Using 12 foot rotor on a 10 foot machine.

[Smithson]

Hi.  You can buy for 60 dollars more a set of 24 magnets, 1x2 but 3/4 inch thick.  Hugh's plans call for a 10 foot machine,  12 magnets per rotor, 14 inch rotor.

The 12 foot machine calls for16 magnets, 12 coils, on a 16 inch rotor.  Could you build the 10 foot machine, adjust the cut in for the 12 foot machine, put 12 foot blades on it and have it work across the wide range of speed?  Have it work as a 12 foot  machine should?

Thank you.     Arch

[Mary B]

depends on how the stator is wound and what your wind speeds are. In a lower wind speed zone I could see doin gthat. Where I live you would be asking for destruction.

[Smithson]

Mary:   The 12 foot calls for (for 24 volts) 12 coils at 45 turns. (2@14).  The 10 foot calls for 70 turns.  2@15.  Arch.  9 coils

[Mary B]

What are you average and peak wind speeds? That is the main question. Here my average is 18mph, peak is 110mph during summer storms. Had 50mph sustained last night for 8 hours. Those conditions would rip apart a 10 foot designed machine with 12 foot blades.

[Smithson]

Mary:  Ok.  This is western Indiana.  No wind here.  I just do this because I like to build them.  Build anything.

You know electricity so let me run this by you.  Mechanically the only difference is the extra blade length.  Electrically there might be a problem.  The flux on 12 3/4 inch magnets would equal 18 1/2 inch magnets.  The 12 foot plans calls for 16 magnets.  So there is extra flux from the 2 (in effect) magnets.  The turns on a (48 volt) machine , 12 foot would equals 1440 turns.  The 10 foot machine would have 1260 turns.  So if you made the 10 foot with 9 coils and 160 turns of 14awg (same awg as in 12 foot) would that not electrically have the 10 foot equal the 12 foot?  1440 to 1440.  Would different frequency effect it?

Of course you would have to make a thicker stator.  But you have the extra flux.  I don't know how else to explain what I was thinking.  Thank you.   I'm sorry I got your name wrong.  I can't remember  if the second part starts with an E or an A when I writing.  Arch

[ChrisOlson]

So if you made the 10 foot with 9 coils and 160 turns of 14awg (same awg as in 12 foot) would that not electrically have the 10 foot equal the 12 foot?  1440 to 1440.  Would different frequency effect it?

You can get the internal stator resistance approximately the same whether the generator is 16 pole or 12 pole.  However, the frequency plays a role too, so with equal resistance between the two designs, the 16 pole will turn out more efficient and produce higher voltage/revolution/amp-turn.

The downside is that without using an MPPT controller or transformer to let the 12 pole generator run at higher voltages, it will burn up.  The primary problem with all the homebrew designs is that they depend, to a certain extent, on running the airfoil quite inefficiently (high angle of attack).  Many people call this "stalled" but it's not really stalled.  Just that the rotor is not making all the power it can make.  If you put a 3.75 meter rotor on a 12 pole design the generator will be inefficient enough to let it really run.  Since the generator won't keep the airfoils at high angle of attack by keeping tip speed down, the rotor will produce way more power than the generator can handle and she'll be burn-out prone.

So I would recommend that if you want a 3.75 meter machine, to stick with Hugh's design on that rotor size.  He has tested them enough, and got the furling set so they power limit fairly well and are pretty reliable.  If you put MPPT on it then you can turn 'er loose and that 3.75 meter turbine will easily produce 3.5-4.0 kW.  But without MPPT, you have to keep the power dissipation down to reasonable levels in the stator or you will overheat the windings.  And that's why Hugh uses the 16-pole generator on that turbine - it has more cooling surface area, and the generator is more efficient and powerful so it can stall the blades and keep things under control.

[Mary B]

I knew an expert would be along who could explain it way better than me! I know some but no where near what Chris and a few others do. I am better at control electronics.

[Flux]

Chris has covered this. There is a limit to what you can do with a fixed amount of magnet. You can get the slower cut in speed with the small alternator by using more turns of thinner wire. You may be able to squeeze in enough turns of the same wire but only by increasing the air gap ( stator thickness as you said). The end result is that the small alternator can never be as stiff as the big one.

If you are in a low wind area and never see seriously high winds then it may be ok. As stated the prop will run higher up the power curve and in the low winds you normally meet it will actually behave better. The problem that both Chris and Mary are concerned with is any wind speed beyond that needed to produce rated output. To stand a chance of survival during even modest storms in a low wind site you will need to modify the furling to come in at 20mph or less, the offset and tail design of the 10ft machine will need modification. You will also need to modify the 12ft machine design if you fit the modified 10ft alternator as it will have to furl a lot earlier.

In the end it all depends on the maximum wind speed you can expect, only you have that information. Yes if you can be sure you won't have winds to kill it, it will work very well but a freak storm could mean destruction.

These machines are not entirely safe left braked ( shorted) in high winds, the 10ft alternator with 12ft prop would not be saved by trying braking, in fact that would be sure death, once the prop gets away the short circuit is many times worse than full load where a significant portion of the energy goes to the battery.

Flux

[ChrisOlson]

These machines are not entirely safe left braked ( shorted) in high winds, the 10ft alternator with 12ft prop would not be saved by trying braking, in fact that would be sure death, once the prop gets away the short circuit is many times worse than full load where a significant portion of the energy goes to the battery.

Indeed, you have to design for the worst case or the machine will be damaged.  I've seen my 3.5 meter turbines, fully shorted against the voltage clipper to prevent them from running, start up in 80 mph winds during a storm, break out of stall and take off running putting out 3.5 kW and burn the clipper out.  I'm afraid the 10 foot generator with 12 foot blades would be much the same scenario - you have enough resistance in the wiring so it can't keep the rotor fully stalled in high winds so it breaks out and takes off running.  Except with the small generator fully shorted the power is dissipated in the generator winding instead of a resistor further down the line.

I've actually gone the other way from Hugh's design on that class of turbine and use 20 pole generator on 14" mag rotors instead of 16.  I've found that the higher frequency provides some pretty hefty overall efficiency improvements over the 16 pole (although the 20 pole generator is quite expensive to build).

[Mary B]

Just an odd though... would shorting relays up at the turbine itself help with keeping it braked? Yes more complexity up the tower and more to go wrong but just a thought.

[Smithson]

Thank you all.

  So when M. Jacobs was explaining matching the blades to the generator and keeping the generator loaded, (in the plow boy interview) across the full power curve he was accomplishing the same thing as stall in the modern machines.  Now you also have Mid Nite Solar's  mppt controller such as Chris uses.  And if you don't use one of those three ways you have an inefficient machine that could self destruct in storm winds.  That what I think.   Arch