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  1. #1
    Join Date
    Nov 2011
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    Two Speed Motors


    I looking for information regarding two-speed motors. I am trying to do a comparison between the existing (two-speed motor) and a VSD for a cooling tower, 30 HP motor.
    My layout for the VSD is as follows

    50% Load = 2 hours/day
    60% = 3
    70% = 6
    80% = 6
    90% = 4
    100% = 3
    Using this (very basic) layout I can estimate consumption for the VSD. However, I am uncertain how to approach two speed motors. The assumption is 50% & 100% load, but I do not think this is correct.
    Please advise.
    Thank you for reading.

  2. #2
    Join Date
    Dec 2003
    Northern VA
    Post Likes

    Vfd programming

    It seems you are asking if on a present two speed motor (can I guess an Evapco tower?) whether the low speed operation delivers 50% capacity all the way around.

    For instance, at low spd. does it turn 850 rpm (50 % of 1725 rpm),does the blower wheel or prop fan thus deliver 50% of its' full speed CFM. and does the heat rejected therefore equal 50% of the tower's rated capacity?

    If this is the question, in light of the law of diminishing returns, I don't think this will bear out.

    In my opinion, to get a more accurate idea of what the low speed perfomance is in reality, I would base my percentages primarily from the motor amp draw at low speed compared with the high speed, and secondly the difference between the temp. split between fluid in / out.
    I would make sure the belts and pulleys are in good condition before I get the amp draw.

    Is this a wet/dry tower with a 1st stage jet pump? You may want to get the same readings in both dry and wet mode and compare.

    Hope this helps.

    And Good Luck.
    "Iron sharpeneth iron..."

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