[Logix] Analog mA going to -1.0 when VFD is called

Hi guys,

I'm noticing something strange in my ControlLogix L72 when I call a VFD to run, and I'm not sure if it's a coding/config problem on the output card or if it's a VFD issue. Here's the lowdown:

PLC - ControlLogix 1756-L72
Analog Output Card - 1756-OF8
Output Range - 0 to 20mA
Output High Scale: 20.0 mA
Output High Engineering: 20000.0
Output Low Scale: 4.0 mA
Output Low Engineering: 4000.0
Output High Clamp - 21000
Output Low Clamp - 3200
Disable All Alarms - Checked

VFD - Allen Bradley SpeedFlex 525
Parameters - Unknown, they were configured by the panel builder so if there's something I should look at, let me know.

What happens is, if the VFD is in Auto but I'm not calling the VFD to run, the analog output value is 4000.0; makes sense, since it's a 4-20mA output. However, if I call the VFD to run, the output value immediately goes to -1.0. If I force the output (4000.0-20000.0) the speed responds properly, strangely.

The way my code works, is that the speed setpoint is 0-100%, and that's scaled to 4000-20000 and then that value is moved to the output tag. Originally I had the speed setpoint as a DINT; if I change it to a REAL, I get -1.$ and the output goes full scale; however, if I force it it does behave as intended.

So I'm not sure if this is a VFD issue or an output config issue or a code issue. My outputs seem to be working just fine for the GE VFDs we also have so I'm not sure what to be looking for. Any thoughts on what could be causing this? I am cautiously optimistic it isn't my code or config, but not sure what would cause this if it is indeed VFD related.

You meant speed flex 525 or powerflex 525 ?
Are you using a PID to control speed ?

widelto said:

You meant speed flex 525 or powerflex 525 ?
Are you using a PID to control speed ?

Click to expand...

Good catch, I meant Powerflex 525.

Not using a PID, it's a chem dosing pump so the output is calculated based on several setpoints like chemical concentration; it's just a compute block.

Defcon:
Just a curious question, why are not you using pflex 525 ethernet capabilities ?? This way you can start, stop, reset, change speed all from ethernet.

widelto said:

Defcon:
Just a curious question, why are not you using pflex 525 ethernet capabilities ?? This way you can start, stop, reset, change speed all from ethernet.

Click to expand...

Great question; I'm not sure why but the designers didn't deem that useful; I was brought on board after the design was complete so what we have is what will be.

I do not get you, when you say you force it you mean you write a value to your output tag ?? If you do not force it what is the values on the output tag ??

just a guess on my part.

since you originally had the data as a dint and you changed it to a real, did you rework your scaling accordingly in all your logic and output card to reflect the real data values?

like I said, just a guess.
back up your program first in case this doesn't work.

james

Output value of VFD or analog module =-1? Are you sure your speed scaling logic is correct?

widelto said:

I do not get you, when you say you force it you mean you write a value to your output tag ?? If you do not force it what is the values on the output tag ??

Click to expand...

Sorry if I was confusing; here are some screen grabs that will hopefully make sense.

Here's how I send the speed setpoint to the VFD, just for reference:
http://i.imgur.com/RjxorH5.png

Each pump has a UDT, and in that UDT I have a Speed_Sepoint_Raw tag. This is the value that is moved to the output card. It's value should be 4000-20000. I've noticed different behavior if Speed_Setpoint_Raw is a DINT, or a REAL.

Here's a screenshot of the actual output tags when the VFD is in local off or local on; i.e., not accepting a speed setpoint from the PLC and is running off a local panel mounted potentiometer:
http://i.imgur.com/Z02rESk.png

As you can see, the raw value is 4000 for the channels that are enabled. This corresponds to a 4mA output obviously, and everything is normal. There are four VFDs total, just for reference.

Here's a screenshot of the same output tags when one VFD is in auto. This is when the Speed_Setpoint_Raw tag is a DINT. As you can see, the output immediately drops to -1.0:
http://i.imgur.com/QNaCSnu.png

Here's another screenshot of the same output tags, VFD is in auto, but Speed_Setpoint_Raw is a REAL. As you can see the output is 1.$:
http://i.imgur.com/3ZSf5Ob.png

If Speed_Setpoint_Raw is a DINT, the VFD responds with a 0% speed feedback. If Speed_Setpoint_Raw is a REAL, the VFD responds with a 100% speed feedback.

If I force the analog output value and manually change it, the VFD responds as expected at 4000 (0%) and 20000 (100%), and every step between.

The GE VFDs that control the larger pumps do not have this problem, it's just the AB Powerflex 525.

Hope that clarifies what I'm seeing; any questions let me know. Thanks!

James Mcquade said:

just a guess on my part.

since you originally had the data as a dint and you changed it to a real, did you rework your scaling accordingly in all your logic and output card to reflect the real data values?

like I said, just a guess.
back up your program first in case this doesn't work.

james

Click to expand...

Great question; I believe everything is working alright; like I mentioned, the GE VFDs are responding just fine and it's the exact same code.

dginbuffalo said:

Output value of VFD or analog module =-1? Are you sure your speed scaling logic is correct?

Click to expand...

I believe so; as I mentioned in another response, the GE VFDs that we have for the larger pumps are working just fine. And more strangely, the -1.0 only happens when the VFD is in auto, not when off or local on; if the scaling was off, I believe this would occur all the time, right?

Nevermind guys I figured it out; god, what a dumb mistake. My code says that if the VFD is in auto, to load the calculated speed setpoint into the output; if it's manual, to ignore the calc. The only pumps in the project that run off a computed setpoints are these chem pumps so I wasn't having the same problem there.

Right now the calc isn't properly configured so it's coming up with weird results. Sorry for the dumb question.

your software is weird, regardless of being in manual or auto mode output tag should always have a value. Or manual means you have to go to the drive and press some buttons.?

No problem! Had a feeling it had to do with how your were scaling and/or calculating the speed command.