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  1. #1
    Join Date
    Jun 2006
    Location
    New Jersey
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    4,283

    Problem with return fans

    Got a couple AHU's that have the return VFD's lockout on over current anytime they lose power. So I got a chance today, to spend some time with them.

    Problem 1- If supply fan stops, it de energizes return fan contactor. But the return VFD is still getting run command from BMS. When supply restarts, return contactor energizes and over current lock out.

    Problem 2- Can't start return fans in bypass. They pull 50-75amps and trip starters. Voltage drops 100 volts. Only way to keep them running in bypass, is to start them on the inverters and manually increase Hz then throw them to bypass.

    Problem 3- Return fans hardly get any signal from BMS. Supply fans have the return fans spinning at 500-600rpms, even when de energized. When energized, the VFD's sit at 33% SP, 10hz, and 25% of rla.

    These are belts drive plenum fans. The rotation is right. Mostly factory wiring. I did take static readings down the AHU's, but they didn't put up a red flag.

    It's almost like there is way to much fan for application, but the supply fans run between 80%-100% to maintain 2.0wc on the supply.

    Any help?

  2. #2
    Join Date
    Aug 2011
    Location
    Los Angeles, CA
    Posts
    34
    1 - Why does the return fan shut off when the supply fan stops?
    2 - What is your accelerate and decelerate parameter set to?

  3. #3
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136
    What controls the speed signal to the return fan vfd. Where is the contactor for the return air vfd that is shutting off. Is it in the drive or external. You say the return fan will start in vfd with a spinning blower but not in bypass. Is the return fan spinning backwards or the correct rotation when this happens.

  4. #4
    Join Date
    Jun 2006
    Location
    New Jersey
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    4,283
    Quote Originally Posted by redcell09 View Post
    1 - Why does the return fan shut off when the supply fan stops?
    2 - What is your accelerate and decelerate parameter set to?
    If the supply current sensor opens, it opens a rib that with open control circuit to return fan. As of right now, my accel/decel times are at 30 secs......I did try increasing them to no avail.

  5. #5
    Join Date
    Jun 2006
    Location
    New Jersey
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    Quote Originally Posted by synergy View Post
    What controls the speed signal to the return fan vfd. Where is the contactor for the return air vfd that is shutting off. Is it in the drive or external. You say the return fan will start in vfd with a spinning blower but not in bypass. Is the return fan spinning backwards or the correct rotation when this happens.
    The return fan spins the correct way always. What controls the speed of the return fan is BMS. I finally got my hands on the BMS specs today. It states the return fan VFD signal is set to maintain differential set point to provide a positive building pressure.(How vague is that!)

    As far as the contactors......there are two contactors per drive. One is inline with the output of the VFD and motor. The other is between the line voltage and motor. The bypass/inverter switch control which one is used. And yes the are external to the VFD.

  6. #6
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,159

    Sounds backwards

    It states the return fan VFD signal is set to maintain differential set point to provide a positive building pressure.(How vague is that!)
    I don't know, but sounds like the return fan runs and the supply fan is OFF Then ( I'm positive that) the building pressure would be negative.
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  7. #7
    Join Date
    Jun 2006
    Location
    New Jersey
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    4,283
    Quote Originally Posted by genduct View Post
    I don't know, but sounds like the return fan runs and the supply fan is OFF Then ( I'm positive that) the building pressure would be negative.
    That's why the return fan won't run unless the supply fan is running.

  8. #8
    Join Date
    Dec 2005
    Posts
    1,822
    So, Mr. Spock. What seems to be the problem here?
    You cannot solve it ? Or the BMS people don't want to solve it.

    Because you already found the problem, which 3/4 the battle. You know that Solving is always easy.

  9. #9
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,642
    If the across the line starter won't work in bypass, I'd fix that problem first. either the overloads are undesized, or there is a problem with the motor.

    the other problems are BMS related. The BMS should be telling the VFD to shut down if it wants the return fan to stop, not opening a contactor between inverter and motor. In my mind, the only thing that should exist between the inverter, and the motor, is a service disconnect, and, if installed with a bypass, the contactor associated with removing the inverter from the loop when in bypass mode. And that should only open up when in bypass, not from some external input.

  10. #10
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136
    Maybe try stopping the return air fan from spinning and then trying to start it in bypass. Could be some krazy back EMF thing caused by the generator effect of the spinning motor.
    I am hung up on that BMS control of the return fan. Differential pressure set point of what. Could it be related to the outside air dampers and the exhaust dampers some how.

  11. #11
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
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    1,642
    Quote Originally Posted by synergy View Post
    Maybe try stopping the return air fan from spinning and then trying to start it in bypass. Could be some krazy back EMF thing caused by the generator effect of the spinning motor.
    Induction motors shouldn't have any problem restarting while spinning, in fact, it should be a much lower starting current.

    Quote Originally Posted by synergy View Post
    I am hung up on that BMS control of the return fan. Differential pressure set point of what. Could it be related to the outside air dampers and the exhaust dampers some how.
    If i had to guess (all i can do at this point), I'm thinking that the supply fan runs on static pressure, but has a flow sensor. The return fan probably has it's own flow sensor, and is run to maintain the same flow as the supply fan, minus a fixed differential - probably the combined flow of building exhaust fans, and whatever extra they determine to maintain positive building pressure.

  12. #12
    Join Date
    Mar 2007
    Posts
    3,054
    The return fan typically lags behind the supply fan by 10-15%. The supply fan is controlled to maintain duct static pressure, and return fan either tracks the supply fan by a fixed percentage based on the air balance of the building, or it's directly controlled to maintain building pressure.

    Some motors are just too big to start full voltage across the line with belts. It's a little easier to start them if they are already spinning, because the startup torque requirement is reduced. Is a solid state, autotransformer, or part winding start in bypass a possibility? How about a time delay on the overload contacts?

    If the return fan is not directly controlled by building pressure, it should be set up to track the supply fan to maintain a slight positive based on the ventilation/exhaust balance. Just my $.02
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  13. #13
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136
    The original post reports a 100 volt drop on start up in bypass. Defently something wrong there. Hence my thought of the spinning motor being an issue. But your right i have seen many a motor start while spinning. Do you not haft to bring in more out side air than exhaust to get building pressure. Thought maybe it was controlling on flow of each and using the differential to control the return fan. If the return fan would ramp up there would not be such hard time making static pressure. Seems like a paralle control signal to both would be the best approach.

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