AB PF755 Drive Problem

[thaipinoy15]

Hello to all,

I am new here in this forum. I am working on a problem that i have regarding Allen Bradly PF755 drive. The problem is the drive does not stop/ ramp down even if the PLC has already sent a stop command on it. Rather than stopping it accelerates more, sometimes up to the Maximum forward speed that i have set on the limits. I already checked the PLC program and i dont see any problem on it. I have also checked the belt that is connected on the motor and it is fine. I am now lost on where this problem came from.

Does anyone know this problem or have experience with AB PF755 drives? I really need your help.

Thanks in advance.

 

[petersonra]

You are not giving us much information to work with.

What do you mean by "stop" command?

Most times the PLC is either connected via hard wired I/O or by some communications link.

A 755 is a bit of a different animal than a typical drive. Often they are used on unusual applications and may not work as one might anticipate even if they are correctly functioning because of how they are configured.

Is this something that has worked fine for several years and suddenly developed this problem? Or is it a new drive that you are having trouble with?

What is the application? Is it part of some machine supplied by someone else? If so it might be wise to contact the supplier. You can also contact AB tech support for help.

 

[Jraef]

I just ran into something similar to this yesterday as a matter of fact. Are you by chance in V/Hz mode and not in Sensorless Vector or Flux Vector mode? If so, are you using it for Decel control, and NOT using Dynamic Braking? Here's why.

In V/Hz mode, there is no need for Slip Compensation in most cases. Slip Comp is used in high precision velocity or torque applications to maintain a slip percentage at any speed. You would not typically use V/Hz control in those situations anyway, so what can happen is that Slip Comp overrides a speed command to maintain that slip percent, which is useful in SVC or FV mode. If you use Coast to Stop, Slip Comp is ignored. If you use DB, then DC Bus Regulation takes over and Slip Comp is ignored. But if you use V/Hz control, Decel, but NO DB, then nothing disables the Slip Comp and it overrides the speed command. EVENTUALLY the motor would slow down, but Slip Comp takes away your control of that and it might take 20 minutes or an hour, you never know.

But the 755, because it is intended to be a high-end drive that most people would use in SVC or FV mode, will default the "Autotune" parameter to "Calculate". If you use V/Hz mode AND you use Decel, AND you do not use Dynamic Braking, then you must disable the Slip Compensation. You do that by going to that parameter and setting the slip RPM to "zero". But with the Autotune left at "Calculate", the drive will immediately recalculate the slip RPM and load it BACK into that parameter, which sometimes people don't notice. So you must go into the Autotune parameter, set it to "Ready", then go back to the Slip Comp and set it to zero.

I'm not sure if this might be a recent change, because yesterday, Tech Support seemed unaware of it. They (and I) identified the lack of speed change as a Slip Comp override, but didn't mention the effect of the Autotune setting thwarting efforts to fix it. I suspect that the Autotune parameter used to be defaulted to "Ready", and in some recent firmware upgrade, they changed it to "Calculate" because people using SVC and FV were forgetting to change it, but did so without realizing the side consequence it has on V/Hz operation with Decel and no DB. I'm going to be bringing it up later today.

 

[thaipinoy15]

I just ran into something similar to this yesterday as a matter of fact. Are you by chance in V/Hz mode and not in Sensorless Vector or Flux Vector mode? If so, are you using it for Decel control, and NOT using Dynamic Braking? Here's why.

In V/Hz mode, there is no need for Slip Compensation in most cases. Slip Comp is used in high precision velocity or torque applications to maintain a slip percentage at any speed. You would not typically use V/Hz control in those situations anyway, so what can happen is that Slip Comp overrides a speed command to maintain that slip percent, which is useful in SVC or FV mode. If you use Coast to Stop, Slip Comp is ignored. If you use DB, then DC Bus Regulation takes over and Slip Comp is ignored. But if you use V/Hz control, Decel, but NO DB, then nothing disables the Slip Comp and it overrides the speed command. EVENTUALLY the motor would slow down, but Slip Comp takes away your control of that and it might take 20 minutes or an hour, you never know.

But the 755, because it is intended to be a high-end drive that most people would use in SVC or FV mode, will default the "Autotune" parameter to "Calculate". If you use V/Hz mode AND you use Decel, AND you do not use Dynamic Braking, then you must disable the Slip Compensation. You do that by going to that parameter and setting the slip RPM to "zero". But with the Autotune left at "Calculate", the drive will immediately recalculate the slip RPM and load it BACK into that parameter, which sometimes people don't notice. So you must go into the Autotune parameter, set it to "Ready", then go back to the Slip Comp and set it to zero.

I'm not sure if this might be a recent change, because yesterday, Tech Support seemed unaware of it. They (and I) identified the lack of speed change as a Slip Comp override, but didn't mention the effect of the Autotune setting thwarting efforts to fix it. I suspect that the Autotune parameter used to be defaulted to "Ready", and in some recent firmware upgrade, they changed it to "Calculate" because people using SVC and FV were forgetting to change it, but did so without realizing the side consequence it has on V/Hz operation with Decel and no DB. I'm going to be bringing it up later today.

We are using Induction FV mode and Coast Stop on stop mode. We have been using these program for a year now and the problem only happened recently. I have already consulted the person who did the program and he did not see any problem on the drive or either the PLC program.

This program is used in the double spooler machine and it is identical to the other spool,but this problem does not happen on the other spool. We are quite lost on where is the root cause of the problem.

 

[thaipinoy15]

You are not giving us much information to work with.

