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  1. #14
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
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    1,651
    Yes, you have to bring in more outside than you are exhausting to make the building positive. The outside air is typically brought in by the supply fan from the economizer. The amount of extra air being pulled in is the difference between supply fan CFM, and return fan CFM. As Rob mentioned, this can also be done with some % difference in speed, or based on building static pressure measurement, but I find that using actual flow is far easier to control, and it's not like flow sensors are any more expensive than static pressure. Besides, having the flow sensor on the supply fan and the return fan provides another diagnostic data point as well.

  2. #15
    I've seen what synergy said about the spinning fan backfeeding to the vfd. The fix was to go into the programming and set a longer delay for restart. It was a 600 ton ahu and the wheel took about 3-5 minutes to slow to a stop depending on rpm. Im fairly confident this is a similar problem, in fact almost identical symptoms.

  3. #16
    Join Date
    Jan 2005
    Location
    Houston TX
    Posts
    873

    Returns to the supply issue

    ascj- If I understand what you are saying that when the supply fan shuts down the return fan output contactor de-energizes while the ramp signal still exist to the vfd. When the supply fan restarts the output contactor for the return fan energizes and your return fan instantly gets the V/F directly from the drive without the benefit of ramp up. You can't start a stalled (off-line) motor with a drive already up to speed. Also open your frequency input to the drive and when the supply fan re-starts let the fan start @ 0 htz and ramp up.-GEO
    Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO

  4. #17
    Join Date
    Jun 2010
    Location
    U.A. (upper Alabama)
    Posts
    832
    Quote Originally Posted by ascj View Post
    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?
    With most newer drives you can enable a flying start parameter. This allows a motor to be spinning at any speed and when drive gets a run command it starts at 60hz then "hunts" the speed of the motor. I think what is happening here is the motor is spinning the equivalent of around 20 hz and when the drive starts at 0 hz it is seeing the amp draw of it running 20hz and trips on overcurrent. You can start a motor that's spinning,across the line all day long. That's just because the motor is spinning slower than at full speed. But start one across the line with the motor spinning faster than rated rpms( although nearly impossible) and something not good is going to happen.

  5. #18
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,330
    A little update- Finally got on the front end. It kinda of a mess, but yes supply fan tracks with static and return fan tracks on percentage of supply cfm. Relief dampers are controlled by return plenum static. Well, that's what it's suppose to be doing. I was able to tweak it abit, but it needs to be programed better. And the reason the return contactor is de-energizing when supply fan loses power, is because the 24v for the airhandler is feed from a transformer off the disconnect of the supply fan. The start/stop for VFD's is controlled by rib's power by remote bms controller. It's not that big of a deal, I'm just going to put another rib in series, that closes when power is applied to the supply fan contactor. That will make sure the fans start/stop together and always accel after lose of power.

    Back to the return fan.....going through the setup, I found something I have never seen. There are line filter station on the line voltage going to the fan control boxes. They consist of a line reactor, pf correction cap, and line filter. Then the voltage goes up to the disconnects. Then it splits, one way to bypass contactor and the other way to another line reactor. From the line reactor to vfd, then through inverter contactor. I wondering if this causing some of my issues. I was told by ABB, that they don't need line reactor before there drives. And if you do and have a problem, try enabling DC stabilizing or remove them. DC stabilizing has worked in the past, but that was with only one line reactor and it was always just faulting supply voltage unbalance.

    As far as the fan not starting in bypass, unless fully up to speed. All I can say is motor megged at 550Mohm. The voltage to contactor and motor goes into the toilet, unless motor is spinning at 1000rpm+. The starting amps just stay high for to long, trying to start under 1000rpm. Just wierd.

  6. #19
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,651
    Sounds like most likely they had an older drive in there, or old power factor corrected starting, and it was replaced with the newer ABB. If the ABB that is in use doens't need the reactor, you might try bypassing them and see if it starts smoother.. that said, I can't see how the reactor's would negatively affect the performance of the drive. Ultimately, it ends up as a relatively smooth DC, so the phase shift really shouldn't matter.

    If the return fan doesn't have a spearate start/stop output from the DDC, you'd be better off slaving the return fan VFD's enable input from the supply fan's status output, than you would be adding another relay to the mess.

