View Full Version : moly-d without the transformer

Mark Wilson
07-18-2002, 03:18 PM
please excuse me if i have misunderstood the discussion here, so i would like to clairify a couple of points. it is my understanding that you need a step down transformer to take 240VAC down to 60VAC to run the moly-d heating elements with 4x the current available on the 240VAC line.

for example, if i feed 240VAC @60Amp into the primary of the transformer, i would get 60VAC @240Amp out of the transformer. if the output of the transformer was connected to 4 heater elements in parallel, each heater would draw 60VAC @60Amps or 3600 Watts. so for 4 elements, you could supply 14.4 KWatts of heat.

it is also my understanding that the normal method used in your furnaces has the heating elements in parallel, and that you use the scr's in the primary (high voltage side) of the transformer so that you don' have to buy bigger, more expensive high current scr's.

if this is true, then i have a question. if you connectecd 4 heating elements in series, not in parallel, you could still use the same scr's but you would not need a step down transformer. in the example stated above, you would have a 60 volt drop at 60 Amps on each heater so you would get the same total output power of 240VACx60Amps or 14.4 Kwatts, except without the cost of the transformer. i know that this is not the best configuration, as if one elements goes down, they all go down, until you repair or jumper around the bad element. but you still get to use the cheaper scr's and you don't need to buy the transformer. and if the moly-d heaters are as reliable as everyone says, then you don't need to worry about an element going down very often.

has anyone tried using moly-d heaters as i have described?

Pete VanderLaan
07-18-2002, 07:10 PM
The trouble with the direct from the wall approach was that the SCR and the contactor needed would get very big and expensive. Enough so to wipe out the potential savings of not using the transformer. We did consider it for mine because I had a spare 200 AMP Panel. Beyond that, I will leave this one to Steve. :dog:

Henry Halem
07-18-2002, 07:48 PM
I believe you can preclude the need for a transformer if the combined voltage of your elements in series adds up to the total operating voltage but you will still need an SCR. As in: if your primary voltage is 220 and each element is rated at 12 volts you would need in the neighborhood of 18 elements. There is in this case and economy of scale but that scale is usually beyond the scale of the studio.

Mark Wilson
07-18-2002, 09:46 PM
you don't have to buy a more expensive scr. for my example, you need an scr that can switch 240VAC and pass 60 amps. that is the exact same scr that you would use in the primary of the transformer and the same one that you would put in series with all of the heaters.

you could put a current transformer around the current path to sense if you have heater continuity. this would also allow you to know the resistance of your elements while they are hot. it would work the same as a clip on ampmeter, except it would be permament and part of your safety system. if an element went out, all elements would go out since they are in series. but the current sensor would know that you have a fault, and you could use the signal to interface with an autodialer to call your cell phone to let you know that you had a problem.

Steve Stadelman
07-18-2002, 10:33 PM
Hi Mark, In these systems the elements are in series in multiples that add up to 60 volts. I have four elements that each take 15 volts to push 170 amps at temp. Your big considerations are the high amperage loading and very low resistance of moly. If you run moly at less than rated amperage it doesnt put out the wattage you want, just like any other heating element. The commonly available four to one stepdown transformers can give you the low voltage that you need along with bumping the current up where you want it. Or you can devote a whole 200 amp panel and use the current limit function on the scr to hold it all back. 240v x 200a gives 48000watts, I know several folks melting 300lb on half of that. Steve.

Mark Wilson
07-19-2002, 06:21 AM
so it sounds like you have what is called a "series/parallel" network. your transformer outputs 60 volts, you elements require a 15 volt drop, so you hook 4 elements in series, on several parallel legs of your circuit.

if this is the case, then what i said the first time is still true. if you put all of your elements in series rather than in series/parallel, you end up with the same control scr, the same elements, the same number of elements, you just loose the transformer.

lets discuss another example. 16 heaters, 4 parallel circuits of 4 elements, each requiring 15 volt drop at 200 amps, so that your total current draw for the furnace is 200 amps at 240 volts. the transformer outputs 60 volts at 800 amps, so that you have 200 amps available for each of the 4 parallel legs of the circuit. therefore each heater operates at 15 volts at 200 amps or 3000 watts. so since there are 16 heaters, you get 48,000 watts of heating. there is a 200 amp 240 volt scr in the primary of the transformer.

you can connect all 16 elements in series with the same 200 amp 240 volt scr, and get the same results without the transformer. you end up using the exact same number of elements, the exact same elements, the exact same scr, you just don't require the transformer.

if you hook 16 elements in series each with a 15 volt drop, you get 16x15 or 240 volts drop with 200 amps passing through all of the elements which the 200 amp scr controls.

of course you don't get something for nothing. you loose the transformer, but you end up with a less fault tolerant system in that if one element goes down, they all go down. but with a current monitor and given the reilability of the moly-d elements, maybe it is workable.

i am not saying that this is the best way to hook up your furnaces, i was just tring to understand the options, and find out if someone has done it before.

