# No Load on a VFD



## Cow (Jan 16, 2008)

Why is running a VFD with no load connected bad? First I've heard this...


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## micromind (Aug 11, 2007)

I've tested tons of VFDs with no load connected. Never had any sort of problem. 

Now it the VFD is operating no-load and a motor load is suddenly added (like closing a disconnect switch), bad things will happen.......


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## JRaef (Mar 23, 2009)

In what's called "V/Hz mode", a VFD doesn't technically even know if the motor is connected or not, other than it will be monitoring current for the purposes of OL protection, so it will think "_Hey, no OL going on here!_ _Proceed as commanded_.".

But now with Sensorless Vector Control, the VFD is looking for a current feedback from the motor, it uses that to determine the proper vector calcs. So no current, nothing to calculate from and the drive will trip on "Load Loss". On the PF700s, V/Hz mode was a true V/Hz mode, no feedback at all, so as long as you are using it in V/Hz, it's going to be fine with not having a load connected. If you are using it in SVC mode or Flux Vector Control mode (i.e. encoder feedback), it will not run; no more than a few seconds in SVC, about 1 second in FVC before it trips. Newer generations of VFDs, like the PF750, still use the current feedback loop for better motor performance even if in V/Hz mode, so they will trip off too now.


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