So is a 240v Variac considered a tool as well?

[Bob Korves]

So, John, what about an electromagnetic chuck? Inductive or resistive, or something else?

 

[hman]

Geez, Bob .... ya nailed me fair and square! Gotta admit, that's the trouble with rules of thumb.

Yes, an electromagnet converts electricity into magnetism. Maybe I can weasel out of the dilemma by pointing out that the magnetism isn't used to produce motion. Though an electromagnetic chuck would indeed be an inductive load on the variac, it should work OK (as would a universal motor), with the holding power being approximately proportional to the voltage.

I guess the difficulty lies with the earlier "blanket" statement that a variac should not be used with inductive loads. Maybe we can make silverforgestudio's life a little simpler by restating the rule as variacs being generally OK to use with resistive loads, sometimes Ok with inductive, and generally not OK with solid state electronics. Yes, there'll still be exceptions. But short of a detailed course in electronics and/or motor theory, I'm hoping it will be useful.

 

[silverforgestudio]

Hman (John) and Bob- I get the rules of thumb do not apply universally- The use of this unit may not be an issue for now as I have no idea what/how to apply it... but the time will come. And the best part is I know enough to NOT use it till I understand it...

And I am seeing a base in electronics/motor theory is very useful!

 

[Bob Korves]

Geez, Bob .... ya nailed me fair and square! Gotta admit, that's the trouble with rules of thumb.

Yes, an electromagnet converts electricity into magnetism. Maybe I can weasel out of the dilemma by pointing out that the magnetism isn't used to produce motion. Though an electromagnetic chuck would indeed be an inductive load on the variac, it should work OK (as would a universal motor), with the holding power being approximately proportional to the voltage.

I guess the difficulty lies with the earlier "blanket" statement that a variac should not be used with inductive loads. Maybe we can make silverforgestudio's life a little simpler by restating the rule as variacs being generally OK to use with resistive loads, sometimes Ok with inductive, and generally not OK with solid state electronics. Yes, there'll still be exceptions. But short of a detailed course in electronics and/or motor theory, I'm hoping it will be useful.

My question was completely practical, not a gotcha. I am planning to use a variac to control my electromagnetic chuck, I am not an electrical/electronic genius, and want to make sure I am not trying something stupid... I am also planning to try to use the variac and a drum switch to reverse the current to the chuck multiple times while reducing the voltage in steps to zero, to try to demag the chuck and parts after grinding or machining or scraping or whatever. Are those reasonable ideas?

 

[chips&more]

My question was completely practical, not a gotcha. I am planning to use a variac to control my electromagnetic chuck, I am not an electrical/electronic genius, and want to make sure I am not trying something stupid... I am also planning to try to use the variac and a drum switch to reverse the current to the chuck multiple times while reducing the voltage in steps to zero, to try to demag the chuck and parts after grinding or machining or scraping or whatever. Are those reasonable ideas?

Hi Bob, You would build a power supply that has a DC output for creating magnetism. And an option on the power supply to change its output to AC for demag. That’s the simple power supply. There are more elaborate power supplies that pulse the AC and such, but just a switch to change from DC to OFF to AC (mag-off-demag) works pretty good…Dave.

 

[Bob Korves]

Hi Bob, You would build a power supply that has a DC output for creating magnetism. And an option on the power supply to change its output to AC for demag. That’s the simple power supply. There are more elaborate power supplies that pulse the AC and such, but just a switch to change from DC to OFF to AC (mag-off-demag) works pretty good…Dave.

Yes, Dave, I am using a bridge rectifier for AC to DC, along with a electrolytic capacitor, etc. to mag the chuck. I think I have that figured out pretty well. I am not quite sure how to wire it for AC as well, I guess just bypass the DC circuit completely with the AC, no capacitor or anything else, and then dial the voltage down to zero with the variac to demag(?) Any important details I am missing?

