Motor disconnect and sizing

Jraef said:
Personally, I have experienced only one rectifier failure in my 30+ years of focusing on VFD projects, and I have never once experienced a complete line-to-load meltdown/short. Nor have I ever heard of one. So I think the new rules are silliness because they are theoretical, not experiential.

But my opinion is meaningless…
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try hooking up a drive backwards. I've seen it. Exploded.
 
was a really simple mistake with one of my guys adding a drive to a fan on a 40-ton RTU. He got the line / load reversed on accident. Bang. You only get 1 smoke test.
 
Jraef said:
Personally, I have experienced only one rectifier failure in my 30+ years of focusing on VFD projects, and I have never once experienced a complete line-to-load meltdown/short. Nor have I ever heard of one. So I think the new rules are silliness because they are theoretical, not experiential.

But my opinion is meaningless…
Click to expand...
Not to most of us here!
 
herding_cats said:
was a really simple mistake with one of my guys adding a drive to a fan on a 40-ton RTU. He got the line / load reversed on accident. Bang. You only get 1 smoke test.
Click to expand...
Had to come into a project where they hired an HVAC contractor to hook up a 250HP VFD as part of a packaged air handler. It wasn't working and the HVAC guy had no clue how to troubleshoot it. I got there and it was blown; soot all around the transistor packs, DC bus caps had spewed their guts all over everything, firing boards were toast. Took a look at the wiring and the line terminals were going to the motor leads, the load terminals were going to the breaker. I pointed that out to the HVAC guy, he didn't see the problem. In his words:

"The motor terminals said "Line", the breaker terminals said "Load", so I connected Line to Line and Load to Load. What's the problem?"
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They wanted me to repair it, I told them there was no field fix for that. They had to replace it and MAYBE send it to the factory to be rebuilt as a spare, but it was unlikely to be worth it. Everyone up and down the line on that project was in a heap of hurt, and the GC tried to blame me, saying that in my testing, I somehow caused the transistors to blow! Kill-the-Messager syndrome I guess... I eventually got the contract from the HVAC contractor to replace the VFD, but I had demanded to be paid up front because I didn't want to get caught up in the blame game and not get paid on time.
 
Jraef said:
Had to come into a project where they hired an HVAC contractor to hook up a 250HP VFD as part of a packaged air handler. It wasn't working and the HVAC guy had no clue how to troubleshoot it. I got there and it was blown; soot all around the transistor packs, DC bus caps had spewed their guts all over everything, firing boards were toast. Took a look at the wiring and the line terminals were going to the motor leads, the load terminals were going to the breaker. I pointed that out to the HVAC guy, he didn't see the problem. In his words:



They wanted me to repair it, I told them there was no field fix for that. They had to replace it and MAYBE send it to the factory to be rebuilt as a spare, but it was unlikely to be worth it. Everyone up and down the line on that project was in a heap of hurt, and the GC tried to blame me, saying that in my testing, I somehow caused the transistors to blow! Kill-the-Messager syndrome I guess... I eventually got the contract from the HVAC contractor to replace the VFD, but I had demanded to be paid up front because I didn't want to get caught up in the blame game and not get paid on time.
Click to expand...
I don't recall why but a long time ago someone connected a DC motor to the brake terminals on a VFD. I heard about it after the fact. No doubt what I heard was nowhere near the truth, as is often the case with such stories.

I used to look at things that were not working and the only thing I ever noticed was it was never doing what they told me it was.
 
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