apparently has two selenium rectifiers (of old). That would suggest a full-wave-center-tapped DC configuration. But I don't really see that ferro-magnetic thingy as a regular transformer. It looks more like a center-tapped choke or an autoformer. The choke would be to take out the "buzz" of AC, and make the chuck run more quietly. There is a small cap on the control board. It might be there to suppress switch arcing, or it could be used as a fly-back to suppress quick-break magnetization of the chuck, or it could be a little bit of filtering to take out more buzz.

The label sez... 115DC out 115AC in. So there's not much voltage "transforming" going on in the choke/autoformer/transformer.

LLoyd

the selenium rectifiers look like they are hooked up as a full wave center tapped rectifier system, but the wires on the secondary also go out to the switch. I would imagine they are just using the ac to feed the magnet while you remove the part from the magnet rather than on other control units that actually demagnetize the part and magnet

John