VFDs wont turn off

Hello everyone,

I am completely new to PLCs no formal training but I work as a process engineer in a plant that uses them and have been trying to teach myself for several months now.

The system was supposed to be set up by a contractor but was never fully finished because funding was cut so the essentials to operate the plant are there. I was tasked with commissioning a 4-20 AO flow meter and adding the reading to our factory talk HMI with a totalizer.

The PLC (Logix5555) that the flowmeter is wired to is only used to receive and send inputs and outputs to the main plc (Logix5575) in the control room. The control room PLC does all the operating logic.

Now, what I did is use RSLogix to first get the Logix5555 to send the input from the flowmeter to the Logix5575 and store it in the tag there. I did nothing new just added it to a previously created list of other inputs that get copied to the PLC that I know already worked. This all went fine.

Then I took that tag and connected it to an analog input function block which I connected to a totalizer function block. I did this based on another flow meter that had already been setup. This seems to work fine too.

I was getting ready to add the graphics to the HMI and I was calibrating the bit data and PVScale when I noticed that the pump I was using was behaving strange in factory talk. All our pumps have HOA switches which can be set to hand, off and auto. The pump ran fine in hand, and turned off, but with the switch in auto, the HMI faceplate still says its running and is in Local mode. So I vent and checked the VFD (connected to Logix5555) and it was on at 0.0% in remote. What I also noticed is that every VFD connected to that PLC was powered on but not running. This is strange behaviour to me and had not seen it before because as far as I know all VFDs are set up to turn off when output is below minimum.

Any thoughts on why this might be happening? or how I could find out what the matter is. I suspect that the changes I made may have something to do with it but I did not alter anything related to the VFD.

Any help is appreciated!

If I understand your question, you are asking that if a VFD is not running it should be powered off?
That is not the case, the VFD will remained energized and only output power to the motor when commanded to.
Cyclicing power off and on to VFDs shorten the life span.

Typically VFD displays are on when the system is powered up, whether running or not. Usually the display is powered from the drive input power and is independent from the drive output.

I don't know your process but I'd change the HMI to show if the motor speed is > 0 as a run indication assuming this is safe for your application.

If the drives are truly in local mode, then you should have no remote start/stop nor speed reference control via your PLC. This doesn't sound like a correct configuration to me. There are many questions that I would have about this situation that would relate to the drive's make and model, the wiring schematic and the drive's parameter setup.

You don't need to power your drives down when they are not in operation, but that doesn't mean that you shouldn't "stop" the drives.


BTW: All drives are not designed to "turn off when output is below minimum", whatever you mean by that. If you are referring to the speed reference from your PLC being less than 4 mA, then some drives will actually fault under those conditions and some drives will simply run at their minimum Hz setting.

It's a basic rule of engineering that you assume nothing !

In my experience, VFDs are typically Started and Stopped by discrete signals, rather than by an analog threshold. But analog threshold starting and stopping is a feature supported by many VFDs.

"H/O/A" and "Local/Remote" mean different things in different contexts; you're going to have to figure out exactly how your drives are wired and programmed.

this is just a guess – based on what I understand of the information you've posted so far ...

it sounds like you're saying the VFD is in "run" mode – but that the speed has been set for "zero" ...

rightly or wrongly - quite a few systems are set up that way ... basically the programmer doesn't technically "stop" the VFD - he just sets the speed command to "zero" - so the motor doesn't turn ...

are you just concerned that the HMI isn't showing the VFD as being "off" – even though the motor isn't actually turning? ...

Thanks for the replies, I will try to give more details.

we're all trying to help ... the more details you give us, the more detailed we can make our answers ...

and it would be very helpful if we can all stay in this particular thread ...

Ron Beaufort: Yes your guess is correct but there is more to the problem.

