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  1. #1

    Help with control sequence

    Okay, I have a eaton vfd (mvx9000) and a veris (px series) differential pressure transducer. This will run a exhaust fan for building static @ or above .05 w.c. The pressure control is 0-5, 0-10, or 4-20 with ranges of .1, .25, .5, and 1.0. I started by using 0-10 with .1 range and setting vfd to allow on @ 5vdc so that the fan will not run until static equals .05 or above with 20 hz min speed for motor safety but I am worried about not having good control. I have no test balance records to work with because it was excluded from job. What would you do or recommend?
    Thanks in advance, svc

  2. #2
    Join Date
    Mar 2005
    Pacific Time Zone
    Where are you getting your outside reference from?
    I like what you are saying for your setup up but I've tried to maintain 0.03 or less and I've seen/heard of others maintaining a 0.01. If your outside reference is in a poor location then wind can really affect the the reading and will "bounce" your pressure reading all over and thus you'll be short cycling your fan and causing issues. I have not used an Eaton VFD before but I would make sure your PID loop for the response to changes is somewhat slow, especially in the beginning and then you can fine tune it later. This will be tough though as you don't sound like you have any way of trending your information. Just some input from my limited experience with building static setups.
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  3. #3
    Join Date
    Feb 2007
    Dallas, Tx.
    i have had similar problems with effects of wind on the outside reference, especially in an area filled with medium or high rise buildings. this can be solved with multiple restrictors in the line. i have found that this works better that any type of shield ad the end of the line.

  4. #4
    Join Date
    Oct 2005
    You might also tee a volume tank into the OA sensing line, to dampen the response. You can make a tank with a 4" or so piece of PVC capped on both ends and tapped with a pressure fitting. The same method works on pneumatic signals if you need to dampen them.

  5. #5
    Thanks for the response, outside ref is outside ref. I am using the same type dwyer as you see on voyagers to prevent wind issues. I am maintaining .05 because that is how the architect spec'd it. My questions are more to find out what the normal sequence of operations would be using the devices stated rather than field variables. I have not yet seen the control in action. I will have to force the rtus into economize to test.
    thanks, svc

  6. #6
    Join Date
    Dec 2006
    Washington, DC
    It appears you have a standalone operation here with just a VFD & a DP transducer, no other control for occupied/unoccupied or start/stop.

    Didn’t know you could start & stop a VFD with the same analog input being used for the PID loop. You may need to do some sort of enable for the VFD, if you want to use the analog signal you can just add a UCS-121. Then adjust the UCS-121 to enable the VFD at the setpoint you want.

    The DP range you have selected seems fine. The PID Loop will probably need to be adjusted. I use a signal generator to simulate, this way I can watch the operation and adjust the PID loop as needed. It saves from going to the units to enable/disable the economizer operation.

  7. #7
    "It appears you have a standalone operation here with just a VFD & a DP transducer, no other control for occupied/unoccupied or start/stop"

    correct. and this particular drive does not have pid! I bet your glad this isnt your job right about now.

  8. #8
    Join Date
    Jan 2007
    If there's no PID then you're dead before you even get started. Is there an external control system?

    I just went to their website and it says there is PID control.

  9. #9
    Found out today that the drives came with a cd. I assume it will have more info than the quickstart guide I was given from my office. Qickstart guide shows nothing at all about pid!

  10. #10
    Join Date
    Dec 2006
    Washington, DC
    The online pdf shows this drive does have a PID loop.

    You are going to need a seperate power source for the controls as the vfd only has 10vdc for control power, not 24vdc.

    It will work with the internal PID loop, but you are going to need a digital input to start/stop the drive. As I stated above, just use a UCS-121. Then you can use the 0-10vdc input signal to enable the vfd and for the analog input for the pid loop.

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