We have an chiller / cooling tower installation that is using these new direct drive / permanent magnet motor application that requires a special VFD that runs at about 7Hz maximum speed. I do know that these were the first of this size in this application. We have three of these installed. Over the four years of their operation, we've experienced four failures, two on each of the two cells. The last failure was about a week ago. The third cell has been fine so far. While searching for possible answers, I came across this thread: which seems similar to my situation. (Hey, itsmoked, if you see this, yes, I am still at skool - and yes this is from there).
What we are seeing is evidence of arcing on the DC bus and blown MOV's on the rectifier circuit. See attached image.
We have several ION meters on the building transformers at this plant from three substation feeds / buses that are all served by the same utility. No other ION meter recorded a disturbance at the time and it doesn't appear to have been caused by an over voltage condition. Rather it appears that the current surges and this is causing the voltage disturbance. What is causing the failure is unknown. Similarly, I don't know if the problem is a result of something on the inverter / DC side of the system or on the input. Normally, the immediate go to culprit is the input power, but in this case the evidence doesn't seem to support it. We sent the last failed unit back to the manufacturer and asked for an analysis, but despite repeated attempts to gain information they have been mute as far as their response.
We would really like to get to the bottom of this. We have considered putting a meter on either the input and or the output and looking at the quality, but given the time frame between incidents, it is questionable whether or not it would show us much of anything.
Any thoughts, comments, or ideas, would be helpful.
What we are seeing is evidence of arcing on the DC bus and blown MOV's on the rectifier circuit. See attached image.
We have several ION meters on the building transformers at this plant from three substation feeds / buses that are all served by the same utility. No other ION meter recorded a disturbance at the time and it doesn't appear to have been caused by an over voltage condition. Rather it appears that the current surges and this is causing the voltage disturbance. What is causing the failure is unknown. Similarly, I don't know if the problem is a result of something on the inverter / DC side of the system or on the input. Normally, the immediate go to culprit is the input power, but in this case the evidence doesn't seem to support it. We sent the last failed unit back to the manufacturer and asked for an analysis, but despite repeated attempts to gain information they have been mute as far as their response.
We would really like to get to the bottom of this. We have considered putting a meter on either the input and or the output and looking at the quality, but given the time frame between incidents, it is questionable whether or not it would show us much of anything.
Any thoughts, comments, or ideas, would be helpful.