 

[chips&more]

Yes, Dave, I am using a bridge rectifier for AC to DC, along with a electrolytic capacitor, etc. to mag the chuck. I think I have that figured out pretty well. I am not quite sure how to wire it for AC as well, I guess just bypass the DC circuit completely with the AC, no capacitor or anything else, and then dial the voltage down to zero with the variac to demag(?) Any important details I am missing?

Hi again Bob, I would first run the 120vac through an isolation transformer (for safety, especially if you ever decide on using coolant). And then yes, just tap of the transformer for the AC source. I would use a 2 pole 3 position switch for the DC-off-AC. The switch would have 1 momentary position for the AC. Momentary means, it’s only on when your thumb/finger is on the switch. Makes it handy for pulsing the AC if you need to. Sometimes just non-pulsed AC is enough to demag. Depends on the type of metal on the chuck at the time. This kinda switch is not your regular garden variety, but can be found. As far as the Variac, don’t know if it’s needed? And I don’t think it isolates either (can’t remember for sure). For me, I would want all the magnetism I can get and not some dialed down percentage of it. Maybe there are some jobs applications out there that work better with less magnetism/pull? Some jobs require shimming because it’s warped. And if not shimmed correctly the magnetism will do its own warping. But that just means it was not shimmed correctly and not because it had too much magnetism. If you have ever had a part fly off the chuck, it’s one of those never forget moments...Dave

 

[Bob Korves]

Hi again Bob, I would first run the 120vac through an isolation transformer (for safety, especially if you ever decide on using coolant). And then yes, just tap of the transformer for the AC source. I would use a 2 pole 3 position switch for the DC-off-AC. The switch would have 1 momentary position for the AC. Momentary means, it’s only on when your thumb/finger is on the switch. Makes it handy for pulsing the AC if you need to. Sometimes just non-pulsed AC is enough to demag. Depends on the type of metal on the chuck at the time. This kinda switch is not your regular garden variety, but can be found. As far as the Variac, don’t know if it’s needed? And I don’t think it isolates either (can’t remember for sure). For me, I would want all the magnetism I can get and not some dialed down percentage of it. Maybe there are some jobs applications out there that work better with less magnetism/pull? Some jobs require shimming because it’s warped. And if not shimmed correctly the magnetism will do its own warping. But that just means it was not shimmed correctly and not because it had too much magnetism. If you have ever had a part fly off the chuck, it’s one of those never forget moments...Dave

Dave, this chuck will be for general use in the shop, perhaps on the mill, and on the bench for holding work for scraping or other hand work. I suppose I might use it very occasionally for holding a special project on the surface grinder. I already have a nice 6x18" B & S permanent magnet chuck for general use on the surface grinder. Part of the usefulness of this chuck will be the ability to lower holding strength for thin, delicate, or other unusual projects. Without the grinding wheel it is no longer a safety issue. With the grinding wheel I will use full mag power. Thanks for the suggestion of the isolation transformer, an excellent idea, and for a switch momentary to AC. Great ideas!

 

[hman]

Bob -
Thanks for your clarification. But even though you weren't intending to "gotch" me, you did correctly point out a flaw in my simplification. No problem whatever (and even if it were intended, I took it as being in good humor).

It's been my long held philosophy that it doesn't matter whether "you" (whoever that is) are right or that I'm right. What matters is that reality is right, and we all benefit from getting there. Anyway, it sounds like you're well on the way toward achieving your goal. Best wishes!

 

[Bob Korves]

Bob -
Thanks for your clarification. But even though you weren't intending to "gotch" me, you did correctly point out a flaw in my simplification. No problem whatever (and even if it were intended, I took it as being in good humor).

It's been my long held philosophy that it doesn't matter whether "you" (whoever that is) are right or that I'm right. What matters is that reality is right, and we all benefit from getting there. Anyway, it sounds like you're well on the way toward achieving your goal. Best wishes!

John, we are all looking for knowledge and understanding here, and sometimes the words get in the way. I did not know the issues with inductive and resistive loads, and I learned something important that relates to a current project from your post. Thank you!