Author Topic: 12v fan recommendations  (Read 4937 times)

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lundis

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12v fan recommendations
« on: August 19, 2006, 02:38:02 PM »
On my list of 120V things to phase out and replace with 12V power are the fans in our house.


We run 2-3 window fans during the night and after storms to suck in the cooler air from the outside into our much warmer non-air conditioned home.


Been looking at several car radiator type fans - 12V at 10-30 amp draw.  Thinking that amount of draw is quite too large for our solar setup to handle.


Wondering what anyone else is doing similar with recommendations for fans / suppliers for such.

« Last Edit: August 19, 2006, 02:38:02 PM by (unknown) »

Norm

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Re: 12v fan recommendations
« Reply #1 on: August 19, 2006, 01:58:14 PM »
  If the fan blades were smaller...would draw

less amps.

  A car heater blower motor will draw less amps

by merely restricting the intake on the blower.

The motor will speed up it won't be blowing

quite as hard...or working as hard.

  A small Ametek might also work well ?

                 ( :>) Norm.


 

« Last Edit: August 19, 2006, 01:58:14 PM by Norm »

Fiddlehead44

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Re: 12v fan recommendations
« Reply #2 on: August 19, 2006, 05:32:36 PM »
Norm.....Give your head a shake.....

A car heater blower motor will draw less amps

by merely restricting the intake on the blower.

The motor will speed up it won't be blowing

quite as hard...or working as hard.


Your restriction would draw more amps as the

motor would be working harder...

Fiddlehead

« Last Edit: August 19, 2006, 05:32:36 PM by Fiddlehead44 »

jimovonz

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Re: 12v fan recommendations
« Reply #3 on: August 19, 2006, 06:08:56 PM »
What is the load on the motor - friction from the air. If you were to restrict the outlet the pressure would increase due to the air 'mounting' up. More air = more friction = more load = more amps. However Norm suggested restricting the intake. The pressure would drop. Less air = less friction = less load = less amps.
« Last Edit: August 19, 2006, 06:08:56 PM by jimovonz »

scottsAI

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Re: 12v fan recommendations
« Reply #4 on: August 20, 2006, 12:30:53 AM »
Hello Norm,

You have it right!


I just took two computer fans rated at 12v 280ma to run an experiment:

Connected to 12v battery, current 263ma. Unrestricted air flow in and out.

Blocked the intake, by laying it down on the table. Current 253ma.

Flipped it over, 274ma, could feel air blowing back out the input, so air was still moving.

The current should have gone down. These fans are not tight, air can get around the blade tips.


Put a box on it Blocking both input and output,. Current 217ma. Validation of above.

The fan speed up, if friction loss were greater, the fan would not speed up.


Connected two fans in series, current 109ma. fans slowed down.

I didn't have any way to measure air flow or RPM. Lower air flow requires less current.


Power is in the moving of the air. Restricting the input or output should reduce the power. Air friction does draw power, but not as much as moving the air.

Blocking the input or output should have the same effect as blocking both in a tight fan. (no air around the tips).

Not as efficiently as slowing down the blade.


Most Car engine Fans are designed to be PWM speed controlled. Lowering the voltage will do the same, doing so efficiently may be a problem.


Wire two fans in series, both MUST be the same motor so they share the current equally. The current dropped more than half in the above experiment. The air flow is reduced, but You have two fans running, using less current.


I run fans in my computer off 7v or series all the time to reduce the current. With 22 computers it gets costly for the power.

Have fun,

Scott.

« Last Edit: August 20, 2006, 12:30:53 AM by scottsAI »

ghurd

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Re: 12v fan recommendations
« Reply #5 on: August 20, 2006, 06:05:07 AM »
Have a look at this.

http://www.fieldlines.com/story/2005/8/27/42713/8716


And DC ceiling fans are nice.

G-

« Last Edit: August 20, 2006, 06:05:07 AM by ghurd »
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Norm

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restricted air intake less amps
« Reply #6 on: August 20, 2006, 05:57:52 PM »
Fiddlehead44.....


      No ....if you restrict the intake it will

speed up because it isn't pumping as much air...

it isn't working as hard so it speeds up ...

   Put a newspaper on the suction side of a box fan...same thing...it'll speed up because it isn't working as hard...put an ampmeter onto the fan motor and you'll see it isn't taking as many amps.

             ( :>) Norm.

« Last Edit: August 20, 2006, 05:57:52 PM by Norm »

nothing to lose

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Re: 12v fan recommendations
« Reply #7 on: August 21, 2006, 03:53:26 AM »
Well I geuss that's good to know, but if you reduce the airflow to save power and then need more fans to get the airflow you need, did you really save power?


Say I use 1 fan in the bedroom window at full speed and that's the correct amount of air. Do I really gain anything if I run 2 fans at half speed and move half the air with each fan?


Of course each fan running at half speed is less noisey than one fan at full speed, and I can point two fans in different directions. Maybe one sucking in cool air from window and one sucking heat off the ceiling blowing it outside. I do that often instead of just mixng up all the hot air and fressh cool air with one fan. Never thought it might be saving power though? Both running on medium speed instead of on high.

