weber wrote: Your anti-parallel diodes are all blown and the IGBT's are going into reverse breakdown at about 20 volts?
Interesting (and terrifying) thought. I took my meter out and measured the Vf for each IGBT. None are open circuit in the forward direction and none are shorted in the reverse direction. Though I do find it odd that I'm getting a drop of 0.33v across both diodes in one module and 0.22v in both diodes in the other two modules. For comparison, a brand new, just removed from the packaging to do this check module has 0.33, so that -may- be noteworthy. Either way, both numbers are very small for silicon diodes, but not unusual for IGBTs (one of my 600a modules also has the 0.33v drop). I'm not worried about the diode drop measurement because of the consistency I'm seeing between phases (see below).
Here are some useful and possibly telling scope traces:
Here is a trace of the voltage between E2 and C2E1 for one module with no load connected:
Here is a trace of the voltage between C1 and C2E1 for one module with my 11kw motor connected:
And for comparison, the gate drive pules for G2:
I am pleased to see that the traces with and without load are practically identical (I checked this across all phases with and without load). What I am not so pleased to see is the 20v deltaY between the phase and the 0v reference when the IGBT is on.
The gates voltages are exactly within spec. +18.2v, -5v, so I don't see how these could be "partially" on, but there's that 20v I keep seeing. I'd double and triple checked that my probes and scope settings match.
Also, I verified with my multimeter that this trace from before reads at about 25vac:
Any more thoughts from anyone?