PowerFlex 755 Current Regulation Issue

[Bmoore14]

We have 4 PowerFlex 755 drives that have been running for years. Each drive runs two motors. Recently, two of the eight motors began cogging at low speeds on a single drive. Runs great at high speeds. We have swapped the output leads from one set of motors to another and the cogging follows the drive.



At this point, we scoped the drive. From my limited knowledge, we were getting all six pulses perfectly. If you look at the picture attached, you'll notice that motor 5-6(RED) current is having a hard time regulating compared to the other drives. I'm certain that this issue is with the drive, but I need some help pin pointing what is causing this issue after years of service. Any suggestions are welcome!

 

[GaryS]

What are you defining as low speed
Below 10 hz depending on the load a standard vfd will cogg
With encoder feedback you can go down to zero speed at full torque on some vfd’s I have
Keep I mine you don’t regulate the current in a vfd you can control / limit / regulate the torque on the motor controlled from a vfd. The current supplied to the motor will be what is needed to control the torque.
The problem you have on your application is that you have 2 motor on each vfd
The vfd you describe it sounds like the load on the one vfd has suddenly increased
Causing the cogging you described. Or it’s possible that a vfd is failing a buss cop failing replace the vfd
The system you described is poorly designed from the start and will never live up to your expiations. It should have been 1 vfd to 1 motor

 

[jraef]

When running multiple motors on one drive, you can only use V/Hz mode, in which case the drive has no ability to control anything differently on each motor. So if the current in that motor is different, it’s something in the motor.

 

[kamenges]

Those current traces aren't for one motor. They are each for a pair of motors. While I agree the drives are set up in V/Hz I would disagree that the issue is necessarily a specific motor. At minimum it would be a motor pair based on the current traces.

My vote is for a failing or failed IPM.

Keith