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Project With Multiple VFD

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  • Project With Multiple VFD

    I have a project that consists of 20 VFD on one RS-485 multi drop cable. The frequency will change based on temperature which is updated every minute. For testing purposes I only have two VFD hooked up. I am able to turn them on and off with no problem. I want to use an edge contact "SC8" to scan for temp changes and change the variable being sent out to each VFD. I am able to accomplish this if I use the rising edge for one and falling edge for the other. If I use the same edge for each it will only communicate with one of the VFD. The send/success indicators will illuminate for that one send command only and not the other. Just looking for any ideas how to remedy this as there will be a total of 20. Thank you


  • #2
    One way is to set up an interlocking set of contacts that will go down the list of drives to communicate with. Each success bit turns on the next drive start bit.

    A second way is to use an integer tag for a status to go down the list.
    Start at 1 when time to start.
    Write to drive 1 - or the drive assigned on the status tag rung
    After success bit for drive 1

    Increment the integer status tag
    Write to the drive on the current status tag
    After success bit for current drive

    Increment the integer status tag
    After last drive success bit makes, go back to zero
    Wait for the time interval to start at 1 again.

    This makes it a bit easier to see what drive is currently being communicated with.

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    • #3
      This post is similar.
      It is for PxK to Solo, but similar to what Roger is describing.

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      • #4
        I struggled all day and then you guys replied. I setup the interlocking contacts and it works perfectly. Thank you so much!!!!!

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        • #5
          Glad to help.

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          • #6
            When you initiate a send transfer, the serial controller will be busy until it finishes and send requests initiated while the transfer is in progress, will be ignored. Otherwise, if you have a send block in your program that can be enabled every time the program cycles, then you could overwhelm the serial controller with unnecessary send blocks, not knowing which block actually transferred and which one skipped.

            Therefore if you need to send to different slaves, you have to do it one at a time, or in sequence. However, beware that at some point some device will fail to receive. For this reason, having the vfd reconfigured at runtime is a safety concern. It could work forever or fail once, and it usually comes at the worst possible instance.

            I believe the programming is not trivial. Shutting down a machine running amok is more difficult than speeding it up. You have a difficult task ahead, I wish you best of luck.

            I wonít spend more time on an issue you say is fixed. The point I wanted to make is about the safety concern regarding plc programming: usually the companies doing plc programming present insurance within 2 to 10 million dollars before meddling with machinery on customers premises. That because usually a shift work can cost hundreds of thousands just to run, not to mention human safety.

            on this forum i had the chance to see over time plenty of people without fundamentals in electrical engineering, doing all sorts of automation. Even for a home project or individual tool-shed hobby, it is very risky to meddle with electrical power. However, getting into modifications that could affect innocent people that operate the machinery could prove to be an attempt to other peopleís life. Thatís all I have to say.

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