# PC fan generator

Discussion in 'General Electronics Chat' started by wannaBinventor, Oct 21, 2010.

1. ### wannaBinventor Thread Starter Active Member

Apr 8, 2010
180
4
I played around last night with using a computer fan as a generator. It is a 120mm fan. I took it apart and soldered leads directly to where the coil windings are joined to the circuit board. The resistance between the two coils (there were four wired in series, these were opposite each other) was about 120 ohms.

In a light breeze, this gave me about 1.3 to 1.4 volts AC RMS -- so I was fairly incouraged by the energy harvesting capabilities of this little fan if I got it in some good wind.

Now, the questions:
I am terrible with the basic theory of electronics -- ohms law and what not. I just never took the time to master it. I'm trying to determine how much energy this is actually producing, so it did this.

Voltage (AC RMS): 1.4 volts
Reistance between coils: 120 ohms
I = V/R
I = 1.4/120
I = 11.67mA

Am I thinking correctly here?

If so (and here is where I get SUPER ignorant) --- say I were to triple the voltage to 4.2, minus a few schottky diode drops to rectify to DC leaves me with say 3.4 volts. Say I use this to recharge a couple of batteries or a capacitor and draw from it at different times (I'm thinking a uC taking ADC measurements and an rf transmitter -- both with a very low duty cycle).

Could I, according to this, draw a max of < or = 11.67mAh with my circuit and expect it to sustain itself? (Assuming the wind keeps blowing at that rate).

Next question:

The fan will not always start to turn at very low wind speeds, but it seems at the that if you help it break the "initial holding force" that it's momentum will keep it turning even at the same wind speed.

So, what is that "initial holding force?" Is it the magnetism between the stator and rotor? The friction of the shaft bearing? Both? What's the best way to help overcome this?

Thanks for the help!

Dec 7, 2008
174
13
Hi,

File size:
1.1 MB
Views:
395
3. ### PackratKing Well-Known Member

Jul 13, 2008
849
218
Tilting at windmills ? are we ??

Yup. it is as you say w/ its magnetism.......a cold day will aggravate the problem by stiffening the grease in the bearings, if ball, worse if solid sleeve or "oilite"

In the event you have a means of injecting a drop of superfine non-detergent bearing oil into them will help a little. [ NOT "3 in 1" 'cuz it will only turn to snot under any heat at all... ]

Last edited: Oct 16, 2013
4. ### wannaBinventor Thread Starter Active Member

Apr 8, 2010
180
4
Thanks for the replies and helpful information guys.

PackRat:

I read that people try and boost the output of something like this by using strong magnets. I'm assuming, then, that while it will boost the voltage once rotating, that it will only make the initial holding force greater. Is this correct? Would adding more windings to the coil be the solution to allowing for more votlage without raising the initial holding force?

Also, can anyone confirm my thinking on my mAh calculation?

5. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Just remember that as you add load to the output it will be harder to rotate the blades.

Also - not all PC fans will work, they've built them in several different ways.

6. ### soda Active Member

Dec 7, 2008
174
13
If you add more winding to the armature, then you have to use stronger magnets, so I will either go for a larger fan if I were you.Your calculations looks correct. It's ohm's law you use.

Why don't you rectify the voltage and then see if you can light up a LED. You can then hook up a avo tester and see what current the led draw. Thought, the led can handle any thing up to 30mA.

I must say, I like your idea a lot and I'm going to try it myself. I'm going to use a larger fan and then add larger blade's to it.

7. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
Fans are designed to drive air, not to be driven by an air stream. They won't be very efficient at generating electrical power. Take a look at wind turbine blades and you'll see what I'm talking about. They have more of an airfoil shape, and more closely resemble an aviation propeller.

If you wanted to get sophisticated about it, you could build a constant speed blade, where the blades can pivot. The angle of incident to the wind will affect the torque applied to the motor.

8. ### wannaBinventor Thread Starter Active Member

Apr 8, 2010
180
4
Thanks for the help guys.

Sgt Wook:
That adjustable blade propeller sounds like something I'll need to spend some time in engineering school to design, lol.

Am I understanding your comment correctly when I take it to mean that the inner workings of the coils/magnets of the fan are fine, but the fan propeller itself is fundamentally flawed?

