PowerFlex Drive output frequency steady for years beginning to jump around.

Good Evening ,

I have a Powerflex drive that has been running steady at about 39 Hz for years , but is beginning to swing 39.6 Hz , 40.5 Hz , 38.6 Hz , etc. Steady speed on this machine is important . What do you guys think could cause this , bad bearing , the drive , etc. ? I imagine the Frequency Command could cause it , but it is just a constant number from a recipe . Most likely, I think it would be bad bearings , motor , gearbox , etc. , correct ?

For educational purposes , what would the frequency drive be trying to do , by swinging within this range ? I imagine it would be increasing speed or torque , to overcome a restriction , then it lets go , so now it needs to cut back , correct ? Sometimes these issues seem difficult to find , with bearings , rollers , etc.

Thanks so much for your help .

You need to check if it is being commanded to make these changing frequencies, or if it is a result of something else.

What model PowerFlex? Most of them have a parameter called "Commanded Frequency". Watch that to see if it is changing and report back.

The charge capacitors in the rectifier have a fixed lifespan, usually about 5-10 years. If the drive is old enough they might be going bad.

If it isn't all that expensive of a drive, and you have spares on hand AND there can be some small downtime on the machine, I would swap it out with a known-good drive and see if the problem disappears. This can both begin your investigation and possibly solve the problem. This is one of the few cases where I would suggest something like that to troubleshoot a problem. You can pull your hair out trying to find out what is causing a problem like that.

If you have a FLIR, these are fantastic devices for spotting problems with bearings, gearboxes, etc.

Again, it sort of depends on which model and what control mode you are using, ie Sensorless Vector Control, V/Hz, etc. Diffetent models have different features that may or may not affect your operation. Post some more details.

Failing capacitors don’t generally affect the output frequency drifting.

One other possibility...

Perhaps for the last however many years, the drive has been set to display the commanded frequency. So all day it sat there saying "39Hz".

Maybe someone recently changed it to display the actual frequency, which might not be quite as "rock steady", but is still within the range of what you might expect.

I once got a call back to a facility where the drive I'd recently put in was "all over the place, the speed is completely erratic". It was using an analog speed reference was set to be running at 50Hz. The display showed the drive flicking between 49.99Hz and 50.00Hz. Because all four digits kept changing and flickering, management were waving their arms above their head in panic and postulating that this drive's erratic speed control was the cause of the jam ups that kept stopping production.

I set the speed reference in the PLC to 50.2Hz, changed the display to show Hz with no decimal places, and voila, the speed was miraculously rock steady!

Then I leaned against the panel and watched about half a dozen jam ups eventuate, worked out what the operators were doing wrong to cause them, and quietly let management know.

Anyway, as others have mentioned, we're all just guessing until we have some more information. Let us know the drive part number, and check the drive parameters to find out where it's getting its speed reference.

ASF said:

... I set the speed reference in the PLC to 50.2Hz, changed the display to show Hz with no decimal places, and voila, the speed was miraculously rock steady!...

Click to expand...

Gotta love it!

Accuracy is defined by how the information is portrayed.

You didn't let them see you do that did you? We have to keep the secrets among the magicians...

While it is true that knowing what drive model we are dealing with would be helpful, in the meantime you might want to take a look at motor current. If it is jumping all over the place while the frequency changes a bit, you could very well be dealing with load related issues and are seeing the VFD trying to react to them. Varying current could be an early sign that the motor is beginning to fail.

As mentioned, the exact source could be a little tough to isolate short of exchanging for a known-good drive.