PowerFlex 755 output frequency not reaching the command speed

[PaulKim1003 3]

Hi guys,

I have 4 of PowerFlex 755 and one of the powerflex is weird.

It can not reach the its command speed. 

When its command speed is 60, it only produces 20.

Has anyone had this issue before?

Thanks

[BobLfoot 301]

The usual suspects come to mind.  Max Frequency and current limiting.  Also accidental set to a preset speed or front panel pot.

[ElectronGuru 81]

There are only about a million things that could cause this. The PF755 gets it's speed command via a parameter called "Speed Reference A" or "-B". When troubleshooting speed problems the first two things I ask myself are:

1. What is the speed reference?
2. What is its value?

So is the speed reference coming from the HIM? From ControlLogix? From an analog input? Once you've identified that, figure out what the value is and determine whether the drive is responding correctly. You said the commanded speed ref is 60 but it's only producing 40. Are you sure there's not a preset speed that's been activated? Has an input been programmed and activated to make the drive switch to Speed Ref B? That's the most obvious stuff to start with, and I imagine you've already checked. 

Use Connected Components Workbench (CCW) or the older Drive Executive to go online with the drive while it's running and view parameters 935 and 936, Drive Status 1 and 2. This will tell you at a glance what the drive thinks it's doing. So if @BobLfoot is correct and you've hit a current limit, it should jump off the page at you in P935.

In no particular order, here are some other ideas;

• Are these drives new, or have been installed for a while and just started doing this? If new, it's likely in the drive configuration. If this is a new problem on an existing system that was previously working, starting looking into what changed. Did someone screw with the parameters? Was someone digging around in a control cabinet and knock a wire loose? Etc
• Don't forget to check for obvious things outside of the drive like mechanical binding, bad motor bearings, etc.
• Do all four drives do the same thing? If so, use Drive Executive or Connected CCW to run a compare tool against a working drive to see what's different.
• CCW has a feature that allows you to view only the parameters that have been changes from their default values. The is a really helpful tool is you suspect programming.
• Continuing in that same line of thought, particularly on new installations, mis-programmed parameters are the likely cause of drives not responding correctly. If the motor name plate says max speed is 1200RPM but you don't want your process to run more than 900, then set the motor name plate parameter value to 900, that will give you a lot of scaling / speed control issues. (Proper to set name plate parameter to 1200, and the separate "Max FWD Speed" parameter to 900). Are you sending a signal that you think calls for 60Hz but the drive is programmed for RPM?
• The same thing applies to encoder programming, if you're using one. Leaving the encoder resolution at the default value of 1024PPR when you actually have a 3K will cause issues. 

[VFD Guy 13]

I have ran into this many times. I’m a big fan of divide and conquer. 1st. Figure out if the drive is doing what you’re commanding. Compare the commanded reference with output frequency. If they don’t match, then you’ve hit a limit. Look at the at limit parameter to see what’s going on. The 750 will reduce the output frequency to reduce the current. You can follow the block diagrams in the manual to see which regular is at a limit

[Ken Moore 23]

If you run in manual via the HIM, does the speed go to setpoint?

[alan_505 24]

look at parameter 2 this is the commanded speed value it should be the same as parameter 1 the output speed.

 

 

Alan