Powerflex 753 not reaching AI Setpoint, no faults or alarms

[Graham EDL 0]

Hi,

We went to one of our sites last week to investigate a Powerflex 753 connected to a 132kW motor that wasn't performing as expected.

 

The AI is a simple 0-20mA. When we give the mA for anything above about 17.50Hz the drive does not output any further and the volts is around 180V.

 

Anything mA lower than this value and the drive operates as expected.

 

When we check in DriveExplorer there are no faults or alarms when trying to drive the motor greater than the value above.  

 

We changed the VSD for a spare and downloaded the program from DriveExplorer. We restarted the drive operates as expected at all setpoints.

Attached is the parameter list for the drive now.

Previously

1:0.1 was 17.50 Hz

1:0.2 was >18Hz

1:0.3 was around 17.50Hz

 whereas as the new listing shows as expected. 

(We are trying to control to a SP of 9kPa and tuning the PID loop in the Micrologix will be the next item on the list so not too worried about the difference at the moment.)

 

Does anyone know why there wouldn't be faults or alarms for the drive not reaching 1:0.2 when >18Hz?

 

Thanks.

[ElectronGuru 84]

A couple of times in my career I've seen a diode in the converter section (input side) of a drive go bad and it didn't alarm or fault because bus voltage was maintained, but performance did suffer quite a bit. I've seen main control boards go bad and not throw alarms or faults because the board (being broken) didn't know it was broke. Also, someone could have tampered with the parameter settings at the drive, such as setting the current limit too low. This would definitely keep you from accelerating regardless of commanded speed reference, and not throw an alarm. When I'm troubleshooting these types of problems on a PF753 or 755 I go strait to parameters 935 and 936, Drive Status 1 & 2. If you want to know what the drive *thinks* it's doing (compared to what it's actually doing) this is a good place to start. If the drive should be accelerating based on the speed reference input, is the drive actually trying to accelerate as indicated by bit 4? Or did it hit a current or other limit as indicated by bits 26 and 27? I think every PowerFlex drive has a Drive Status parameter but the 750s give the most information, and have been very helpful to me when troubleshooting non-alarm/non-fault performance issues.

I can't remember if the outdated Drive Explorer (last release was 2011) has a compare tool or not, but I often run compares in Drive Executive or Connected Components Workbench (CCW) when I'm considering these things. Most often, people are not randomly changing parameter values, but running the compare tool against a known good program is an easy way to be sure, which allows me to quickly check that off the list of possibilities. The image below is from Drive Executive, which has to be purchased from your local Rockwell distributor. CCW is free and can be directly downloaded from the Rockwell site without a license. I've found that both are more intuitive and easier to use than Explorer.

Since replacing the drive seems to have fixed your problem, the cause is almost certainly in the original drive. But that doesn't automatically mean it was a hardware failure. If it were me, I'd go through it with a multimeter and scope to check input diodes, output IGBTs, etc. If I got no joy there, I'd power it up at the shop, run a compare tool against a known good program, and then start putting it through it's paces.

All that said, while nothing breaks without a reason, I've thrown away a drive or two without ever having found out what the true failure cause was. 

Hope this helps.

[VFD Guy 16]

Always check 945 for at limit status. Simplest way to see if it’s a hardware issue is to remove output wires, set to vhz the run to full speed. Measure output v with low pass filter meter and check you have full voltage. Not a guarantee that if voltage is good drive is good. But if voltage is not balanced and full then you immediately know a hardware issue

[Graham EDL 0]

Thanks guys. OPs are doing a service exchange on the drive but I'll try to find out the cause.

 

My initial thought was it is something in the Power electronics. 

[Graham EDL 0]

On 11/09/2024 at 11:50 PM, ElectronGuru said:

...If the drive should be accelerating based on the speed reference input, is the drive actually trying to accelerate as indicated by bit 4? Or did it hit a current or other limit as indicated by bits 26 and 27? I think every PowerFlex drive has a Drive Status parameter but the 750s give the most information, and have been very helpful to me when troubleshooting non-alarm/non-fault performance issues....

It was always accelerating. We could see on the new drive that it changed to accelerating, decelerating and maintaining speed (I think it said but it was longer ago than yesterday so hard to say).

[ElectronGuru 84]

If the old drive was always accelerating, then it almost certainly was hitting a limit. Again, I'd refer to parameters 935 to see if you've hit the current or other limit, and as @VFD Guy suggested, 945 if you've hit any other limit. 

Usually when a drive is bouncing rapidly between accelerating, at speed, and decelerating, it's pretty much "at speed" and just not regulated well. I see this "hunting" a lot in V/Hz mode or when a low resolution encoder is being used. Unless you're in FV mode with a higher resolution feedback, it can be difficult to lock in and maintain an exact frequency output, and a constant At Speed indication. 

 

Edited 20 September by ElectronGuru
Accidentally submitted before finishing comment.

[Graham EDL 0]

On 20/09/2024 at 5:06 AM, ElectronGuru said:

If the old drive was always accelerating, then it almost certainly was hitting a limit. Again, I'd refer to parameters 935 to see if you've hit the current or other limit, and as @VFD Guy suggested, 945 if you've hit any other limit. 

Usually when a drive is bouncing rapidly between accelerating, at speed, and decelerating, it's pretty much "at speed" and just not regulated well. I see this "hunting" a lot in V/Hz mode or when a low resolution encoder is being used. Unless you're in FV mode with a higher resolution feedback, it can be difficult to lock in and maintain an exact frequency output, and a constant At Speed indication. 

 

That would make sense that it's not regulating well. We are trying to maintain a 9kPa methane gas pressure and can see the PID PV and CV in the compact logix is all over the shop. I think it was just chucked in there without much tuning.

I don't know who would have set any limits, the OPs guys wouldn't go near it and the parameters have been transferred to the new drive without any problems.

Thanks for your help.