Here's one for the books.......
I get a call from a customer on Friday night. They had just installed a new VFC and it had been running off and on for the past day and a half but was now having problems.
It seems the drive was faulting out on "Under Voltage" and would not run. The fault would clear but the minute that they gave the unit a start command, it would trip back out on undervoltage.
I asked the customer to measure the input line and it checks out okay.
I asked them if they could hear the precharge contactor picking up when the unit was energized. They reported to me that they could hear it.
I asked them to measure the bus voltage and watch it as they gave the unit a run command. They report back that the bus voltage was dropping by 30 percent.
That tells me that the pre-charge contactor is not picking up and that the voltage to the bus is still running across the pre-charge resistor.
Saturday morning comes and the customer has brought the unit to our shop. We test it and the precharge contactor is pulling in and all seems well. Then with further inspection we find that although the pre-charge contactor is pulling in, the contacts are not changing state. Wierd!
Just another one of those things that happen from time to time that I thought you may want to know about. We change it out and the unit is now reinstalled at the site and working well. I usually rely on my ear to tell me if the pre-charge is doing it's job.....this time that didn't work.