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Old 08-31-2015, 01:03 PM   #1920 (permalink)
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Originally Posted by MPaulHolmes View Post
So, the main contactor is basically welded closed? When did that happen?
Not sure, but likely right after the firmware was flashed on the AC controller. I'll get the exact sequence written up and posted when I get home and see my notes.

It's possible that the resistance could be almost 0Ohm. My fluke multimeter sometimes says 0.1Ohm when it's really almost zero ohm. Fran told me once that the mosfets can fail shorted if the capacitor bank fills up too fast. I don't know the mechanism for why, but evidently he's seen that. So, do you have something like this:

+125v Battery Pack --------- 10 Ohm ---------- switch ------------ AC and DC cap +

+125v battery pack -------- 0.1Ohm (contactor) --------------- AC and DC cap +

+125v pack ground --------- AC and DC controllers ground
Yes. The gigavacs don't have a set of 12V contacts that show when the large contacts are closed. Motor starters in the 3 phase world normally do. I've only had a dozen failures on AC motor starters across 700+ motors in our mill during the last 12 years, but the contactors do weld themselves shut on a rare occasion.

That would mean the precharge part isn't really doing anything, since all the current would be hogged by the 0.1Ohm part of the circuit. That could explain why the failure happened after closing the DC breaker, but before closing the main contactor.
That's my guess so far. Further investigation tonight.

I wouldn't do anymore tests with the AC unless you remove the contactor from the circuit, and maybe fill up the cap manually by some other means. Maybe something like this:
permanently wire 10Ohm resistor from +125v pack to the AC cap + (well, permanently just for testing). No breaker in the way:

+125v battery pack ------- 10Ohm resistor ----------- AC cap +

Then, for the high current path:
+125v battery pack ------- dc breaker ---------------- AC cap +

+125v ground -------------- AC Cap minus.

Then, because the caps are always full, you can open and close the breaker any time you want, and don't have to worry. Careful about running the code with the breaker open, though, since I caught a precharge resistor on fire by doing that. haha
That will work. I will likely replace the gigavac with the one from the precharge circuit and leave the resistor across the gigavac as you show. The 80 amp DC breaker I have on the pack (the one that tripped during the failure) is also my pack switch.
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