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Thread: Variac speed controller on fan - whats the output now !?

  1. #1

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    Default Variac speed controller on fan - whats the output now !?

    Hey everyone

    I just installed a variac control on my 800cfm Rhino fan and have the following question, given that the five settings for the variac are 110v, 140v, 170v, 190v and 230v (UK here), is there anyway to work out the output of the fan based on this.
    Eg. 110v is approx half of 230v and so could one assume the 800cfm fan is kicking out a little under 400cfm.

    I know I will likely be directed to the manufacturer's website but there is scant information on this question.

    Thanks in advance

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  3. #2
    MrChedHead Guest

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    at a guess id presume @ 170v it would kick out roughly 400cfm


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    Just what I found by having a Google...


    Calculate the variac’s maximum power-handling capacity. Multiply its amp rating by 84.8. For example, a 10 amp rating x 84.8 = 848 watts

    R.M.S. power is a conservative way to figure equipment’s power-handling capacity. It’s .707 times the peak power. In the case given here, 120 volts times the current rating times .707 equals 84.8 times the current rating. If you use less than the calculated R.M.S. power, the variac won’t be overloaded.

    if the motor speed is reduced 20%, motor horsepower is reduced by a cubic relationship (.8 X .8 X .8), or 51%

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    I'm thinking it might be more toward the higher voltage as TT suggested. Karsa, never mind overloaded the variac, you just overloaded my brain

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    Karsa (28-09-17)

  7. #5
    MrChedHead Guest

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    Quote Originally Posted by kieron90 View Post
    Hey everyone

    I just installed a variac control on my 800cfm Rhino fan and have the following question, given that the five settings for the variac are 110v, 140v, 170v, 190v and 230v (UK here), is there anyway to work out the output of the fan based on this.
    Eg. 110v is approx half of 230v and so could one assume the 800cfm fan is kicking out a little under 400cfm.

    I know I will likely be directed to the manufacturer's website but there is scant information on this question.

    Thanks in advance
    Hang on mate i just re-read that!
    Id hazard a guess that ...

    230=100%...800CFM
    190=80%.....640CFM
    170=60%.....480CFM
    140=40%.....320CFM
    110=20% .... 160CFM

    just a guess tho lad

    so assuming the above id say


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  8. #6

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    I'm being told that half the voltage means half the current, and that equates to a quarter of the power. That's make my 800cfm fan run at 200cfm if I am lucky (likely less as it's probably more efficient at higher speeds, as someone suggested), that means it's likely 175cfm which drops to 140cfm or so when you take into consideration the filter and the ducting. That's 140cfm for a can rated at 176cfm, at least when I run it on the first setting, I checked with some smoked at the passive air ducts, it gets sucked right in, so there's negative pressure there. Hopefully it'll scrub the air as quickly as I want !

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