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Dave Hilty
01-27-2012, 10:22 AM
Did a charge & cook cycle last night and came in to find the furnace down to 1400 F. and no power to elements.
Tracked a blown 63AMP bussmann fuse in the safety breaker which fuses one leg of the 240 primary in the Moly cabinet. Not seeing anything obvious I used the spare that shipped with the furnace and its powering the elements.

Question is what caused it. I took the transformer front off to try to find anything obvious like the burnt lug or whatever that happened once before. Nothing looked particularly odd but here's what I got for voltage readings. After the fuse replacement I've got 240 to the primaries as I should.
The secondary voltage tap is marked 60V but I'm only getting 50V reading across the secondary legs. When I read one secondary to ground I get the full 50 volts and when I read the other secondary to ground I get no volts.
The cables to the elements show a draw of 207 amps.

Do I have a fried transformer? Or is there something else going on I don't understand?

Thanks for any help here.

Pete VanderLaan
01-27-2012, 10:24 AM
Doesn't that SCR have two fuses or is it a din-o-mite?

It should be the case that one of the 240 legs is actually just passing through the SCR but is not being affected by it. It just looks like the SCR is managing both legs. Also, do you know for a fact that you were actually getting 60V before the incident? Your transformer has compensating taps on it which will allow you to up the output voltage.

Lastly, how do the elements look to you at 207 AMPS. Are they yellow white hot, or dull?

Dave Hilty
01-27-2012, 11:26 AM
Doesn't that SCR have two fuses or is it a din-o-mite? It's a Din-a-Mite. The fuse that I replaced was not in the SCR but the safety fuse on one leg of the primary but in the cabinet. And I do have good primary voltage on both legs to the transformer so the SCR is passing full voltage on both legs.

It should be the case that one of the 240 legs is actually just passing through the SCR but is not being affected by it. It just looks like the SCR is managing both legs. Also, do you know for a fact that you were actually getting 60V before the incident? You've got a point as I can't remember checking voltage at the secondary or if I did I didn't write it down so I may always have been getting 50V.

My biggest concern at this moment is why I only get that voltage from one secondary leg of the transformer since when I measure the other secondary leg to neutral, I'm getting almost no volts. This makes no sense to me.

Your transformer has compensating taps on it which will allow you to up the output voltage. I see those taps and perhaps moving the primary to a 230V tap rather than the current 240V tap would give me more power? I don't really want to mess with that until I understand whether the transformer secondaries are normal or not.

Lastly, how do the elements look to you at 207 AMPS. Are they yellow white hot, or dull?

The elements are good and hot, not dull at all.

Pete VanderLaan
01-27-2012, 03:43 PM
If I recall correctly, that's a 75 lb furnace . You use a 6/12 250/450 element and have two banks of three each? Is that correct? If it is, then it optimally generates 6270 watts per bank or 12.5 KW total. Each element should be drawing 12.5 volts or 36-37 volts per bank in series wiring. Is that correct?

If that's the case, I'm confused about the secondary rating on your transformer being ( you say) 60 Volts. My calculations suggest it should be drawing 37-38 volts on the secondary or do I just have your entire furnace wrong in my head. The fifty volts I don't understand and it makes me think I have this wrong.

Help me out here Dave. There should be a reason that the breaker blew.

Steve Stadelman
01-27-2012, 04:08 PM
This might be a "hot" 80 pounder. Having the electrics from a 100 pounder. The amperage draw is actually a bit high. Run with it. I would upgrade to an 80 amp fuse. Dave, feel free to call me if you would like. 503-709-9922

Pete VanderLaan
01-27-2012, 05:03 PM
So did it have 300/450's then?

Dave Hilty
01-27-2012, 06:32 PM
Steve is right. Brent gave me the elements/transformer for the 100# furnace to give me quicker recovery for charge/melt cycles. This unit has a total of 4 elements. We determined that the elements are 6/12, 250 Le, 450 Lu - that is what you shipped me in 2010 to replace my spares that the damn cat knocked to the floor and broke. A month ago I pulled the crown & replaced the crucible. Didn't lose an element so haven't put the spares into service yet.

Just checked the furnace again and it's idling at set point 1900F. Secondary Volts at this idle are 35.3 and amp draw is 145. I guess I can hope that the fuse just decided to give up rather than blowing from an overload...?

