DC Fan modification

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SentinelAeon

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I have a question about a 12V pc fans. Lets say we have a 90mm fan, 12V 0.5A. What would happen to this fan if we replaced the 90mm blades with 120mm, 140mm or even 200mm ones. Lets assume that the fan motor is strong enough to actualy turn the blades. Would this mean:

a) the fan motor will overheat and stop working quite fast
b) the fan motor will work but will be more noisy
c) the blades wont even spin properly
d) something else ?
 
Assuming you are not increasing the static friction, it would run (unless you went crazy with blade size and it was a very crude brushless motor).

The speed would be lower and it would take more power. How much lower / more is total guesswork..

Quite a few small fan motors are stall-proof, they cannot draw enough current to overheat and cause damage even if the blades are locked. That's very unlike a normal DC motor, which will normally take high current and overheat quickly if stalled or the speed much restricted with full power applied.
 

Why would you not replace the whole fan?
 
This is purely theoretical for now. I always wondered what would happen yet never tried it. For now.

But does have some practical implications. I will need some 200mm fans for a room where noise is not a problem, only electricity usage is. And my room fan uses to much electricity even on minimal setting. I even tried downvolting it from 240V to 60V and it simply isnt a very energy efficient fan. Obviously i could just buy a 200mm DC fan but they are very expensive and it would be neat to transfor 1 of my 90mm 1A DC fans since i have like 30. And i have 200mm blades from a DC fan that isnt working. I also have some huge blades from room fan that is broken and it would be neat to see if any of my fans can actualy spin it. The strongest fan i have is 12V 1.6A. It is a beast, i am almost sure it would spin even the huge blades of a room vent.
 
Your 12V 1.6A fan motor would spin 200mm blades - just extremely slowly, compared to its proper blades, and probably move no more air due to the increased air friction.

You would be better off using an "Air multiplier" type setup.
They move large volumes of air, powered by a much smaller but higher pressure fan.
eg. Like the Dyson bladeless fans.

List of Dyson products - Wikipedia

en.wikipedia.org
 
Hmm ... i didnt know dyson thingy is for anything but shows ... i thought it wasnt as efficient as a normal fan. Might have to try that.

Also, i have to ask ... if i just use 9 90mm fans or 4 120mm fans instead of a big one, am i losing efficiency due to having 4 or 9 friction points vs only 1 ?
 
100² x 3.1416 = 31416 mm² - 15² x 3.14159 (hub) = 707mm² = 30709mm²
45² x 3.1416 = 6362 - 707 = 5655 x 9 = 50892
60² x 3.1416 = 11309 - 707 = 10603 x 4 = 42410.

9 x possibly double the noise from the smaller fans = pain. For a 60% uplift.
4 x somewhat more noise each, for 40% more?
 
If you use four 120mm fans with liquid bearings EG (note only US$5 each). They will move nearly 200 cfm and will be super silent (24dB) and only use ~300mA.

Mike.
 


Thank you for the calculations but i don't quite understand them and how to corelate them to noise & electricity usage. For simplicity lets compare just 2 things:

1x 200x200mm fan vs 4x 50x50mm. Both this things put together are 200x200, but 1 only has 1 motor and one has 4 motors. At the same power consumption of 10W, which will move more air and which will be louder ?

The reason i am asking is ... right now i am using a big room fan undervolted from 220 to 80V. It barely moves any air and still uses 10W. And i have plenty of 90mm fans lying around and i am wondering if i could just put a few of those fans together. Noise is not a problem in my case. I am just looking at air moved vs electricity consumption.
 
SentinelAeon said:
Noise is not a problem in my case. I am just looking at air moved vs electricity consumption.
Click to expand...
Then find soec. sheets for the fans you have and/or those you are considering using and look at their power requirement.

Eg. The specs on the amazingly cheap 120mm fan pommie linked above show it shifts 44CFM and uses 0.84W:


Look up the specs for your 90mm and compare energy usage and airflow.

On my PC, I went from 2x80 to 2x120 to get better cooling; but then switched to 1x200 because of the noise level. Bigger fans generally turn more slowly for a given throughput and slower generally means less noise.

The 2 x 120s were markedly more noisey than the two 80s, not because they were inherently noisy, but because the two did not spin at exactly the same speed, and so their sounds interfered and produce a low frequency, puslating, throbbing or thrumming noise that you were barely aware of at first but slowly impinged itself upon your conciousness and became extremely irritating.
 
I wasnt talking about specific fans, the ones i have dont have noise rating written on them, most are from OEM computers. I was talking about general rule. I am guessing 1 big fan will usualy be more efficient and quiet than 4 small fans due to only 1 moving part and the big fan moving slower. But i am wondering if this rule applies to the big room fan also. For some reason i think they are less efficient than those small computer fans. Or am i wrong ?
 
The small computer fans appear to be much more efficient than mains powered fans. This could be due to room fans using a (very) old design.

Mike.
 
There are quire a few manufacturers around of mains fans which are made very efficient.
And can be selected by degree of air flow.
Orion.
Comair.
Papst
Just for a few.
 
Here in Australia the room fans are super cheap (approx US$10) and made in China. Or super expensive (approx US$250) made by Dyson. One is probably very efficient the other not so.

Mike.
 
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