Problem #1: All pumps in the plant have a three state discrete switch with states "hand", "off", "auto" that is wired to the PLC and VFD. The pump piped to the flow meter is a diaphragm pump driven by a VFD but the pump also has manual adjustment for stroke frequency. I ran the pump in "hand" which runs the VFD at 60 Hz and it was fine, it stopped on "off" then I set it to "auto" where the PLC sends the AO set by the user on the Factory Talk Faceplate to the VFD. Work fine for 100% or 60Hz then I tried 50% and the VFD faulted. I reset the VFD and the switch was left in HOA switch was left in auto "auto" now the FactoryTalk faceplate for the pump no longer has the A and M but has L at the bottom as if the pump was being run in "hand" mode locally and the faceplate is also saying that the VFD is running at 0.0% which is true if I physically look at the VFD. No matter what I do I cannot get control of the pump from Factory Talk.

Problem 2: When I was trying to solve Problem 1 I noticed that all other VFDs connected to that PLC are energized but are @ 0.0% and are supposed to be stopped on the HMI. This is not the behaviour that I normally see with the other PLCs. When those VFDs are stopped from the HMI or are set below the minimum of 25% output the screen turns completely off. I thought that the two problems could be related.

The VFDs are ABB ACS400 drives btw.
Sorry if I am using incorrect terms or not proper terminology but as I said I am new to this.

And again I apologize for the duplicate threads, I am not sure how it happened I must have hit refresh but I assure you it was not on purpose.

Hello

Problem 1 - "Work fine for 100% or 60Hz then I tried 50% and the VFD faulted."
I would check the fault code, you can setup the drive so if there is a minimum AI value that the drive will fault. Check parameters 1304 and 3001. As for being able to run in hand, in local you can setup to run off the keypad or presets my guess would probably a preset. As far as your faceplates do the faceplates change when the drive is in local, maybe a wiring or programming error maybe the programmer had other ideas and never finished.

Problem 2 - "When those VFDs are stopped from the HMI or are set below the minimum of 25% output the screen turns completely off."
Maybe there is a contactor used to supply power to these drives. I seen this before where it was supposed to be setup to remove power in the case of an E-Stop condition dont under stand why BUT....

Hope This Helps
Tom

It is often very useful (and occasionally overlooked ) details of which HIM features are masked that can make the difference.

On our networks Powerflex drives, we write to the local/auto control bits every scan. The logic uses MOV instructions with constants for each desired state, ie. stop, start fwd, start rev, reset fault, etc. If you need local control and the ability to return to remote from the PLC code, you can do that too, it takes a bit more manipulation of the bits based on events that you program to control the conditions for remotely returning to remote.

Also, don't get burned by sleep mode. We had a drive that would shut itself down if it ran below 20 Hz for more than 60 minutes! I have no idea who owns the fat fingers that set it up that way, but it was an oven conveyor in the middles of a string of them luckily it happened during clean mode. Yes, folks, like your VCR, the VFD has a sleep mode. Are they shooting for an Energy Star sticker or something?

OkiePC said:

We had a drive that would shut itself down if it ran below 20 Hz for more than 60 minutes! I have no idea who owns the fat fingers that set it up that way, but it was an oven conveyor in the middles of a string of them luckily it happened during clean mode. Yes, folks, like your VCR, the VFD has a sleep mode. Are they shooting for an Energy Star sticker or something?

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Probably just trying to avoid melting the motor windings. If you're going to run a motor for an extended time under load at very low frequency, then the motor will need auxiliary cooling, since it doesn't spin fast enough to dissipate it's own heat under these conditions.

Sounds like you could use a good experienced controls/drives specialist to sort this out. You may even find one that is willing to turn this into a training session to get you up to speed.

DickDV said:

Sounds like you could use a good experienced controls/drives specialist to sort this out. You may even find one that is willing to turn this into a training session to get you up to speed.

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That is excellent advice. Hire an expert to walk you through getting a single drive to function properly, and then use that experience to apply the solution to the other drives.

When I was trying to solve Problem 1 I noticed that all other VFDs connected to that PLC are energized but are @ 0.0% and are supposed to be stopped on the HMI.

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When any VFD has truly been given a valid STOP command, it will not show "running" or % speed" on the HMI. On the other hand, many programmers do not actually give stop commands, but simply run the speed control to 0. I like the STOP method.

Remember that there are different types of "Remote" VFD control: from a remote drive keypad/HMI, from a remote analog input device, from a remote operator HMI giving either or both analog speed signals AND VFD terminal block inputs. You have to find out what are all the remote possibilities.