Power Flex 755 question

ceilingwalker

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Mar 2010
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Hello, I have a PF755 that is doing something I don't understand. I have the drive tuned and it is working well however, sometimes when I tell it to drive fwd, it will drive in rev for a few feet, then drive fwd. It's position is fed back from an encoder on the motor, to an encoder card on the drive. I am certain nothing in my logic is telling it to do this. Also, it never does it when drive in rev, only fwd. My guess is that I don't have something set for the encoder module correctly but I am unsure of what it might be. Thank you
 
Is there a chance that this drive is jogged or ran manually in the field sometimes?
I have encountered this when the drive is run outside the PLC logic, in Reverse. The logic did not have anything to ensure the drive was in FWD, prior to starting. I added a rung with a rising edge to set the FWD bit when the drive status went to auto.
 
Is it pushing up against something resilient?

Take a look at the commanded torque parameter. That will tell you if it is driving itself backward or if something else is pushing it backward.

Keith
 
Is there a chance that this drive is jogged or ran manually in the field sometimes?
I have encountered this when the drive is run outside the PLC logic, in Reverse. The logic did not have anything to ensure the drive was in FWD, prior to starting. I added a rung with a rising edge to set the FWD bit when the drive status went to auto.

I am driving this from Studio5k for now, it is a start up. I make sure it gets direction first, then start.
 
It is possible that the drive cannot produce enough torque to move the load in the proper direction, but since its not an overhung load then probably not. Are you using torque proving?

Do you have an XIO contact for the REV.O on your rung for the FWD.O? If not I would put one and see if the issue goes away. If it doesn't then most likely the logic is fine, it would be something in the torque. Do you have more then one REV.O and FWD.O in your program? This is not good practice but I still see people do it from time to time.
 
First thing; check to see it you have the “Flying Start” enabled. If you have an encoder, you do not need or want that feature enabled because this is the sort of thing that can happen. Flying Start involves the VFD output putting out a “sweep frequency” to try to determine if the motor is turning already and if so, what rotation. When the encoder is used the drive already knows that so the sweep may end up giving the encoder input some false feedback that it takes a few seconds for the drive to evaluate and override. If the Flying Start is disabled, that won’t happen.
 
I have had something similar when I did not properly tune the motor to the drive. On the one I was using, it did not have an encoder, though. As I recall it had something to do with the control mode of the drive being set wrong. I had the motor set up for SV instead of FV or vice versa.
 
Check your flux current value I have seen this happen when this value is too high usually when you use autotune on non standard frequency's.
Flux current for a 4 pole motor should be around 30% of FLA.

Alan
 
Originally posted by alan_505:

Flux current for a 4 pole motor should be around 30% of FLA.

...for larger motors. Small motors (1/2HP, 1HP, etc) may have flux current of 50% or more of nameplate.

You drive calculated flux current should be somewhere in the ballpark of you motor no load, or idle, current. If you can find that value.

Keith
 
Is the reference coming from another drive? Any chance you're running in torque mode ?

I had a PowerFlex 70 drive give me negative torque feedback for a couple of seconds in a torque-sharing application because of wind-up in the gearboxes, making the "leader/follower" method unusable at the start.

Also, about Direction command bits; I think that if you assert both REV and FWD at the same time, the drive will use the previously selected direction.

Check that direction control logic very carefully to be sure it's logically exclusive. Use "storage bits" if there's a concern about asynchronous I/O update in between rungs.
 
Just wondering, if the application is safe, is it okay to let it do that? I believe I have similar issue right now on my machine, but my question is, is it really an issue for relatively safer applications, or would this be considered a bad practice.
 
If you are asking if this motion will hurt the drive or motor, no it won't. It may hurt the drivetrain, driven load or the process but the motor and drive won't care. As long as it is safe and doesn't cause the load any issues, I wouldn't necessarily worry about it.

Keith
 
I am embarrassed to say this however, it was something in the program. I thought I perused the program thoroughly however, I ended-up finding it. Sorry about sending you guys on a wild goose chase.
 

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