VFD Altivar Hoist Problem

Interestingly, this is the fourth time in a month that I have had questions brought to me about Schneider ATV71 drives and problems with hoist applications.

One thing I discovered in the other issues was that Schneider has a different way to doing what is commonly called "Torque Proving" with other VFD mfrs. Torque Proving is a necessary feature for hoisting applications where there is a mechanical holding brake. What it is supposed to do is that when a movement command is given, the brake must be released, but BEFORE that happens, the VFD puts out a torque command and then TESTS the motor circuit to PROVE that it is actually providing the necessary torque to hold the load. That way when the mechanical holding brake is released, the VFD is ALREADY capable of holding the load in place with the motor.

What it appears Schneider does differently is that they ESTIMATE the output torque ahead of the brake release, then adapt to the actual load afterward. The danger there is that if the estimate is wrong, the load can START to drop and then it takes MORE torque to catch and stop it than it did to hold it in place. When I worked on the cranes at Boeing that lifted and moved 747 and 777 aircraft, that would have NEVER passed muster.

But back to this situation, the issue might be with that "estimate" system, in that if the load is drastically different from one moment to the next, it may be estimating based on erroneous data. So has anything changed in the way you are USING these hoists? One danger I can think of is that the VFD estimates the torque based upon the torque it was applying when the brake was last set. If you set the brake, then add load to the hoist, the estimate is now wrong.