Pete VanderLaan
07-19-2002, 08:27 AM
Jon indicated to me that if I wanted to run direct from the wall that I needed a completely different set of elements to do it. They were available. As Henry pointed out, as did you, that when you get into a big pile of elements to balance the load, the savings goes out the window. :dog:

Steve Stadelman
07-19-2002, 09:12 AM
Mark, I think you are right about what you are saying. The transformer also keeps more panel space open in small electrical panels. (i use a 60 amp breaker for my furnace). And I am a plant from the kennicott copper mine and we are bent on WORLD DOMINATION!!!!. So you must all buy scrapyard transformers and bend to our will!!.

Mark Wilson
07-19-2002, 10:05 AM
Originally posted by Pete VanderLaan
Jon indicated to me that if I wanted to run direct from the wall that I needed a completely different set of elements to do it. They were available. As Henry pointed out, as did you, that when you get into a big pile of elements to balance the load, the savings goes out the window. :dog:

in my examples, you use the same elements and the same number of elements, so it does not cost any more. and in both of my examples, your electricity use is exactally the same, so the operational cost is the same. the only trade off is transformer vs. a little bit of fault tolerance.

it is good that you can get elements that operate at different voltages and currents. that gives you more things that you can use to optimize the furnace design. if a 3000 watt heater operating at 15 volts and 200 amps cost the same as one operating at 60 volts and 50 amps, then where does the savings go out the window?

Pete VanderLaan
07-19-2002, 07:04 PM
my best understanding is that the larger SCR and the contactor costs a bunch more. ( keep in mind I haven't built this yet) :dog:

Steve Stadelman
07-19-2002, 09:34 PM
Mark, Kall kanthal and procure yourself a kopy of thier super element handbook, The give them away and there is lots of really neat math in it for your numerological pleasure. Also it talks about ALL of the control and element configurations. They even printed it with some color photos for the firefighting crowd. (guys like Pete and I need the good visuals). Steve.

Mike Firth
07-21-2002, 12:24 AM
Well, step 1, Mark is totally correct, from what has been said on the board, same number of elements, therefore same cost, same amps on SCR, therefore same cost, therefore, why have a transformer. No answer yet.

Docs for Super Kanthal are downloading as I write 2.9 meg from www.kanthal.com so maybe answer is available faster than calling them.

Best answer I can think of is if these things age unevenly, putting 16 in series with 240 volts at each end might result in one element (colder one? hotter one?) getting a lot more current and burning out, while cranking down to 60 volts means the max on any element would be much lower and risk lower.

Jon Myers
07-21-2002, 12:38 AM
Hi Mark, Sorry not to chime in sooner but I've been away from the computer for a while. I origionaly was going to run my furnace direct from the wall as I have 3 Phase 200 amp of power to my shop (208v). To do this you would need around 50% more elements (either size or number) than you would to run them 120V. For instance I2R quoted me a furnace when I was just starting to think about the moly's that used 6 28" elements(6 28" elements need at least 156v to fire correctly.) run in series to provide 37.5 Kva. In fact I built my furnace to accomadate just this setup. (I have run 6 28" on 120v and they are pokey.) I've also run them as two sets of three elements and (two parallel sets of three)(3 element(28") set needs @ 80v) it ran great. Problem was you had to limit the power. I ran it this way (2 sets of three) until I needed the controller I was using for a high limit for an annealer.(and I found out about the Peak demand charge)(see below) I then put it into series (needing 156v) and ran it on 120v. It was pokey. (It was what I was running when Pete was here.) I've since bought another Hi Limit controller and am now running 4 28" elements in series, essentially the same as 6 18") (the extra 4" is to make up for the radius in each of the 18" elements) and they draw 85-90 amps (170-180 amp 120v) max. I like the longer elements because you can tuck them into the corners of a square furnace and add several inches between the elements and the pot. (It probably makes no difference but it helps me rationalize why I built the obviously less efficent square format furnace instead of round:) ) Here in OR we have a demand charge on our electricity bill that takes your Peak Demand and multiplys it by a factor (which depends on your electrical contract). We built this furnace (and signed our commercial power contract) after the Cali power problems so our peak demand charge is high, often $250-300. That is with the shop pulling an avg of 19kva and a high of 29 kva. Before I found out about peak charges we would run peaks of 35, 40 kva (charging, running the annealer and drying clothes at the same time etc) We would have $500 peak fees. You would have this level of fee every month if you were running the straight out of the wall variety furnace. Sure you could limit the power with the scr but what would be the point. I've been unable to find a 200 amp contactor and a 200 amp SCR for less than I could find a 150 amp contactor and a 150 amp SCR AND a transformer (New no less). And that would be running the SCR at its rating...bad idea. With a transformer all the components are 2/3 their rating (assuming we're talking about 100 amp 220v and 150 amp components) Now I'm sure that someone has figured a way to heat these things with $15 worth of parts and some tape but I'll bet it took them a while to figure it out($$$) and I'll bet it isn't very reliable (also$$$). The neat thing about these elements is that they work well for a long period of time. It's like having a gas furnace. You go to the shop every day knowing there'll be hot glass. With the wire furnace (at least the one I had) you went to the shop hoping there would be glass. Speaking of which I need to get back to work. I've spent my whole Internet time for today on this one thread/post...jeeze I need to learn to type faster.:)

