So. I get called to look at a 'desktop furnace' that has a problem. It's a 'tabletop' model they tell me. I think to myself, "What can the problem be with some Nichrome wire, a PID controller, and a 'solid state relay'"? Silly me.
I get there and I don't even recognize the 7 foot tall 19" rack enclosure, three feet away, is the power supply.
Turns out the thing's 20kW.
It has the PID and that's all
of what I expected.
The PID puts out 0-5VDC to a HALMAR phase controller (PC).
The PC dices up the supposed 240V into back-end sine-waves that feed a HUGE transformer that fills the bottom of the rack.
That turns the three phases into two phases of lower voltage DC at up to 200A (100%). These two outputs are crammed thru two banks of 4 elements thru fat braided straps off of bus bar.
Can someone explain why they didn't just run elements at LINE voltage but instead ran it thru this massive transformer?
It just happens that the current is twice the power setting percentage which is nice.
When the power setting gets up to about 60% something shifts.
For example if you turn it up to 75% 150A are running thru the elements. After about 5~7 seconds they drop by about 40A. Just spontaneously drops. Simultaneously the two currents to the heaters stop being nearly identical. One might drop to 30% of the other. This manifests itself as two different voltages at the bus bars. One bank of elements may stop glowing red. If you switch the cables feeding the two banks of heaters the problem follows.
Also notable is that the ever-present humming gets an angry tinge to it. The front panel breaker gets hot after a few minutes instead of just above room temperature after hours at 100%.
This all sounds to me like one or more of the six SCRs are cutting out or back. They said they "checked them" but I know for a fact they don't have the chops to do it correctly.
Have you seen this type behavior on a phase controller? Do you recall what the problem was?
I'm going to start with the Gunnar SCR tests but it would be nice to hear any other ideas. I've pulled the PC and have it at my office. I can hook it to 3ph 240 eventually to run live tests.
kcress - http://www.flaminsystems.com