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  1. #1
    Join Date
    Apr 2006
    Location
    Columbus, Ohio
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    753

    Tridium Niagara AX - VFD Floating Control Sequence

    So my company has chosen to install VFDs in place of some vortex vanes. The vortex dampers were being controlled by floating type control. (Open Damper, Close Damper, and Hold Position) Someone in my office sold the job to reuse the floating type control as inputs to speed up and slow down the VFD and hold its speed. I really don't like this particular control sequence, and I have never seen a VFD controlled in this manner.

    The controls engineer has instructed me to write a custom program in Tridium that determines the VFD percentage based off of time. So I am assuming that if I have a 60 second acceleration/deceleration time on the VFD. I can write some logic stating the number of seconds the speed up or slow down output is active. I would count up during a speed up command, and count down during a slow down command. I would hold the count when in the hold command. I am trying to get my head around doing this sequence. I feel that this particular way of doing this can become very inaccurate at times. I know I have to incorporate a min/max so that the count doesn't go over 100 or below 0. I am also planning on using a PID to output the count. I am wondering if anyone has written anything similar to this before? If so can you give me some pointers on how I need to approach something like this. I told the office it would of been a lot easier if they would have just set up a analog output on the AHU controller, but they are XL5000 controllers and I guess our company doesn't have the CARE tool. So here I am with this situation.

    Thanks for any advice you may have.
    J. King

  2. #2
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    You don't have to calculate the percentage. Nearly all VFD's that I have seen (at least recently) have a 0-10V analog output that is or can be programmed to follow the VFD speed. There's your reference.

    Just write your code to monitor the up/down comands, and increase/decrease your new AO to the drive so many percent-per-second, and use the drives speed output wired to your AI to display speed. You aren't trying to send the up/down command directly to the drive, are you? Either way, you can use the drive's speed output to monitor % speed.

    Any attempt to calculate this will result in a constantly increasing error, that's why floating VAV controllers do a periodic resync to 0 or 100% - which could be pretty spectacular if you try it with a VFD running an air handler....

  3. #3
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,289
    Why not just use a Kele PWA-1T?

    I've used them in some application that I ran out of AO's and had some DO's.

    I know McQuay has controlled there VFD's with floating point on alot of there RPS rtu's.

  4. #4
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Quote Originally Posted by ascj View Post
    Why not just use a Kele PWA-1T?
    Or an AUD module:

    http://workaci.com/products/aud-flot...-analog-output

    If I read the OP correctly his main problem is how to display the speed though.

  5. #5
    Join Date
    Aug 2011
    Posts
    75
    I used one kele pwa-1t this week with good results.

  6. #6
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,289
    I took as a salesmen sold a vfd to replace some guide vanes. And then the controls engineer is over thinking it.

    I going to assume the original logic was a stepped increase or decrease to maintain a set point static. You can easily setup in some drives or use a tranducer, like what I or you suggested.

    If you want speed.....just reference it from the drive to a open AI.

  7. #7
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,289
    Duplicate

  8. #8
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    You might check your drive as well. Some, like the ABB ACH550, are programmable to do floating control directly via two DI's (I have not personally tried this though).

  9. #9
    Join Date
    Aug 2011
    Posts
    75
    Cutler hammer/ johnson controls drives will accept floating control (motor pot up pot down)

  10. #10
    Join Date
    Apr 2006
    Location
    Columbus, Ohio
    Posts
    753
    Quote Originally Posted by DDC_Dan View Post
    You don't have to calculate the percentage. Nearly all VFD's that I have seen (at least recently) have a 0-10V analog output that is or can be programmed to follow the VFD speed. There's your reference.

    Just write your code to monitor the up/down comands, and increase/decrease your new AO to the drive so many percent-per-second, and use the drives speed output wired to your AI to display speed. You aren't trying to send the up/down command directly to the drive, are you? Either way, you can use the drive's speed output to monitor % speed.

    Any attempt to calculate this will result in a constantly increasing error, that's why floating VAV controllers do a periodic resync to 0 or 100% - which could be pretty spectacular if you try it with a VFD running an air handler....
    I would like to use a analog output out of the VFD for speed reference but do I need CARE to set up the XL5000 controller for this additional speed reference input?
    J. King

  11. #11
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    830
    I see an issue trying to calculate VFD speed output by monitor timing of the increase or decrease commands outside of the controller that is actually controlling the timing. Will the JACE be capable of reading the change of state within a couple of ms? Even then a few missed here and there and the calculation is off. I would use the AO of the drive as a speed reference as mentioned previously and add another control or IO module for it.

  12. #12
    Join Date
    Apr 2006
    Location
    Columbus, Ohio
    Posts
    753
    Quote Originally Posted by DDC_Dan View Post
    You might check your drive as well. Some, like the ABB ACH550, are programmable to do floating control directly via two DI's (I have not personally tried this though).
    Our install crew has already installed and set up the VFDs for floating type control. Now I am left to figure out how to display the speed in percentage without having the capability for using analog inputs and outputs off of the AHU controller. This is the way our controls engineer and salesman figured it. I would of just paid someone to add the necessary logic in the XL5000.

  13. #13
    Join Date
    Apr 2006
    Location
    Columbus, Ohio
    Posts
    753
    The AHU controllers are on a CBus network and the VAVs are on a Lon network. What do you recommend I add to read the AO off of the VFD. Is there some sort of generic Lon IO module I can add?

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