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Thread: Motor Question

  1. #1
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    Motor Question

    Not sure if this the place for this , but I bet I get an answer....I had to do a start up for another contractor on an exhaust fan at a waste water treatment plant...I didn't do the install. Small fan, dont even know the HP, but it doesn't matter here. Here's the problem. Motor starter trips after about five minutes. (overloads they installed were too small)
    Motor data as follows:
    460v/3ph fla .55 service factor 1.0
    The overloads installed were .32 to .35 amps. (Sq D BO.51...I think)
    Actual amp draw .6/.7/.6 (each of the three legs).
    My question is, if I put in bigger overloads on the starter to handle up to .7 or .8 amps, will I hurt the motor, or am I being too fussy? I know I would be over the service factor, and I'm not too comfortable about it.
    Don't want to mess with the adjustable shieve just yet, the cfm here is really important here and the contractor doesn't want me to do that just yet. Everything is explosion proof and I guess the air exchanges are important.
    What do you guys think?
    Thanks!
    once you think you know everything, you'll never learn another thing!

  2. #2
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    Your first problem is the motor is over amping. Rated at .55 drawing .6 / .7 / .6 Why?

    Was the blower replaced but hooked up to the existing starter?

    Whoever installed / designed this should be making the decisions concerning heater size!

  3. #3
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    Quote Originally Posted by pecmsg View Post
    Your first problem is the motor is over amping. Rated at .55 drawing .6 / .7 / .6 Why?

    Was the blower replaced but hooked up to the existing starter?

    Whoever installed / designed this should be making the decisions concerning heater size!
    Brand new building, new installation, all brand new equipment. Not sure why the motor is overamping at this point. Would like to adjust the sheive to try and lower the amperage, but they want to keep the cfm up. Didn't design it just got hired by another contractor to start it up and make sure it's right...something his guys can't do. Just wondering if that amperage will hurt the motor over time. If yes, my advice may be to re-design and install it properly at a cost no one wants to incur.
    once you think you know everything, you'll never learn another thing!

  4. #4
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    The actual current draw on this motor is 33% over the motor full load rating. Reducing the fan speed by 10 % would be a solution; however the new current draw would still exceed the rating on your overload.

    I think you need to look at design elements; system requirements and fan curve would be helpful.
    Local 30 New York, New York Operating Engineer

  5. #5
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    Quote Originally Posted by ddalberto View Post
    Brand new building, new installation, all brand new equipment. Not sure why the motor is overamping at this point. Would like to adjust the sheive to try and lower the amperage, but they want to keep the cfm up. Didn't design it just got hired by another contractor to start it up and make sure it's right...something his guys can't do. Just wondering if that amperage will hurt the motor over time. If yes, my advice may be to re-design and install it properly at a cost no one wants to incur.
    You state the contractor wants to keep the cfm up. What cfm is the exhaust system producing? What was the design cfm?

    You are damaging the motor by over amping it.

    You were contracted to start it up and make sure it's right. Well it's not right if it's over amping. So the next step is to measure airflow and compare it to specced air flow. Then you have a leg to stand on and know how to proceed.

  6. #6
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    Cool

    As the other techs stated above your first problem is over current.

    Yes, adjust the drive to bring your amperage to .5 or lower temporairly. Your overloads should be sized for .5 amps, no higher.

    Also as stated above , find out what cfm the fan should be exhausting. Then measure your cfm.

    At this point you can figure out if the fan or the motor is capable of doing the job it was designed for.
    "My hands are for sale"

  7. #7
    Make sure the rotation is correct. On a centrifugal fan, it will still move air, but the amp draw will be higher.

  8. #8
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    Any chance the motor is wired for the wrong voltage? gotta ask.
    Did I just brain fart? Seems right.
    Last edited by btuhack; 02-20-2010 at 11:02 PM.

  9. #9
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    Fan is rotating correctly, wiring is correct. Don't know the recommended cfm. Municipal job specced by an engineer. Have to get my hands on the prints to see the recommended cfm, measure it and see if I can ahieve with the shieve, while trying to get my .55 amps. If not, the engineer will have to start from scatch...maybe a larger fan (which will entail duct work too).
    My company was hired by the mechanical contractor to start up a bunch of stuff. We didn't design, spec or install it. I was just wondering if the motor would get hurt being so close to the service factor. I think I got my answer....yes. All I can do is make a recommendation at this point.
    Thanks for the feedback.
    once you think you know everything, you'll never learn another thing!

  10. #10
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    Roof Monkey,

    That’s a bold statement regarding motor current and rotation. Could you elaborate a little more on your conclusions?
    Local 30 New York, New York Operating Engineer

  11. #11
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    i understand about the cfm in places like that...i have one myself that i take care of.

    what was the voltage? lower voltage will equal higher amp draw.

    as far as heater sizing...like others have said...pass it on to the installing contractor.
    How to make the perfect "Half-Hitch" knot or any other boyscout knot in 3 easy steps...

    1. Remove your meter leads from the meter and very carefully return them to their storage case or bag.
    2. Wait 2 seconds
    3. Very carefully remove your meter leads from their storage place and enjoy your new knot!

  12. #12
    Kdocsr05,
    Please refer to point #2 under "operation" in the attached link. This explains it better.

    http://www.captiveaire.com/MANUALS/E...BLAST_OIPM.pdf

  13. #13
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    Reverse rotation with radial and forward curved bladed impellers will give reduced performance and power consumption but can cause severe overloading of the motor in the case of backward bladed impellers.



    http://www.engineeringtoolbox.com/fan-types-d_142.html
    Local 30 New York, New York Operating Engineer

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