The idea of resistance

[taylorp035]

The general equations of V = I*R and Heat(Watts) = R*I^2   <-- I think (makes sense)

Therefore, as a windmill generates more power, more heat is generated, but based on the first equation, if V=12, then R must go down as I goes up.

So as to my real question: I see people mentioning the idea of adding resistance to their lines, therefore the amperage would be lower, meaning an easier time getting to high rpm's ?

If I am wrong, please explain why adding resistance helps.

[RP]

Adding resistance allows the blades to come out of stall (if they are stalled) and thereby better couple to the wind.

The extreme case would be where the mill is shorted (zero resistance) and the blades turn at only a few rpm in a good wind.  Almost no energy is being produced.  By adding resistance the blades begin to turn and with enough resistance the blades get closer to an appropriate TSR and start producing greater amounts of power.

[ghurd]

Or could look at it starting at the end, and working backwards.

Battery volts, plus line voltage drop = windmill voltage.

The extra resistance lets the operate at a tiny bit higher voltage with the same amperage, meaning it is operating at a slightly higher RPM.

And the higher RPM is better matching the wind speed to the power output.

Same result from either end.  Gets it out of stall.

G-

[wpowokal]

There are two voltages in play, the generated  and terminal volts, one could think of the terminal volts (battery less diode and line volt drop) as constant, and up to cut in they can be considered the same.

After cut in the generated volts must rise to cause current (amps) to flow, it is easiest to consider resistance as remaining constant although some small decrease occurs because of heating. Here is where stator heating begins to rise with current, and efficiency drops (heat in stator) the squared part of the equation.

allan

[taylorp035]

Between the 3 of you, the all of my electrical lessons have come around and made my circuit complete (light bulb turned on!).

So basically, the only reason on why I would need more resistance when it comes to my windmill would be to use the power from higher wind speeds, which in a couple of hours could be helpful with my new 4' diameter blades that I just made.

Just curious, how many ohms are "generally"  used for a windmill, as I have two 5 ohm resistors.  I didn't really feel like calculating this at this current moment, nor making an educated guess with no real knowledge to go along with it.

[RP]

I believe I'm seen Flux recommend inserting ~2ohms from time to time.  Put your 2 5ohm units in parallel and then put the pair in series with your battery and mill.

Put a switch across the resistor pair so you can easily compare current output with and without the added resistance.

[taylorp035]

Good idea!

Thanks

[ghurd]

You have entered a gray area...

(take an aspirin now)

High winds can over power the blades and it breaks out of stall.

Like it will go up to 2A, and almost refuse to go any higher.  Or faster RPM.

Until the wind gets a LOT faster, then it jumps up to maybe 10A.

The problem is it is making 2A all day when it should be making 8A all day.

Treadmill motors usually have a fairly high coil resistance anyway.

I doubt you will stall in a manor where adding line resistance is a great idea.

If the treadmill motor stalls, you made the blades too short or too fast.

Might find something more relative to stall related to a treadmill motor here,

in opo's motor conversion,

http://www.fieldlines.com/story/2009/10/15/17331/618

http://www.fieldlines.com/story/2009/11/30/194624/37

G-

[Flux]

I am not sure I follow this.

Are you charging batteries or are you trying for heat in a resistor?

The explanations given above all refer to a machine charging a battery. You only seem to have a small machine and normally for battery charging the generator/alternator is small and fairly inefficient, such machines rarely stall and you don't need to go adding resistance.

With large axial flux alternators you have to use lots of magnet and copper and you reach a situation where the alternator is too efficient to let the speed rise and blades stall. This resistance trick is purely one to keep the blades working at decent efficiency, from the electrical point of view it is bad but something that helps the overall result.

If you are loading into resistive loads then all this is total nonsense and it is a very different situation.

Not knowing what you are doing means that I can't be of much help but for battery charging with a small machine you shouldn't be adding much in the way of resistance especially at 12v, the problem is normally one of the blades running too fast.

Flux

[taylorp035]

A couple of things...

1. I never stated what I was actually doing.
   
   
2. As I think about the issue, I was thinking about the resistance of the whole system, as I should of been thinking of the resistance of just the motor.
   
   
3. After graphing the equation of the power output of my blades and the power output of my motor, it is obvious that the motor will stall the blades up until 8-9 m/s, in which then the extra power will go to the rotational speed of the blades.
   
   
4. My actual set-up (as of 10 min ago), is a treadmill motor with some new 4ft dia blades that are meant to spin faster than my 67" blades.  My expectations are about 200 watts at 12 m/s and 100 watts at 9.5 m/s.
   
   
5. Smaller dia means less stress on my weak tower
   
   
6. My windmill is currently set up so the whole mechanism slides right on top of a 4" pvc pipe, therefor the whole thing can be taken down in about 10 seconds with my 4-wheeler and my twin brother.  I do not have a battery that is dedicated to the windmill, but I can use the lawnmower battery for the winter.  I also have a 400/750 watt inverter.

   
   

7.  Adding resistance to my windmill is pointless as I need all of the volts I can get.
   

[Rover]

Hi Taylor,

I think what everyone is asking, including myself, is what are the goals here. Are you charging batteries? direct use of the power through resistance, something else?

Everythuing changes based on what you are doing with the power.

 

[taylorp035]

Oh... I see.

I really don't have a use for the power, this more of a fun hobby that an 18 year old kid has been active in for about 8 years now.  As I said earlier, I don't have a battery that is suitable for storing more than a few watt hours, so this windmill is not going to be left up for long periods of time, especial not during storms or other things.  Maybe in the future, I will buy some batteries to power a D.N.S. server that I have or maybe a water pump ( as I also have a 15 watt solar panel to make it a hybrid system).

This summer the windmill may stay up for weeks on end, but as for now, it goes out on moderately windy days, maybe for a week, but it does not have a quota of kwh's that it needs to make.

So making a windmill for low wind speeds to charge a battery is not what my goal is, my goal is to have fun and make some power while I watch my volt and amp meters (I have a non-digital 60 amp meter that will get installed into the line soon.)

Most likely, I will have the new blades on it tomorrow (they are sitting completed in my living room), and most likely I will take a video of it if the wind gods are nice (they never are).

[wpowokal]

So in short you are addicted already!

[taylorp035]

You have that right!!!

Today: the wind gods are not being nice, but tomorrow is supposed to be extra windy!  Always good to start off the year in a god way!

Here is my local wind meter ( a bergy 10 kw about 10 miles from my house)

http://view2.fatspaniel.net/NorthCoastEnergySys/greenetownship/HostedAdminView.html?&eid=132262