Does a frequency drive need external overload protection (Per UL)?

[Pat H]

I've got a control panel I'm building with a 25HP frequency drive for a single motor application. I've got fuses to protect it from short circuits. The freq drive has built in electronic overload protection. I remember reading the NEC a while back and it said a drive needs external overload protection even if the drive has it built in. I'm not sure if this is still true, or if it is relevant to UL 508A panels.

If I put in a manual motor starter, I would have external overload protection along with short circuit protection. Both forms of protection from the MMS would be redundant though.

Any thoughts?

Pat

 

[david luchini]

If the drive has motor overload equipment included, no additional overload protection is required. 430.124(A).

If there was a bypass circuit, overload would need to be provided in the bypass circuit. Perhaps that is what you remember reading.

 

[Jraef]

There is a slight glitch in this, not commonly understood.

IF the manufacturer was honest enough to get the UL listing for the VFD as a "Motor Controller", then they must provide not only the motor thermal OL protection in the VFD, but also the motor Short Circuit protection as well. Because a VFD can be programmed to control a much smaller motor than it is rated for, the fuses AHEAD of the VFD often end up incorrectly sized for the motor anyway, even if you ignore the fact that there is no longer a direct connection to input and output current. So years ago, UL began requiring VFD mfrs to provide both. This will be the case with all of the major VFD mfrs here in the US and most of the larger European and Japanese ones.

But, some "less than scrupulous" bottom feeder 2nd and 3rd tier VFD suppliers in Asia have found a loophole in that they do NOT get UL listing as a "motor controller", but rather as a "power converter", same as a UPS or a ballast. That listing does NOT have the same load circuit protection requirements, leaving the end user to have to provide that themselves. Their manual MUST state this, so if you are suspicious, look in the manual. If it tells you that you must provide an external OL relay, that is why.

Then the lowest of the low bottom feeders, the cheap-o Chinese drives making it onto Fleabay lately who's name sounds like vomiting, are not UL listed AT ALL, so even if they SAY they have it, I would not trust it. But then again, those drives are the worst POS things out there so I would never be in that position any