« Last Edit: August 21, 2006, 03:53:26 AM by nothing to lose »

Norm

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What I did...
« Reply #8 on: August 21, 2006, 06:47:52 AM »
   The easiest way to run a fan on 12 volts is

to just plug a 20" window fan into an inverter

the window fan is brushless and probably more

efficient anyways...I think they use something

like 1 or 1.5 amps and you probably already

have the fans...so that would be about 20-25

amp/hrs for 2 box fans in 8hrs.??


            ( :>) Norm

« Last Edit: August 21, 2006, 06:47:52 AM by Norm »

Drives

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Re: 12v fan recommendations
« Reply #9 on: August 21, 2006, 10:35:29 AM »
Fiddle:


Sorry, your incorrect, Norm is right.  On a centrifugal pump or fan, if you restrict the input, power (amps) will go down,  

For a positive displacement pump or blower, you are correct.  Current will go up, but a car blower is a centrifugal blower.

Try it out...it will surprise you.

Before VFD's we used to use throttling valves, and dampers to control the flow of fluids, and gases in building HVAC.  

« Last Edit: August 21, 2006, 10:35:29 AM by Drives »

Nil

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Re: What I did...
« Reply #10 on: August 21, 2006, 10:52:41 AM »
Isnt that 1-1.5 AH @ 120v? So 2 fans @ 1 amp=2 amps, 2 amps for 8 hrs=16 AH, 16ah* 120v= 1920 Watt Hours? Ouch


I'd guess a normal box fan isnt the most efficient either. Since when it runs they have Slip losses. Something like a brushless motor that uses magnents and coils has to be better than normal AC type motors.


Just to confuse things a bit

« Last Edit: August 21, 2006, 10:52:41 AM by Nil »

Nil

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Re: What I did...
« Reply #11 on: August 21, 2006, 11:02:10 AM »
A bit more to add. If those fans use 1920 watt hours, assuming you use a inverter to bump that to 120v. To convert the energy for your fans for 8hrs you'd have to pull 160AH out of your 12v batteries.


Once again assuming the 1 amp figure (and my math)is right.

« Last Edit: August 21, 2006, 11:02:10 AM by Nil »

scottsAI

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Re: 12v fan recommendations
« Reply #12 on: August 21, 2006, 06:17:28 PM »
Hello Nothing to loose,


I would not suggest to reduce airflow to save power, not any where near as effective as reducing the fan speed.


In large pumps, the intake is shut off during start, the starting current is much much lower. I had a guy forget to shut off the intake, popped the breaker due to over current during the start! Man did that cause some crap...


My choice would be to use two fans in series.

In my experiment the current is 109ma, less than half the normal running current. (263ma.)

If the air flow of the two are equivalent to one full speed then there is a huge power savings:

Series fan 1.308 watts, full speed fan 3.156 watts. Just shy of 60% less power!!! Wow!


By the way, stacking two fans in series only increases the pressure not the volume of air moved. Two fans in parallel, will double the air flow. Learned a lot about fans working on improving the temperature in the case of the SETI computers. Very frustrating work. Lot's of miss information out there. Designed a very cheap CPU water cooler for my next upgrade.


I too was wondering about how much air flow he needs.

The slower moving large fans often are more efficient than the high speed small ones.

Someone mentioned Ceiling fans. The idea is to move the air, not jet it out!


For my computers I use 92mm fans, run them on 7v. (12v to 5v) Power drops fast with voltage. So does the noise. I use the 7v so if one fan fails the other still works. Many cheap computer fans do not work on 6v.

Have fun,

Scott.

« Last Edit: August 21, 2006, 06:17:28 PM by scottsAI »

Sue Karber

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Re: 12v fan recommendations
« Reply #13 on: August 22, 2006, 10:38:53 AM »
Home Depot has some highly efficent 12volt attic ventilation fans for about $80 US,

designed to run on Solar panels. You would have to build a mount to install them in a window. Harold & Sue
« Last Edit: August 22, 2006, 10:38:53 AM by Sue Karber »

hohenwald48

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Re: 12v fan recommendations
« Reply #14 on: August 23, 2006, 02:51:38 PM »
I went to www.surpluscenter.com and bought several appropriate speed and current draw D/C motors.  Then went to Wal-Mart and bought 4 cheap box fans (about 2' square)and replaced the A/C motors with the Surplus Center D/C motors.


Draw about 3 amps DC, cost about 20 bucks total each and i have one that has been running full time for almost 2 years with no problems.

Chris

« Last Edit: August 23, 2006, 02:51:38 PM by hohenwald48 »

ghurd

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Re: 12v fan recommendations
« Reply #15 on: August 23, 2006, 03:15:27 PM »
Got a part number for the motor?

G-
« Last Edit: August 23, 2006, 03:15:27 PM by ghurd »
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nothing to lose

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Re: 12v fan recommendations
« Reply #16 on: August 24, 2006, 09:07:29 AM »
"I would not suggest to reduce airflow to save power, not any where near as effective as reducing the fan speed."


Not really sure which you meant.

 If I run a 12" fan on a lower speed it moves less air, reduces airflow.

Someone else mentioned restricting inputs or outputs reducing airflow I think.


I will have to dig out my fans and play with them, you may have me rebuilding a few things again soon. Mostly I use 80mm computers fans for everything and some run fullspeed on full volts and others I series wire for lower volts/speeds. Not my normal house fans, but stuff I build for other uses like vans or camping uses etc...

« Last Edit: August 24, 2006, 09:07:29 AM by nothing to lose »