Replacing infinite switches

[Callie Beller Diesel]

So this was REALLY not how I wanted to start a Monday morning. 

My kiln had been having issues (second hand 40 year old Cress with the thumbwheel, 6 elements,2 infinite switches, 2 relays and a cone sitter/timer. Wiring diagram here, if you’re interested. It usually turns itself up slowly through some mechanical/electrical math magic that I don’t fully understand, but think is kind of cool when it’s working.  2 weeks ago, it stopped turning itself up in the middle of a bisque. I finished the firing manually, and began a bunch of troubleshooting. Between my local clay supplier in Canada, and a few phone calls to Arturo at Cress and a few DM’s to Neil, we figured it was one of the infinite switches. Because the kiln is 40 years old, the same switch wasn’t available, so a different one was used as a replacement.

The electrician that does kiln repairs for my clay supplier here in Calgary swapped out one of the infinite switches, and I ran a dry kiln test for about 4 hours. It seemed to be working, if running somewhat fast, but I attributed it to a new part being a bit more efficient (?) than the old one that had failed.  So I loaded the kiln to fire a glaze overnight.  I turned it on at midnight, set an alarm for 7 AM, as the firing setting I had it on usually takes about 9 hours. Normally this timing allows me to keep an eye on the last few hours of the firing, and do a manual drop and hold soak.

Long story short, the kiln over fired by a lot. Probably cone 9+ judging from the colour of the clay, and the cone pack. The cone in the sitter must have gotten stuck, and the thumbwheel seems to have also stopped progressing itself at about 6 (out of 10). Apparently the fact that the cone pack couldn’t be seen wasn’t bad placement on my part, they were just all flat.

So my question is, are all 240V infinite switches created equal, or do they conduct varying amounts of current? Is the variance due to a new part that I now need to compensate for, or is it because the part is faulty? Or is there a problem with something else entirely?

 

 

[neilestrick]

@Callie Beller DieselIf the over firing was cause by the switch, I would expect it t only be overfired in one section, like maybe the section with the new switch got hotter faster than the section with the old switch, or vice versa. Which switch did you replace? Where was the cone pack? Where is the sitter tube located- right in the middle, or toward the top or bottom?

[Callie Beller Diesel]

@neilestrick The two switches work in tandem to control the entire kiln, not just one section. The sitter tube is right in the middle, and I put the cone pack on the bottom, because it’s usually the coldest part of the kiln.

[neilestrick]

3 minutes ago, Callie Beller Diesel said:

@neilestrick The two switches work in tandem to control the entire kiln, not just one section. The sitter tube is right in the middle, and I put the cone pack on the bottom, because it’s usually the coldest part of the kiln.

The wiring diagram shows that the bottom element is powered directly by the bottom switch.