If so, could I just chop off the blades of the fan, epoxy on a good sized model airplane propeller and see more RPM (and more voltage) over the regular fan blade for the same wind speed?

9. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
A model airplane propeller would not have the right shape either.

If you examined a cross-section of your fan blade, you would notice that it has a distinct airfoil shape; like an airplane's wing. That helps the fan blades to be more efficient. However, the shape used was meant for driving air through the fan, not for air to drive the motor's spindle; it's opposite of what's needed for efficient use of prevailing winds.

It would take time and lots of experimentation to come up with a really decent design. However, since the blades will be rotating at much lower speeds than when used as a fan, the airfoil shape can be much more pronounced.

If you took a real airplane propeller and examined the cross sections at various distances from the hub, you would see that the airfoil shape is most pronounced at the hub where the relative surface speed is the slowest, and it becomes nearly knife-thin at the very end. The angle of incidence changes as well. This helps to optimize the "lift" effect across the entire length of the blade.

10. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Yea, a lot of people don't realize that an airplane's propeller actually pulls the plane through the air like a corkscrew.

11. ### ke5nnt Active Member

Mar 1, 2009
384
15
If I remember correctly from ground-school, it's P-factor. Propeller driven aircraft's naturally tendency to bank left when torque is applied.

12. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
Well, P-factor comes into play when an aircraft's angle of attack is high. However, that's not really terribly relevant to the thread, unless one wants to consider efficiency when the turbine blade is off-center from the wind that's driving them.

http://en.wikipedia.org/wiki/P-factor

I remember seeing a design for a turbine blade that resembled a squirrel cage fan. It was very interesting to me, as the turbines' shaft was oriented vertical, and was mounted in a fixed position. The squirrel-cage design would be rotated efficiently by airflow that came from any direction.

13. ### PackratKing Well-Known Member

Jul 13, 2008
849
218
Summed up in an HVAC rule of thumb.........air digs, Water slaps...

Last edited: Oct 16, 2013
14. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
I remember seeing a design for a turbine blade that resembled a squirrel cage fan. It was very interesting to me, as the turbines' shaft was oriented vertical, and was mounted in a fixed position. The squirrel-cage design would be rotated efficiently by airflow that came from any direction.

I remember seeing that too, I think it was years ago in issue of Popular Science or Mechanics but they were probably just doing an article on the concept.

Wonder what ever happened to that idea?

15. ### DonQ Active Member

May 6, 2009
320
14
Also, since you are trying to get power out of the wind, remember that P=I*E.

When you implement what you suggest as a 'circuit', the only way you'll have a total of 120 ohms is if you have zero ohms external to the fan. Then you will have the maximum current, but zero external voltage, with the net effect of zero external power. With that circuit, all of the power will be dissipated in the fan itself. Not much help.

With an infinite external impedance, similar to what you had when you measured the voltage with a voltmeter, the voltage is at a maximum, but the current is at zero. Net effect of zero power to the external device.

The maximum power is transmitted to the external load when the impedance of the load is equal to the impedance of the source, in this case 120 ohms. At that point, the voltage will be divided across the source and the load, so you will have half the voltage across the load. Start your estimate there.

Now, if you are using this for a battery charger, you will not have an impedance match without some intermediate circuitry, so you have to assume less than the max power transfer. How much less depends on a lot of things. Adjust your estimate.

Also, once you start to load the generator, the rotation speed of the blades will start to decrease. In a perfect world, the air-flow equivalent of impedance matching would also apply to the fans rotation. Max speed, no torque. Max torque, no rotational speed. Max power somewhere in-between. The end result, is that the rotational speed will be reduced to get power out of the fan blades. Adjust your estimate again.

This effect will be further skewed toward the 'not good for you' end because of the inefficiency of the fan blades at extracting power from the wind. They also have problems of stalling (an aerodynamic stall, meaning the wind would stop turning the blades) if you attempt to get even a relatively small amount of the total energy available in the wind. Adjust your estimate yet again. (Better blades can get more energy out of the wind before stalling.)

Bottom line, lots of math, and lots of unknowns even if you try do the math. Better to just realize the limitations (at least the magnitude of the limitations), go with it, and see what you get. Improvements can come later.

16. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
719

Commercial Version