PF 755, but process requires max torque

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VTSandman

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Hey guys, first time posting a thread here but I feel like this would be a good place to ask for help since I've learned a lot just from reading here. I have a Powerflex 755 drive application that is going to be running a screw conveyor press. This just squeezes moisture out of plastic waste product on a screw against a pressurized cone to drain. 60 HP induction motor. The vendor for the product met with us today and claims that the motors require a safety shut off in Nm or ft/lb. I said that we can use a max amperage shut off or motor torque percentage cut off, but they're adamant that it must be a set measurable torque amount. I just wanted to make sure that I'm not missing anything that the PF755 is capable off. I've scanned Rockwell literature all day and I don't see it. I know with ABB 1880 this is a selectable parameter, but I'm not sure with PF's. They also request that the drive run in speed control as opposed to torque control. I'm a little at a loss here and not sure how I can get around this with the drive I have on site. Any input appreciated. I'll do my best to check back tonight and tomorrow. Thank you all
 
VTSandman said:
Hey guys, first time posting a thread here but I feel like this would be a good place to ask for help since I've learned a lot just from reading here. I have a Powerflex 755 drive application that is going to be running a screw conveyor press. This just squeezes moisture out of plastic waste product on a screw against a pressurized cone to drain. 60 HP induction motor. The vendor for the product met with us today and claims that the motors require a safety shut off in Nm or ft/lb. I said that we can use a max amperage shut off or motor torque percentage cut off, but they're adamant that it must be a set measurable torque amount. I just wanted to make sure that I'm not missing anything that the PF755 is capable off. I've scanned Rockwell literature all day and I don't see it. I know with ABB 1880 this is a selectable parameter, but I'm not sure with PF's. They also request that the drive run in speed control as opposed to torque control. I'm a little at a loss here and not sure how I can get around this with the drive I have on site. Any input appreciated. I'll do my best to check back tonight and tomorrow. Thank you all
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Also I should mention, there is no encoder feedback. The feedback would be simulated.
 
In the event anyone reads this for reference, only very specific PF's are capable of producing a read out in Nm or ft/lbs. Not your typical 755's vers14 fw. It only took me a day to speak with RW tech who could clarify this for me thankfully. Just a word to the wise for anyone who is overseeing an application where you must provide an overtorque value to generate a main fault. Our mill had gotten into a situation where the vendor required this specific protection in order to validate their safety certification. Our audible is going to be an ACS 880 if we can find them in stock.
 
IIRC, the torque value these drives display is just the current scaled by some factor. The drive cannot actually measure the torque being produced by the motor directly.
 
Understood. And I explained that to them, but they would not sign off on it unless I could enter a hard value for the limit. Was kind of a first for me hearing that. It is what it is at this point.
 
VTSandman said:
Understood. And I explained that to them, but they would not sign off on it unless I could enter a hard value for the limit. Was kind of a first for me hearing that. It is what it is at this point.
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This an interesting topic.

You are put to task by your client to tackle a problem not commonly encountered by electrical or industrial engineers.

Your client requires control based only on speed-- while torque control takes a backseat and purposely meant only for monitoring purposes.

The criteria limit your discretion/option on how you could effectively control a motor that would be easier and less complicated.

It is possible but requires more homework.

There are four primary types of motor control methods for induction motors connected to VFD:

1. V/f (volts-per-hertz).

2. Volts/f with accompanying encoder.

3. Closed-loop vector.

4. Open-loop vector.

Option number (2) V/f with encoder is out of the question because the client clearly doesn’t want.

Clearly, you are left with option number (1) to work with.

As for options 3 and 4 they would require sensing components.

So, as mentioned earlier. . . . it is possible but more complicated.




………………………..
 
The PF755 also has an option for Encoderless Flux Vector Control. It's full Flux Vector Control but uses the internal current sensors as the feedback device by detecting the rotor bars passing through the stator fields.

Torque and current are basically synonymous in a Flux Vector controlled VFD. The PF755 has what's called a "Shear Pin" trip that is based on the motor current. That's how the ABB ACS880 works too. If they need to have a specific value of torque for that setting, you could probably write a little routine in the embedded DriveLogix software of the drive to calculate that value for you. If you don't want to write that yourself, you can probably engage Rockwell Services to do it for you. Parameter 707 is the place where it shows a PERCENTAGE of rated motor torque, you could also adapt DriveLogix to display that as a hard number by simple converting that percentage value to a numeric value based on the motor nameplate data using the basic motor torque formula.
 
But without a real torque sensor, you are depending upon the motor model being used by the drive to estimate/predict torque.

The motor characteristics change with things like temperature and saturation, and the sensorless vector model is probably inaccurate during short transients.

Which all goes to the torque value the drive presents is just an approximation and if you want to use the drive to protect the machine from damage you are probably better off setting a current limit which you justify based on the expected torque at that current value.

-Jon
 
Thanks everyone for the feedback. We've explored all of the options that were made available to us. End of the day the vendor would not certify the press without having their preferred drive. Peter is correct, this is more of a machine damage issue than anything else. I offered to write the devicelogix in order to give them some added comfort, but their EE would not sign off on using the PF. Even though it's creating the same style feedback as would any other big name drive.

I have a feeling the same EE doesn't have much experience with drive motor control as they shut down my attempt to use FVC. I have always thought this was the preferred method of control with non fan/pump applications. Although this would have been helpful information from the vendor to share with our oversite engineering dept upon product review. We'll move along.

I've reached out to our other screw press vendor and clarified this before commissioning of that equipment has started. And what do ya know......they're completely ok with FVC and the shear pin protection on the PF755. I think I could go the rest of my life without doing a plant start up again. I really appreciate all of the input and help. A lot of good information for me to carry forward.
 
Sometimes when an engineer stumbles across something that works from a particular manufacturer, they just insist on it regardless of the fact that the same technology is used by everyone else. It’s kind of lazy really, but I also understand it to some extent. He likely had other things to think about and that one was, in his mind, already solved.
 
Jraef said:
Sometimes when an engineer stumbles across something that works from a particular manufacturer, they just insist on it regardless of the fact that the same technology is used by everyone else. It’s kind of lazy really, but I also understand it to some extent. He likely had other things to think about and that one was, in his mind, already solved.
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I can see that too. If I ask will what is shown work and the answer is yes I would ask then why are you in here?
 
We're actually going to try the approach of having RW reach out to the vendor with their reasoning on why this satisfies the vendors safety needs. Worst case scenario, we purchase another drive to appease the vendor. Best case scenario RW engineers talk some sense into the vendor. Feel like this was the best avenue we could take in order to leave our current set up as is. Having it be the word from the supplier instead of the end user.
 
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