Magnetic Chuck Control Circuit Wanted

[Bob Korves]

I already have, on hand, two variacs.

The first one shows either:
Input: 120V 50/60hz -- Output: 0-120v -- Amps 2.25 -- V.A. 270
or
Input: 120V 60hz -- Output 0-132V -- Amps 2.25 -- V.A. 297

The second one:
Input 120V 50/60hz
Output: 0-140V KVA =1 (big mutha')

I also have a 450V, 470 μF electrolytic cap, 105C

I also have a drum switch and a more than capable rectifier bridge, and a bleed resistor...

Whitmore and Mark, tell me where I might go next with this start...

 

[markba633csi]

Bob what are the specs again on your mag chuck? (Input voltage and amps or watts)
MS

 

[whitmore]

I already have, on hand, two variacs.

The first one ...
Input: 120V 60hz -- Output 0-132V -- Amps 2.25 -- V.A. 297

The second one:
Input 120V 50/60hz
Output: 0-140V KVA =1 (big mutha')

I also have a 450V, 470 μF electrolytic cap, 105C
...a drum switch and a more than capable rectifier bridge...

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.

 

[Bob Korves]

Bob what are the specs again on your mag chuck? (Input voltage and amps or watts)
MS

No tags. Resistance is 86 ohms across the the plug on the electrical cable. Everything is properly soldered in the chuck, so that number should be usable. I am sure it was meant to be fed 120VDC. I am guessing(!) that I would not want to run more than 120 watts (1 amp) to the chuck at 120VDC. The variacs will give me some wiggle room there...

 

[Bob Korves]

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.

Thanks, whitmore! That sounds like a very good way to test the chuck for how much power it can accommodate before it saturates. I will do that and report back, may take a while to find enough components to make a reasonably safe and worthwhile test run. The chuck body has no connection to the power wires, already tested...

 

[markba633csi]

Sounds good. You don't HAVE to have an isolated supply to run your chuck, but it is safer from a shock hazard standpoint. Code would require it. But in your own
house you can decide. Just make sure the chuck is well grounded whichever way you go.
Cheers,
MS

 

[Bob Korves]

Sounds good. You don't HAVE to have an isolated supply to run your chuck, but it is safer from a shock hazard standpoint. Code would require it. But in your own
house you can decide. Just make sure the chuck is well grounded whichever way you go.
Cheers,
MS

I plan to change the cord to a three wire one, and ground to the chuck casing. The existing cord is at least 50 years old and is petrified anyway, but interestingly has no cracks or other issues, just very stiff. To hell with code, I do it to standards that are considered safe by those in the know... It is my tail that is on the line! Thanks!

 

[Bob Korves]

Well, MozamPete says the 8x24 incher (192 square inches) takes 130W,
so your 5x11 (55 square inches) ought to be comfortable with 38W.

6x18" chucks typically draw about 100-120 watts, maximum.

 

[MozamPete]

No tags. Resistance is 86 ohms across the the plug on the electrical cable. Everything is properly soldered in the chuck, so that number should be usable. I am sure it was meant to be fed 120VDC. I am guessing(!) that I would not want to run more than 120 watts (1 amp) to the chuck at 120VDC. The variacs will give me some wiggle room there...

The chuck operating power should pretty much just be based on the DC resistance of the winding - in steady state operation on dc the inductance of the winding has no effect. So

I = V/R and
power = V²/R

At 120Vdc and 86 ohms take would imply

Current = 1.4 Amps
Power = 167 Watts


For comparison my 8" x 24" chuck name plate is 110Vdc, 1.3A, 144W and measuring the dc resistance of the chuck I get 92 Ohms.

 

[Bob Korves]

The chuck operating power should pretty much just be based on the DC resistance of the winding - in steady state operation on dc the inductance of the winding has no effect. So

I = V/R and
power = V²/R

At 120Vdc and 86 ohms take would imply

Current = 1.4 Amps
Power = 167 Watts


For comparison my 8" x 24" chuck name plate is 110Vdc, 1.3A, 144W and measuring the dc resistance of the chuck I get 92 Ohms.

Yes, and that sounds like too much power to me. Perhaps the voltage needs to be lower. The variac can do that easily. I like whitmore's method of testing for saturation, and I will do that.