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Unread 10-21-2003, 09:01 AM   #1
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Default undervolting fans. cfm/volt?

I'm wondering if anyone has done any quantitative testing on the effects of undervolting fans and the relationship between the voltage supplied to the fan and the cfm output of the fan.
Is the relationship linear? e.g. a 100cfm fan dropped from 12v to 7v is about 59% the power of the fan, so would that mean that the fan is producing now 59cfm?

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Unread 10-21-2003, 11:00 AM   #2
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Nothing quantitative, but it seems to me that the pressure output of the fan decreases rather more than predicted with linear relationship. P-Q curves as a function of voltage would be MOST helpful; bug JoeC at overclockers about it as he is setup to do something like that now.
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Unread 10-21-2003, 11:14 AM   #3
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the mfgrs of fans reted 6 - 14V sometimes have P-Q curves showing the output at 2 or 3 voltages
not linear at all
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Unread 10-21-2003, 12:59 PM   #4
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I don't see how it could be linear. The faster the blade is spinning to more resistance it will incounter from air. The slower it turns the less resistance it will incounter by air.

As an example (not real numbers):

Say it takes 12V to get 100CFM

It may only take 5V to get 50CFM being there is less resistance.

OR this shows how little I know about airodynamics. Or airodynamics may not even play a part.

I will go back to shutting up now......
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Unread 10-21-2003, 03:07 PM   #5
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Its not all about voltage. The fans I use pull 1.5A at 12V. I was expecting this to go up as I dropped the voltage. Surprisingly it also dropped. At 5v they only pull 0.3A. So at full power they use 18W however I run them at 1.5W. I don't have any solid number but I would guess they are moving roughly half the aiur they used too.
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Unread 03-11-2004, 08:25 AM   #6
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Hmm, not sure about airflow, but I do know that the RPMs follow a linear relationship (although not with slope such that halving voltage halves the RPMs.)

So, if anybody else has a relationship between RPMs and CFM, it might be possible to string together a better relationship. (But I agree that by the P-Q curves, it is probably not going to be linear.) -- Paul

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Unread 05-09-2004, 10:52 AM   #7
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Some useful info
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