Lawrence Duckworth
01-27-2012, 07:59 PM
I came in wednesday morning to purdy much the same deal. it turned out one of the 80 amp bussmanns was shot...after seven charge cycles :confused:

btw... i didn't know even though all four of the fuse terminals read 120v to ground...it didn't mean they're any good.



thanx again for the help Pete....

Steve Stadelman
01-28-2012, 12:32 AM
I would still like to touch base Dave.

Dave Hilty
01-28-2012, 06:41 AM
I'll give you a call later today Steve.

Lawrence Duckworth
01-30-2012, 01:35 PM
Dave, any idea yet what caused the fuse to blow. Mine was clean with no sign of burn on the wire or fuse casing....:and I've ckecked everything I know to ck. :confused:

Bin running just fine since jumping the fuse too.....

Dave Hilty
01-30-2012, 02:21 PM
Lawrence,

I have not identified anything in particular leading to the blown fuse. This type of fuse, as I learned in my discussion with Steve, is designed as a fast-acting type fuse as opposed to the slower acting house circuit type fuses we are all more familiar with. This fuse is designed to protect the SCR. Steve suggested that I check carefully that all the wiring to the fuse receptacle is tight and to the SCR. He suggested increasing the amp rating for the fuse that blew from 63Amps to something higher and I have 80 amp replacements coming from Graingers.

Downstream poor connections (say at the element strap connections) would not in my understanding cause greater current flow or overloads, but the opposite. So other than a transient transformer short, I don't understand (other than failure of the fuse itself) why this would occur.

Like you, I'll keep my fingers crossed and my connections tight.

Steve Stadelman
01-30-2012, 04:02 PM
The transformers don't go out. If there was a transformer short it would not be transient. Check all the connections to the fuse holder and buy the biggest fuse you can. Since this is a fast acting semiconductor fuse that does not protect life or property you can use a fuse that is up to 250 % of load.

Lawrence Duckworth
02-01-2012, 10:28 AM
Did a charge & cook cycle last night and came in to find the furnace down to 1400 F. and no power to elements.
Tracked a blown 63AMP bussmann fuse in the safety breaker which fuses one leg of the 240 primary in the Moly cabinet. Not seeing anything obvious I used the spare that shipped with the furnace and its powering the elements.

Question is what caused it. I took the transformer front off to try to find anything obvious like the burnt lug or whatever that happened once before. Nothing looked particularly odd but here's what I got for voltage readings. After the fuse replacement I've got 240 to the primaries as I should.
The secondary voltage tap is marked 60V but I'm only getting 50V reading across the secondary legs. When I read one secondary to ground I get the full 50 volts and when I read the other secondary to ground I get no volts.
The cables to the elements show a draw of 207 amps.

Do I have a fried transformer? Or is there something else going on I don't understand?




Thanks for any help here.


this strikes me more than a coincidence or loose wire in that we both blew fuses on the squeeze cycle. I replaced the 8amps with 100s and am charging again now. I think I'll try ramping down to 1950 slower.

Pete VanderLaan
02-01-2012, 10:49 AM
The rate of your ramp won't make a damn bit of difference.

Lawrence Duckworth
02-01-2012, 11:32 AM
The rate of your ramp won't make a damn bit of difference.

If I ramp down slower.....I'm thinking it will take more time.

Jon Myers
02-01-2012, 12:27 PM
We've been running these bussman fuses for a long time and we've had several die for no obvious reason over the years. We went from 125% of load to 200% and no more problems. We still have a breaker before the transformer at 125% of load, it's never tripped.

Dave Hilty
02-01-2012, 01:58 PM
Lawrence,

I think what Pete (and I also) are saying is that it may be interesting that the fuse blew during the squeeze cycle, this in itself shouldn't trigger a high current state that would blow the fuse. The squeeze cycle is by definition, minimum draw as the set point is below the process temp until the furnace cools to the set point and then modest draw to maintain 1900 or 1950 f.
The only two conditions that I can imagine that would cause these fuses to blow (other than crappy QC in the fuse manufacture) is a short downstream from the fuse causing high current to heat the fuse element to melt it and blow. Downstream from the SCR & fuse are the transformer, the connections to the welding cables and the elements and their strapping. If there are no shorts in the transformer or in the element circuitry then it points back to the fuse itself.

Pete VanderLaan
02-01-2012, 02:06 PM
I actually think a voltage spike is quite enough to do it Dave, as in the kind from electrical storms. They blow really fast.