Magnetic Chuck Control Circuit Wanted

Hello to you, and welcome! Do you have a surface grinder?
 
whitmore said:
Well, MozamPete says the 8x24 incher (192 square inches) takes 130W,
so your 5x11 (55 square inches) ought to be comfortable with 38W.
To start, get a fuse (1/2A, maybe less) and run the Variac output
through the fuse into the rectifier. Check that the chuck terminals (there
should be two, and a ground) don't have a short to ground, then
connect the rectifier output to (1) a DC-voltmeter and (2) through a
DC-ammeter, into the chuck. MOV at the chuck, if you have one.

Start at something safe (20V?) on the variac, and note current and voltage readings,
preferably on graph paper... take small steps (2 to 5V) and plot a few V-versus-I points,
as you turn up the Variac.

If the chuck starts to buzz, it's probably magnetically saturating (and further
current increase is counterproductive). Note the variac reading, and DC voltage.

If you get all the way to 120 on the Variac and the fuse hasn't blown, shut down.

Now, look at the graph: if it's just a straight line, no problem. If it starts
straight, and curves UP to higher current at some voltage, note the onset
of that curve and subtract 10 percent. The upcurve happens when the magnet
starts to saturate, you do NOT want that to happen in operation. It's
equivalent to overloading a motor, which hums, heats, emits smoke...
The ripple current after the rectifier will upswing sharply at saturation.

Speaking of which, even if the fuse doesn't blow, if you smell something
from the magnetic chuck, shut down. Obviously.

The idea here, is just to get a current and voltage operating point.
It would be great if you could put an AC ammeter in series with the DC
ammeter: the AC ammeter will show a strong indication when magnetic
saturation occurs, and the DC ammeter will tell you the operating current to stay well under.
The AC ammeter will also jump every time you adjust the Variac.
Click to expand...
 
Whitmore, I'm not sure that you will get the desired indication of saturation in your graph if you are relying only on a DC ammeter. Granted, at saturation the incremental inductance disappears (or is reduced immediately or gradually, depending upon the hysteresis curve of the magnetic core), but because the DC current is determined only by the applied voltage and the DC resistance of the wire in the coil, not by the inductance, I would think the DC I/V curve would be immune to magnetic saturation effects. There would, however, be an indication on an AC ammeter looking at the ripple current if you are using an unfiltered, simple rectifier power supply. Having a sensitive tong type AC ammeter on the coil wire would give a useful indication of saturation, assuming an unfiltered supply. AC tong tools are relatively cheap these days.

You do mention the desirability of having an AC ammeter in the circuit and the fact that it will indicate saturation, but I think an AC ammeter, not the DC I/V plot, should used as the primary indicator of magnetic saturation.

However, I think the thermal dissipation of the coil would be the primary limiting factor on permissible applied DC voltage, probably long before saturation in a properly designed chuck. This is fairly difficult to evaluate non-destructively unless you have temperature sensor imbedded within the coil windings, something that you can only accomplish during winding of the coil. There would be an upward bend in the DC I/V curve due to heating of the wire, but thermal damage might occur before you saw the effect. One might get a useful indication of maximum permissible current by inserting a temperature sensor as deeply into the windings as possible (probably being only at the surface, depending upon the tidiness of the windings and the presence of any potting resin), and VERY GRADUALLY increasing the voltage/current while watching the temperature and feeling the surface of the chuck. By gradually, I mean over many hours, as the temperature will be maximum in the middle of the windings and it will take time for the surface to indicate possibly limiting temperature in the core.

Be aware that I am not an electrical engineer (or even a magnetic or thermal engineer - or even a professional machinist).
 
Recognizing that all power dissipation in the core is thermal, I think earlier discussions in this thread of scaling permissible power input from the dimensions of similar chucks with known ratings (and preferably, identical vintage since insulating materials have evolved) is a reasonable way to estimate permissible power dissipation in a chuck of unknown rating. Whether the scaling should be on surface area or volume is beyond me. Starting with a known standard of nearly identical dimensions would be ideal.
 
awright said:
Whitmore, I'm not sure that you will get the desired indication of saturation in your graph if you are relying only on a DC ammeter. ... the desirability of having an AC ammeter in the circuit and the fact that it will indicate saturation, but I think an AC ammeter, not the DC I/V plot, should used as the primary indicator of magnetic saturation.
Click to expand...
My concern was that AC ammeters are sometimes RMS, sometimes peak-and-correction, sometimes rectify-and-correction; it's usually unclear what exactly is being
measured (a transformer-coupled oscilloscope would be ideal, though).
Hopefully, all DC meters average over a few tenths of a second.

If the magnet saturates, the current rise time gets faster, so the peak current
happens sooner than when the full magnet inductance was present.
Because inductive current is phase lagged, that means more power transferred
to the load, and should cause a deviation on the I/V curve. I think.

Maybe a true-power meter on the AC input is a cleaner choice?
 
Pardon, if this is not germane to the discussion, but I have a Brown & Sharpe Micromaster Surface grinder that came with a Neutrofier chuck control; when it degaussing feature is in use, one can see the grinding particles on the chuck poles lean one way, and the other way in a diminishing strength as the cycle does its thing; this suggests to me that the cycle is using direct current and diminishing the voltage as each reversal cycle is gone through, until there is no residual magnetism present in the chuck or work piece, at least not a discernible amount. From the noise it makes, it would seem that an electric motor drives a switch that does the current reversal. I also had a Chinese control on a 12 X 36 Thompson grinder; it was solid state and was tiny compared to the Neutrofier . It seemed to work the same way, by reversing DC input in a diminishing series of reversals. Both worked excellently.
 
That sure sounds like what a PTC would do in an AC circuit - reversals of diminishing strength - except that when using AC, the rate of reversal would be too fast to readily discern by watching the particles.

I'm still waiting for the PTCs I ordered (slow boat from China), but as soon as I have them, I'll announce it here and offer them to anybody for just the cost of postage.
 
benmychree said:
Pardon, if this is not germane to the discussion, but I have a Brown & Sharpe Micromaster Surface grinder that came with a Neutrofier chuck control; when it degaussing feature is in use, one can see the grinding particles on the chuck poles lean one way, and the other way in a diminishing strength as the cycle does its thing; this suggests to me that the cycle is using direct current and diminishing the voltage as each reversal cycle is gone through, until there is no residual magnetism present in the chuck or work piece, at least not a discernible amount. From the noise it makes, it would seem that an electric motor drives a switch that does the current reversal. I also had a Chinese control on a 12 X 36 Thompson grinder; it was solid state and was tiny compared to the Neutrofier . It seemed to work the same way, by reversing DC input in a diminishing series of reversals. Both worked excellently.
Click to expand...
Hi, John, welcome to the thread! John (benmychree) is who I got the chuck we are talking about from, and a B & S surface grinder to go along with it. ;)
Edit: John, I will see you Saturday morning at the mill.
 
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