Yes there is torque loss - and, understandably so... but I have never seen any indication that the VFD is affected by operation this way, and its pretty surprising just what a low hz setting will do - It will just simply stop, and when load is removed it will again operate. And, even on occasion it will throw a default, but here again, I just hit the 'reset' button and away it goes.
It has been mentioned that over heating of the motor can occur when using low hz due to the fan not turning as fast as necessary - and, logically this would seem to be the case, but, I have often had a situation where I was operating at low hz for an extended period and would frequently place my hand on the motor housing to check for heat build up --- have never even had a hint of heat.
As I mentioned, probably 80-90% of my use is in the 30-50hz range.
Did I mention I LOVE my VFD's!!:thumbsup: