Motor/VFD Protection 1

[Power2all]

Does a VFD provide both overcurrent and Oveload protection? The situation is that I have 125hp motor with a 200hp VFD. I do not want the upsteam breaker to be the limiting factor if it does not have to be... so I would like to size it for 200hp not 125hp. Is the motor still adequately protected?

 

[LionelHutz]

Yes, every VFD I'm familiar with has a hardware or hard coded current trip (typically 2pu -3pu of the drive rating) and a built in motor overload.

 

[Power2all]

Great Thanks!

 

[jraef]

Using HP indicates you are probably in North America, which means the VFD is likely UL/cUL listed. Part of the UL listing necessities is that the drive itself MUST provide full protection for the motor connected to it, both short circuit and running thermal overload. This is because in essence, the VFD power going to the motor is more like a separate source, somewhat isolated from devices ahead of the VFD by the conversion process (AC to DC back to Pseudo AC). I don't know for sure if IEC requires that, but I can't imagine a VFD mfr making a product now that cannot be used in every part of the world.

But that said, because it becomes slightly more difficult to measure current accurately over an extreme range, you sometimes see restrictions in the MINIMUM size of motor that you can connect behind a VFD. In my experience it varies between no smaller than 70% of the drive rating, to no restriction at all. So you MUST check your installation manual because your motor is 62.5% of the drive rating and if you find the one that is limited to 70%, you can't do it. You are probably OK, but just check to be sure.


"Will work for salami"

 

[rhatcher]

jraef has made a good point (lps) to answer your question. Check the VFD manual to determine the minimum overcurrent setting that is allowed.