Mike Firth
07-21-2002, 12:56 AM
Well, I read the brochure which is nice and colorful and only moderately useful. There is a chart out on page 15 that shows these things have a 6:1 ohms resistance change from room temp to glass temps (0.4 ohm/mm/m to 2.4) which is very close to a short circuit at cold temps. It's getting late so I won't type what I am thinking and be stupid.

Steve Stadelman
07-21-2002, 11:49 AM
Mike, I guess what sums this up is without a transformer you need a 200amp panel, contactor, and scr, with a transformer you can use 50-100 amp components saving money and electric panel space for other equipment. There are lots of ways to skin this cat.

Pete VanderLaan
07-21-2002, 11:57 AM
At this point, I will go with the transformer and leave the extra panel sitting there waiting for the day you quit the fire department and have time to come to the high desert.. I actually think that the breaking point is the big contactor and whether your local inspector lets you build with or without it. If it was without, it might be cheaper but those puppies are expensive.
It really isn't clear to me that this will get inspected anyway since the building department really is only interested in the panel and the outlets. Since it has no flu type exhaust, the mechanical guys don't get involved either The fire department will probably ask for a 20lb ABC extinguisher. The gloryhole is another matter and that drives them crazt. They cannot cope with a tool that doesn't have a flue. :dog:

Mark Wilson
07-21-2002, 02:03 PM
pete, it doesn't require more current from your 240VAC line, and it does not require a higher power scr or triac. and it doesn't require a higher current line and contractor to your furnace.

for example, lets say you have at 240VAC 100Amps line fed into a 4:1 step down transformer. the transformer steps down the voltage, but steps up the current to a 60VAC 400 amp output. if you connected to 4 parallel sets of elements( 1 or more elements in series on a leg) each parallel leg has 100 amps running through it. if you put all of the elements in series, the same 240VAC 100 Amp circuit will work the same way without the transformer.

i think it would help if i draw up and post a schematic. i will probably draw it up for a triac as that is much simpler to build and costs less.

another thing that i have been wondering about is using an scr on the high voltage side of the transformer. true this allows the scr to run with a lower average current, but then the scr is connected to the inductive load of the transformer. this might casue problems even if the switching occurs at the zero crossing threshold. in the case of the elements in series, it is totally resistive and has virtually no inductance so there will be less "switching transients" which should lead to the scr or triac running cooler and longer.

Mark Wilson
07-21-2002, 10:03 PM
Originally posted by Mike Firth
Well, I read the brochure which is nice and colorful and only moderately useful. There is a chart out on page 15 that shows these things have a 6:1 ohms resistance change from room temp to glass temps (0.4 ohm/mm/m to 2.4) which is very close to a short circuit at cold temps. It's getting late so I won't type what I am thinking and be stupid.

the 6:1 change of resistance with temperature is compensated for by changing the average voltage applied across the elements. the power avaliable to heat the glass is given by the formula

P = V^2/R

so ar R increases with temperature, the average voltage must increase sligthly. it is a slight change since the power goes as the square of the applied voltage. the triac or scr circuit change the average voltage by eliminating a percentage of the power of the AC cycle. this change of resistance with temperature is no different with or without the transformer.

Pete VanderLaan
03-10-2003, 11:05 AM

Steve Stadelman
07-07-2005, 09:24 PM
I have held off for a long time in adding to this thread but it is bugging me, so here goes.

Power factor.

If you don't use a transformer and you build a furnace around 300 pounds and have to use a 200 amp panel exclusively for it you will have a poer factor of .5 AT THE VERY BEST.

You will end up paying for the transmission of hundreds of amperes even though you limited the voltage to a fraction of the available potential.

It cannot work well in the end and the power bills will be huge.