    In the end, this situation looks like an upgrade gone wrong. Leaving in unnecessary equipment, leaving the older slaving controls in-tact, even after the VFD addition/upgrade, etc. Starting and stopping should be done with the VFD, not the contactors. Get that unnecessary power factor correction out of the mix. Simplify things, and it'll either start working right, or it'll be a LOT easier to find the real problem.

  7. #20
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,330
    Quote Originally Posted by CraziFuzzy View Post
    Sounds like most likely they had an older drive in there, or old power factor corrected starting, and it was replaced with the newer ABB. If the ABB that is in use doens't need the reactor, you might try bypassing them and see if it starts smoother.. that said, I can't see how the reactor's would negatively affect the performance of the drive. Ultimately, it ends up as a relatively smooth DC, so the phase shift really shouldn't matter.

    If the return fan doesn't have a spearate start/stop output from the DDC, you'd be better off slaving the return fan VFD's enable input from the supply fan's status output, than you would be adding another relay to the mess.

    In the end, this situation looks like an upgrade gone wrong. Leaving in unnecessary equipment, leaving the older slaving controls in-tact, even after the VFD addition/upgrade, etc. Starting and stopping should be done with the VFD, not the contactors. Get that unnecessary power factor correction out of the mix. Simplify things, and it'll either start working right, or it'll be a LOT easier to find the real problem.
    LOL.....the building is 3 years old. Why these line correction boxes were installed is beyond me.

    The start/stop is done done through the rib's to to the vfd's from the bms. Why didn't power the bms controller of the air handler is beyond me. I just want the start/stop to both vfd's when service disconnects are turned off. Hence adding the rib to to the start/stop chain.

  8. #21
    Join Date
    Mar 2003
    Posts
    7,321
    most drive mnaufacturers like to see the output of the drive turned off as opposed to leaving it on and using a contactor or disconnect switch between the drvie and motor. This is bad practice. If there is any means of disconnecting the output from the drive before it reaches the motor, a current sensor or other device should be installed to open the drive output. They just dont like the dc bus energized with no load.

    Now, on to the motor spinning prior to the drive being energized. There are ways to address this such as dc braking, or speed search feature. Speed search would seem to be the most logical for your application, and as previously described, it essentially takes the drive to 60 htz, and backs on down to meet your actual speed and keeps the fan spinning. dc braking stops it, then restarts it. ramp time adjustment wont help due to the fan being freewheeling from the influence of the supply fan if its running.

    With regard to the overload situation in bypass, is it possible that your bypass phase rotation is wrong? one would think this would be the easiest problem to fix. or is your overload sized wrong? with some of the newer starters, those overload settings can be easily messed up, especially those with two and three dials like mammoth used for a while.

  9. #22
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,330
    Quote Originally Posted by flange View Post
    most drive mnaufacturers like to see the output of the drive turned off as opposed to leaving it on and using a contactor or disconnect switch between the drvie and motor. This is bad practice. If there is any means of disconnecting the output from the drive before it reaches the motor, a current sensor or other device should be installed to open the drive output. They just dont like the dc bus energized with no load.
    Agreed.....that's why I'll be adding a relay to kill start/stop to both VFD's when power is shut down to either motor.

    Now, on to the motor spinning prior to the drive being energized. There are ways to address this such as dc braking, or speed search feature. Speed search would seem to be the most logical for your application, and as previously described, it essentially takes the drive to 60 htz, and backs on down to meet your actual speed and keeps the fan spinning. dc braking stops it, then restarts it. ramp time adjustment wont help due to the fan being freewheeling from the influence of the supply fan if its running.
    These are ABB drives. They were already configured for scalar flyst, which is flying start. I try fly + boost, which didn't make a difference. I didn't think it would.

    With regard to the overload situation in bypass, is it possible that your bypass phase rotation is wrong? one would think this would be the easiest problem to fix. or is your overload sized wrong? with some of the newer starters, those overload settings can be easily messed up, especially those with two and three dials like mammoth used for a while.
    The rotation is correct. And the solid state overload setting is correct.
    It's been better sense I messed around with some of the parameters on the drives. But I still can't start them in bypass(unless they are up to speed) and when they are set to inverter, they will flash overvoltage all day long.

    I really think my next step is to bypass these motor filters and see what happens. Been dealing with other problems. The ahu's are running.......so I'll update when I get to them.

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