What do you mean by "stop" command?

Most times the PLC is either connected via hard wired I/O or by some communications link.

A 755 is a bit of a different animal than a typical drive. Often they are used on unusual applications and may not work as one might anticipate even if they are correctly functioning because of how they are configured.

Is this something that has worked fine for several years and suddenly developed this problem? Or is it a new drive that you are having trouble with?

What is the application? Is it part of some machine supplied by someone else? If so it might be wise to contact the supplier. You can also contact AB tech support for help.

Stop command is when the PLC send the signal for the drive to stop.

We are using this system for a year now and we only experience this situtation recently. I have already worked with the supplier who made the program and he said that everything is okay both the PLC and the drive.

As for the AB tech support here in Thailand they are all new and they dont have that much experience yet. I invited them to the plant i am working to see what is going on and they did see, but they do not know why it happened.

 

[drktmplr12]

Since you have a functioning drive I would suggest the following (if feasible):

1. Copy the config from the working drive to the defunct drive. Run a test to see if the issue is resolved. If yes, someone was screwing with the config and unwittingly changed a parameter.
2. If 1 does not solve the problem, is it possible to switch the drives with one another? See if that translates the problem to the other scroll. If yes, you have isolated the issue to the drive itself. If no, it could be a mechanical or PLC programming issue.

These are quick suggestions and are the steps I would take. Unfortunately, I don't know the size of the drives or what the required up-time is for the process they control.

edit:
if copying the config, be wary of communication parameters (write them down!!!) if the drives are not hard wired.

 

[thaipinoy15]

Since you have a functioning drive I would suggest the following (if feasible):

1. Copy the config from the working drive to the defunct drive. Run a test to see if the issue is resolved. If yes, someone was screwing with the config and unwittingly changed a parameter.
2. If 1 does not solve the problem, is it possible to switch the drives with one another? See if that translates the problem to the other scroll. If yes, you have isolated the issue to the drive itself. If no, it could be a mechanical or PLC programming issue.

These are quick suggestions and are the steps I would take. Unfortunately, I don't know the size of the drives or what the required up-time is for the process they control.

edit:
if copying the config, be wary of communication parameters (write them down!!!) if the drives are not hard wired.

On your suggestion #1, I already tried copying the configuration from the working drive and the problem still remain. I have changed the drive also with a new 1 but it does not correct the problem. As for the PLC part i compared it with the original program when it was built and it is the same, no changes has been made. I have already consulted this with the maker of the program and he said that he do not see anything wrong with the program.

I have also checked the belt tension and compared it with the tension of the other belts and they have the same tension.

This problem is really confusing as i have already checked what is needed to be checked but i cannot see the root cause.

 

[petersonra]

It is really hard to debug this kind of problem from a distance.

Most times these kind of things end up being either a PLC programming issue or a drive configuration issue, or sometimes a combination of both. But it is not always that easy to find.

Now and then a firmware or other drive or PLC issue is the cause, but they are relatively uncommon. Have you put the latest firmware in the drive and PLC? I would also get the latest drive profile and install it in the PLC.

 

[Jraef]

On your suggestion #1, I already tried copying the configuration from the working drive and the problem still remain. I have changed the drive also with a new 1 but it does not correct the problem. As for the PLC part i compared it with the original program when it was built and it is the same, no changes has been made...

this indicates that the problem is not in the drive then, and if the PLC program is exactly the same, it is not likely in the PLC program. So that leaves a potential issue with the PLC hardware or something in the wiring.

When you said "Stop command is when the PLC send the signal for the drive to stop." earlier, does this mean you have an input of the VFD wired to an output of the PLC that is the Run / Stop command? Or are you using the communications of the PF755 to get all of the commands to/from the PLC? If it is a hard wired Run/Stop command, then you are likely picking up an induced voltage on the signal wiring that was not there before. That can happen when your shield ground connection is broken or loose. If you are doing all of the VFD commands via Ethernet, it's most likely a bad connection / connector somewhere or noise getting into the Ethernet cable. That can happen often when connectors are made up in the field.

 

[drktmplr12]

Assuming a AB PLC, you should be able to run a trace on the output signals you think are the culprits. See how they change when the problem is recreated. Do they behave as expected? Then trace the feedback signals to see if their values are expected.

Check the calibration of any instrument that helps provide control to the drive. Maybe a recalibration or replacement is in order?

If the program is using a PID or PIDE instruction for this drive, I would look closely at it's arguments.

Is the feedback loop causing this issue when sharp changes in speed/torque occur? According to your posts, this only happens when given the cost to stop command. My dynamics isn't steller, but if a motor is driving a machine and is given a signal to coast to stop.. the torque should drop off pretty quickly. If any control loop is still acting on the motor because it did not acknowledge the stop, it will try to speed it up to compensation.

Just trying to offer some ideas, sorry if they aren't helping!

 

[petersonra]

have you tried going to the ab web site and using the online chat feature?

 

[thaipinoy15]

Dear All,

Thank you very much for all your inputs. I appreciate it very much. I finally found the root cause of the problem and it is very simple but i ignored the possibility. The problem was on the transmission belt.:jawdrop: I do not exactly know why because i have checked the tension of this belt and it was good. It was exactly on the standard value of tension. But after i changed the belt it works perfectly fine. :slaphead:

I hope this experience of mine will be able to aid our group members if they encounter the same problem.

Godspeed to all.

 

[drktmplr12]

Glad to see that your problem is solved.. now onto my problems for today!