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I agree with those (5) drive techs. I mentioned 18-pulse for awareness of the technology, as there is an install base of them out there, but I never recommend them for new installations. With an 18-pulse, it essentially rectifies (9) phases to make the DC bus, instead of the typical (3) phases. Even though many expect the voltage drop across the phase shifting transformer, and additional/balancing reactor, to result in a lower DC bus... with 18-pulse we end up seeing a bit higher DC bus with the rectification of (9) phases instead of (3). Had they let you power it down to troubleshoot, and assuming the jobsite's 480V power wasn't low, the first thing I'd be looking at is those (9) phases after the transformer. Other than the obvious voltage magnitude test points between input and DC bus, I'd also be casting a critical eye on the performance of those (9) phases. Each was likely fused, check the (9) fuses to see if any were open. After the fuses, I'd check the 18 diodes, were any of those bad/open. When in doubt, current is the great equalizer of troubleshooting - check for balanced current across the (9) phases. (Some of those tests are easier to do than others - from a safety aspect, I'd have started with the fuses and diode check as those are power down tests). Complexity of troubleshooting - another reason I personally don't like 18-pulse:)