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How to control holding force of electromagnet

Sea Sick Steve

Cast Iron
Joined
May 1, 2011
Location
The Buckeye State USA
I have an electromagnetic chuck on my surface grinder. 6X12 Electro-Matic with an old tube powered dc rectifier. My question is can I add a device in line to control the power of the magnet? Would a variac control work? If so would it be used on the ac input side or dc output side. Appreciate any help or suggestions. I will add that this is for a hobby application so Im not looking to spend a bunch of $$on a new chuck and or rectifier.
Thanks
 
I have an electromagnetic chuck on my surface grinder. 6X12 Electro-Matic with an old tube powered dc rectifier. My question is can I add a device in line to control the power of the magnet? Would a variac control work? If so would it be used on the ac input side or dc output side. Appreciate any help or suggestions. I will add that this is for a hobby application so Im not looking to spend a bunch of $$on a new chuck and or rectifier.
Thanks

Variac should work - ac side only. A variac is a type of autotransformer and transformers only work with ac.

An autotransformer is a transformer with a primary winding only (no secondaries). There are taps for different voltages (which may be higher voltage than the input). Instead of taps a variac has a low resistance wiper that runs across an exposed section of the windings.
 
The tube unit should have a means of controlling the force other than just on or off. Also, most units had a demagnetizing mode that resets the iron to neutral. Straight dc units will leave residual magnetism. The demag circuit momentarily applied a AC voltage to the magnetic decayed gradually.

Tom
 
I would think that feeding your old tube powered dc rectifier the wrong input voltage (as you would get from a variac in-line with the input power) is likely to damage it. That tube-powered rectifier has transformers in it which (for example) provide power to the filaments of the tubes. Feeding in the wrong voltage will then give the wrong filament voltages to the tubes, and they won't work correctly. As Tom suggests, look for another solution.
 
Steve,

Can you just eliminate the old original power source to the chuck, I'm assuming it's an external unit(?).

On my Thompson 6F grinder, I just use a variac rectified by one of those little cheap 4 terminal rectifiers. I also mounted a volt gage on the machine, but wasn't necessary as it seems to read the same as variac dial. Been working a treat for 30 years.

Those chucks don't require many watts of power.

Best of luck to you/ Gus
 
I would think that feeding your old tube powered dc rectifier the wrong input voltage (as you would get from a variac in-line with the input power) is likely to damage it. That tube-powered rectifier has transformers in it which (for example) provide power to the filaments of the tubes. Feeding in the wrong voltage will then give the wrong filament voltages to the tubes, and they won't work correctly. As Tom suggests, look for another solution.

Good point. I forgot he said it was a tube rectifier.
 
The tube unit should have a means of controlling the force other than just on or off. Also, most units had a demagnetizing mode that resets the iron to neutral. Straight dc units will leave residual magnetism. The demag circuit momentarily applied a AC voltage to the magnetic decayed gradually.

Tom

I have attached a pic of the rectifier I do not see a means by which the force can be adjusted The thing in the lower left is a broken bulb that indicates power on
Is there a component that i can add?
IMG_3411.jpg
 
Steve,

Can you just eliminate the old original power source to the chuck, I'm assuming it's an external unit(?).

On my Thompson 6F grinder, I just use a variac rectified by one of those little cheap 4 terminal rectifiers. I also mounted a volt gage on the machine, but wasn't necessary as it seems to read the same as variac dial. Been working a treat for 30 years.

Those chucks don't require many watts of power.

Best of luck to you/ Gus

Can you post a pic or specs on the rectifier? My rectifier is 50 watts
 
I have attached a pic of the rectifier I do not see a means by which the force can be adjusted The thing in the lower left is a broken bulb that indicates power on
Is there a component that i can add?
View attachment 295360

It would take a like calculating but adding a potentiometer, one end connected to DC supply, the other to ground or through a resistor, and the wiper to the chuck. The reason for the "not sure" part is that the pot will add additional drain on the rectifier.
The resistance value and wattage of the pot will have to be determined. Also, the range of the pot can be limited by connecting the bottom end of the pot to a fixed resistor and then to ground. Mechanically, this is not difficult, but would require some knowledge of basic electricity and making some calculations and measurements.

If you want to go with a whole new design of a variac and a bridge rectifier, then an additional component is needed, a bidirectional switch that switches between hold and demagnetize. In the demagnetize mode, the output of the variac (AC) is coupled to the chuck instead of through the diode bridge. To demagnetize, the variac is gradually reduced to zero. This will reset the chuck to zero retentive flux.

Tom
 
It seems that you could make a magnet control with 2 light dimmer switches,
1 double pole double throw switch, and a full wave rectifier. The magnetize
circuit would have one dimmer connected to the rectifier then to the DPDT
switch then the magnet. The de-magnetize circuit would have a dimmer connected
to the DPDT switch then to the magnet.

One dimmer would apply DC current and the other dimmer would supply the AC.
You could adjust both the AC and DC independently. The whole thing should
cost $20 to $30.
 
It would take a like calculating but adding a potentiometer, one end connected to DC supply, the other to ground or through a resistor, and the wiper to the chuck. The reason for the "not sure" part is that the pot will add additional drain on the rectifier.
The resistance value and wattage of the pot will have to be determined. Also, the range of the pot can be limited by connecting the bottom end of the pot to a fixed resistor and then to ground. Mechanically, this is not difficult, but would require some knowledge of basic electricity and making some calculations and measurements.

If you want to go with a whole new design of a variac and a bridge rectifier, then an additional component is needed, a bidirectional switch that switches between hold and demagnetize. In the demagnetize mode, the output of the variac (AC) is coupled to the chuck instead of through the diode bridge. To demagnetize, the variac is gradually reduced to zero. This will reset the chuck to zero retentive flux.

Tom

Tom thanks for the reply. This will be beyond my electrical capabilities but I have a friend who is pretty good with this stuff and I will run this by him and see if he can help me out
 
It seems that you could make a magnet control with 2 light dimmer switches,
1 double pole double throw switch, and a full wave rectifier. The magnetize
circuit would have one dimmer connected to the rectifier then to the DPDT
switch then the magnet. The de-magnetize circuit would have a dimmer connected
to the DPDT switch then to the magnet.

One dimmer would apply DC current and the other dimmer would supply the AC.
You could adjust both the AC and DC independently. The whole thing should
cost $20 to $30.

Thanks for the reply as I posted above trying to engineer something from scratch will be a bit more than my skills will allow I will however show this to my friend and see if he can help me out with this
 








 
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