Modbus controlled VFD: what are the best safety practices for when Modbus comm fails?
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    Default Modbus controlled VFD: what are the best safety practices for when Modbus comm fails?

    Are there any industry 'safety best practices' for safely shutting down a Modbus controlled Variable Frequency Drive, in case the Modbus master or the communication cable to the VFD slave fails?

    Background: I have cobbled together a 1 HP three phase motor and a VFD unit. The VFD is controlled by a Windows laptop running a home brewed control software based on LabView. The Modbus connection between the VFD and the laptop is a RS-485 serial cable.

    The motor will drive a 'horse walker', i.e. an excercise machine for horses. Which means there will be unsuspecting humans and animals within the machinery that need to be kept 100% safe under all circumstances. While the software runs fine, I am worried about what happens when the comm link to the VFD goes down due to a Windows crash or cable failure. As it stands now, it would just keep running indefinitely at the last set speed.

    I haven't found anything resembling a 'dead man's switch' in the 185 pages of the VFD's manual. I am quite sure requiring a VFD to auto shut down on Modbus failure detection must a common request in any industry application, but i can't seem to find anything by googling...

    Would you have any recommendations or experience on how to auto shut down VFDs? Thank you!

    kind regards,
    Michael Z

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    You have not yet, nor are you likely to get responses to this query here - for the simple reason you are
    discussing life safety issues. Both for the humans, and the horses. You will probably need to discuss this
    with a local licensed, insured professional engineer.

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    Easiest approach to increase safety of your system is as follows.

    Generate a heartbeat pulse in labview and write to an I/O (output) point on the drive (not sure what drive you are using, but this is possible with many drives that are capable of Modbus RTU comms).

    Wire the output that is turning on/off to an underspeed relay.

    Wire the underspeed relay dry contact to the drive enable input (or better yet, safety rated disable input if so equipped).

    Done.

    If comms fails, the underspeed relay will open and the drive will be disabled.

    You can double up relays for redundancy (parallel heartbeat to two relays and put relay output contacts in series).

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    Quote Originally Posted by jim rozen View Post
    You will probably need to discuss this
    with a local licensed, insured professional engineer.
    Haha, I am living in a country where my 'local engineers' need to be physically threatened to even do a proper grounding wire installation.
    But seriously - my question was already taboo in an engineer's forum? Why? For legal liability reasons?

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    Thank you, motion guru! Yes, that is what I had roughly in mind, too, if there's no built-in function. The VFD does have a couple of digital outputs, accessible by Modbus comm. I will certainly go the road you have described. That will certainly work.
    Thanks a lot for taking the time to write it up!
    kindest regards,
    Michael

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    You might additionally consider a pull lanyard that shuts down the drive that is easily accessible to the operator. . .

    Google “HIRA” and get a sense of possible risks and mitigate with an engineered solution. I have found that safety is a mindset that doesn’t get defensive about the design, but rather embraces ideas from all perspectives on what might go wrong. Consider a torque limit and also a timer that needs to be reset every xx minutes to keep things going purposefully by the operator.

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    My walker is not MODBUS controlled, only controlled from a local control panel at the walker gate. The E-STOP chain goes through the run command contact, there is a E-STOP at the control panel, which is also reachable from the arena. There is also another E-STOP over at the tackroom. I ended up having a variable speed pot, and two preset speeds for walk and trot. I do have overcurrent trip sets low and short timed to prevent an accident. I can trip the drive if I grab a panel and hold on.

    Motion's MODBUS solution is the way I would go also. I just kept mine simple, I really want to add the MODBUS so I could set a program where they have walk, trot, walk.....on timing, and also add indexing to help load/unload. But that is another project for another day.

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