# can i use a variac to control a melting furnace



## Ebel440 (Aug 9, 2014)

Hi I have been casting with a charcoal fueled furnace but the mess it makes is starting to make me want to switch to electric heating.  I plan to build a new furnace with kanthal elements and was wondering if I could use a variac to control the temperature. I don't think I have ever seen anyone use a variac to control a furnace.  Would it provide enough adjustability? Would it be safe for the variac to be used with the resistive load? I picked up a big 30 amp variac cheap and have not come up with a use for it but I don't want to damage it.


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## John Hasler (Aug 9, 2014)

Ebel440 said:


> Hi I have been casting with a charcoal fueled furnace but the mess it makes is starting to make me want to switch to electric heating.  I plan to build a new furnace with kanthal elements and was wondering if I could use a variac to control the temperature. I don't think I have ever seen anyone use a variac to control a furnace.  Would it provide enough adjustability? Would it be safe for the variac to be used with the resistive load? I picked up a big 30 amp variac cheap and have not come up with a use for it but I don't want to damage it.



It'll work fine as long as you don't overload it.  A resistive load will do it no harm at all.  30 amps at 120 V is only 3600 watts, though.


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## strantor (Aug 9, 2014)

A variac that size isn't cheap (except in the case that you bought it I guess). Its probably worth more to you in selling value than as a heater controller. And on top of that, as a heater controller, theres no feedback to adjust the output. Variacs are preferred by those who need clean sine waves at precise amplitude. A resistive heater cares nothing about the cleanliness of the power delivered to it. It will work just as well on DC or AC or anything in between. If it were me, I would sell the variac and use the the money to buy a cheap ebay temp controller and solid state relay. And a case of beer. And some tooling. And maybe some new socks if there's anything left.


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## Ebel440 (Aug 9, 2014)

Thanks do you have any idea how many watts electric furnaces typically use? My variac may be 30 amps at 220 which would give me a little more power to work with I'll have to dig it out and check


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## John Hasler (Aug 9, 2014)

Ebel440 said:


> Thanks do you have any idea how many watts electric furnaces typically use? My variac may be 30 amps at 220 which would give me a little more power to work with I'll have to dig it out and check



An example:
http://www.cnczone.com/forums/casting-metals/158922-electric-metal-melting-furnace.html
Evidently it works despite being spectacularly inefficient.


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## Ebel440 (Aug 9, 2014)

Yes I had read they take some time to melt but with setting up the charcoal and getting the fire going it's not much different. Plus the cost of charcoal is pretty high where I am anyway. But I checked my variac out and its only 30 amps at 120 so I may look into a different method of controlling it.  In the link someone said you could replace the triac in a dimmer with a larger one to control the furnace. I may look into that or see if I can put something together I know I have a big SCR I picked up a few years ago


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## John Hasler (Aug 9, 2014)

Ebel440 said:


> Yes I had read they take some time to melt but with setting up the charcoal and getting the fire going it's not much different. Plus the cost of charcoal is pretty high where I am anyway. But I checked my variac out and its only 30 amps at 120 so I may look into a different method of controlling it.  In the link someone said you could replace the triac in a dimmer with a larger one to control the furnace. I may look into that or see if I can put something together I know I have a big SCR I picked up a few years ago



Actually I was surprised at how fast he gets a melt given his modest power level and minimal insulation.  Go to 50 amps, add a couple of layers of reflective insulation and an enclosure and that thing would melt three pounds of Al before you can turn around.  Of course without temperature control it would also tend to melt itself...

With the large time constant of a furnace  and the lack of a need for extremely precise temperature control you don't need a dimmer circuit.  A big solid-state (or even mechanical) relay and a bang-bang control would work fine.


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## strantor (Aug 9, 2014)

Seriously,  if you can read chinglish, you can have a PID autotuning TEMPERATURE CONTROLLED (as opposed to power controlled (dimmer circuit or variac) or uncontrolled) solution for less than it costs to take a family of 4 to an all-you-can-eat buffet for lunch.

http://www.ebay.com/itm/Digital-PID...931?pt=LH_DefaultDomain_0&hash=item4165b39043

http://www.ebay.com/itm/K-type-High...964?pt=LH_DefaultDomain_0&hash=item233dffc254

Thats all you need right there


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## pdentrem (Aug 9, 2014)

As mentioned use a temperature controller with a thermalcouple, much better than anything else. 

At the shop, I used a RMJ/H 005 from Rautomead, not the biggest made but still not exactly for home use. It will dim the lights in the neighborhood if it wasn't for the raw power feed to our own transformer!:sorry2:
Pierre


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## Cheeseking (Aug 10, 2014)

Flea bay yourself a pile that looks like this and be done
	

		
			
		

		
	





Sent from my iPhone using Tapatalk


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## Ebel440 (Aug 10, 2014)

Yes looks like buying the controller will be the best way I didn't know they were that cheap


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## RWL (Aug 11, 2014)

strantor said:


> Seriously,  if you can read chinglish, you can have a PID autotuning TEMPERATURE CONTROLLED (as opposed to power controlled (dimmer circuit or variac) or uncontrolled) solution for less than it costs to take a family of 4 to an all-you-can-eat buffet for lunch.
> 
> http://www.ebay.com/itm/Digital-PID...931?pt=LH_DefaultDomain_0&hash=item4165b39043
> 
> ...



That's an excellent choice of PID.  I've been waiting to find one with those features at such a reasonable price, and a solid state relay that handles 25A.  Plenty for my 110V kiln.


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