Using a variac to modulate pot heat control

[Quigley_up_Over]

I have been thinking about buying a variac to modulate thermal control on my Mag-20. What I would do is peg the stock thermostat to #10 and use the variac to finely control the heater. Has anybody does this with any success? I am not really happy with the stock "bang-bang" heat control on the pot. I have considered using a PLC to run a PID but before I get that far I want to try a simple variac.

(FYI, for those who don't know, a variac is basically an adjustable transformer that allows the user to control output voltage, in my case, from zero to 120 volts AC)

[alfloyd]

You will still need a thermonture (sp) to watch the temp.
The temp will go up as the pot level goes down even with the variac.
A PID is the only way to control the temp at a set temp.

I use a router varaiable speed control to adjust the temp on my lube/sizer, which is using
the same pricipal.

Lafaun

[runfiverun]

looks like it was answered.
i was gonna ask how you'd know where to set the temp on the varible.
if it's with a thermometer the dial on the pot will work just as well.

[arjacobson]

I used to use one on a guitar amp.......

[uscra112]

As long as you have a way of monitoring pot temperature, a Variac will do nicely. No need for messing with PID controllers. I have bought used Variacs on evilBay for fairly cheap money. Used them for a number of things, including pot control, after the thermostat on my pot went south. Buy one that's two or three times bigger than you need, and it'll never ever go out on you. This from a guy who built countless PID controlled environmental chambers for high precision gages in his career. It's not worth my while to make another just for pot control.

[blikseme300]

There is no substitute for a PID. Consistency is key to good casting. No matter the opinions physics always wins.

Casting actually becomes quite boring when everything is done right.

Bliksem

[1bluehorse]

Wow, a PLC to run a PID, a veriac to modulate the temp control, a thermometure to monitor the temperature, a variable speed to adjust the temp.....man, sometimes I think I'm from a different planet...... and where did that "casting actually becomes quite boring when done right" thing come from.. for who ???

[JonB_in_Glencoe]

I have been thinking about buying a variac to modulate thermal control on my Mag-20. What I would do is peg the stock thermostat to #10 and use the variac to finely control the heater. Has anybody does this with any success? I am not really happy with the stock "bang-bang" heat control on the pot. I have considered using a PLC to run a PID but before I get that far I want to try a simple variac.

(FYI, for those who don't know, a variac is basically an adjustable transformer that allows the user to control output voltage, in my case, from zero to 120 volts AC)

without temperature feedback, your variac will not work any better than the stock thermostat. because as the pot volume gets lower, you have different energy needs.
I've never understood the need for a PID, the stock thermostat has worked well for me with a thermometer in the melt. as the pot gets lower, I adjust the thermostat a little lower, and also I usually need to adjust the lead flow rate at the same time.

Jon

[mpmarty]

I just run the pot wide open and cast fast and add cold ingots as I watch the thermometer to see if I'm casting fast enough.

[popper]

Some individuals here use lamp dimmers,SS variacs. They don't give any temp feedback so just control amount of heat to the pot. You can build the PID for about $80, I think a good variac is about the same $, 1KW dimmer will be about the same. A decent thermometer will cost at least $40 delivered. Not much diff in $ for GOOD control.

[youngda9]

The PID setup is quite easy to do, cheap, and works GREAT.

"Set it and forget it" likeRon Popeil always said.