7 powerflex 700 drives on controlnet, 1 drive runs faster than other

I've got 7 Powerflex 700 drives running a varying number of motors on each drive. I have one drive that is consistently running higher speed than the others, enough to cause trouble. The speed reference is ControlNet port 5. I've confirmed that the speed being sent to the drive is the same as the one next to it, but their Hz are differing by as much as 25-30%. It's constantly tripping out for overvoltage due to the speed difference. I've reset the drive and reentered all the parameters, and verified everything is set up the same as the rest. I'm using V/Hz since the drive is running two motors.

My temporary patch is to go to the ControlNet adapter parameter 27 and set it at 75% to get it close to what it should be, although its still running too fast.

Controller is PLC5.

I'm just out of ideas.

How are you monitoring the output frequency, exactly ? Or are you physically measuring a different shaft speed of the motor than you expect ?

I think that if the drive were actually putting out a frequency that corresponds to the physical speed of the motor, there wouldn't be regeneration and it wouldn't trip on high bus voltage.

What you're describing sounds to me like an overhauling load forcing the motor to turn faster than the output frequency of the drive.

I'm getting the speed difference from the Commanded Speed parameter.

What are they driving? I recently ran into a situation almost just like this where they were driving blowers. 10 blowers run from 5 VFDs, 2 blowers on each VFD. Of the 5 VFDs, whichever one came on last would trip off on over voltage, it didn't matter which VFD is was, they all did it. A real puzzler.

What I discovered was that all of the blowers fed a common header and there was no flow prevention. In addition, the use was such that with 8 of the blowers running, they already had more than enough flow. So when they turned on the last VFD / blower pair, the flow from the other 8 was causing a venturi effect with enough force that it caused an over speed of the blowers. Once the VFD came on line and energized the windings, that's what turned them into generators.