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  1. #1

    Excessive fan speed

    First time posting i have been following you guys for awhile now just seeing if you guys could point me in the right direction the problem I'm having is on a York rtu m# b3ch120a47a in the afternoon my fan speed ramps up along with my amps and also voltage the building I'm at does have a major problem with voltage spiking past 500 volts ac during the day starting off slightly below 460 in the morning we do have phase protectors on every unit due to the fact the company that was in before us was replacing compressors every week since we install the phase protectors very few compressors went out but now with my problem the evap blow motor is 3 phase and is wired for 460 that checks ok Amps on the motor name plate check Out at 3 Amps I'm pulling 4.2 with 497 volts to the motor i would think that this will speed fan motor rpms up and the complaint is that to much air noise late in the day keep in mind the phase protectors cut out at 500 volts just would like some input here on what to do and or check next the pulley and sheeve are at the min adjustment for lowest fan speed I'm thinking about using a tach to check true RPMs are

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  2. #2
    Join Date
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    Quote Originally Posted by totalhcgeo View Post
    ...evap blow motor is 3 phase and is wired for 460 that checks ok Amps on the motor name plate check Out at 3 Amps I'm pulling 4.2 with 497 volts to the motor...
    if you are pulling 30% over nameplate, then the power issue isn't your biggest problem...resheave and recheck. your motor amps will change with voltage, but they will also change with load.
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  3. #3
    That's what I'm going to check in the morning call the supply house and check what size pulley and sheave part numbers should be in the unit its just different how nothing was adjusted and the problem its only later in the day there are no dampers and also no zoning so its really straight forward i really do feel that the fan motor itself is picking up speed the motor is only 1725 RPMs and high and low voltage 230/460 guess I'll just look into it more thanks for the input

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  4. #4
    Join Date
    Feb 2011
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    Sacramento, CA
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    147
    Higher voltage = lower amps. I agree with jayguy check the pulley if it is adjustable open it up and you should see your amps drop. If it is a fixed pulley, call the factory and see what pitch pulley was installed at the factory and start there. Because im sure that a start up and air ballance report are nowhere to be found.
    I have done so much with so little for so long, that now I can do almost anything with nothing at all.

  5. #5
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    Quote Originally Posted by totalhcgeo View Post
    ...i really do feel that the fan motor itself is picking up speed the motor is only 1725 RPMs...
    it shouldn't be picking up much speed, maybe 10-20 rpms at most. the only way a motor could pick up a lot of speed (other than a failed motor) is by UNLOADING it but it sounds like your motor is getting more loaded.

    check the Hertz (most digital meters can easily perform this function)...perhaps the utility is messing with stuff (frequency changes much more often than most people think). motor speed is determined by the number of poles (fixed by the design and manufacturing of the motor) and line frequency.
    It's tube brushing season...93% done (39.35 miles of tubing)...only 1,242 tubes to go!

    Original member of the Raccoon Brotherhood.

    EVERYTHING IS AWESOME!!!

  6. #6
    I returned to that call today and checked everything out checked for the right pulley from factory and sheave everything checked out what i did find is there maintenance guys did change duct work 10ton unit cooling a 2000 sq ft space with all 12 inch flex ran to supply vents yes the duct work is sized right and take offs but running a 12 in supply that's good for i believe 700 cfm over top of an office space with 8 foot ceilings your going to have hellish air coming out this its what i narrowed it down to i just didn't have to time to look into yesterday what do you guys think and what should i recommend the unit is definitely over sized and they will bout spend the money to change out the unit i was possibly thinking about putting a bypass damper in that could just blow air back into the return ceiling space

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  7. #7
    Join Date
    Dec 2008
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    Dixiana, AL
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    My personal opinion is that some punctuation would have a calming effect and possibly help you to slow down and .......... breathe .......... breathe....

    There - now don't you feel better?


    Now look at what you said: you have a forward curved centrifugal fan that's pulling too much current. If you install a bypass damper, you load the fan by allowing it to push more air. You need to restrict the flow to unload the fan. Is there the possibility that someone tried to make this system a VAV system and the ductwork is oversized? I don't disagree with the fact that your voltage is a tad high, but not excessively so. There are millions (well, maybe 100's of 1000's) of 460 volt motors out there running in the 490's and not pulling 30% over nameplate. You have problems elsewhere .

  8. #8
    Join Date
    Oct 2004
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    The Zone
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    392
    Once the pulleys are set motor speed has more to do with hertz than power.
    Is it possible returns are increasing by doors in various areas opening?
    The free return can cause a motor to move more air.
    Have you checked velocity?
    It's Hammer Time!

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