Question on Furling

[Hank]

A quick question.

What will the effect be on furling if I increase my prop (rotor) size from 8 ft dia to 12 ft dia and do nothing else relative to tail size and weight?

Will the unit tend to furl earlier (lower winds) or later (higher winds)?

The reason I'm asking is that my current PMA had to be detuned significantly to operate properly in delta with an 8 ft rotor (it would stall in star).

It's been up and running now for two years and I'll be taking it down for inspection etc. this spring.

At that time I'd like to retune it so it's more efficient and will be installing a 12 ft rotor.

Just want to know if I will need to change the tail as well.

All comments appreciated,

Hank

[Countryboy]

Hi Hank,

  You will be more than doubling your swept area going from an 8' to a 12' rotor.  You didn't say what your stator was designed for - going to a 12 rotor may require a new stator too.

I think the mechanical aspect that is going to affect it the most is rotor offset.  An 8' rotor will probably have 4 inches offset.  You should have about 6 inches of offset for the 12' rotor to furl right.

My advice - tune your 8 footer better, and keep it an 8 footer.  You might want to open the airgap to allow it to run faster too.  That might solve a lot of your problems.  If you want the power from a 12 footer, build a 12 footer from scratch - don't try to cobble together a 12 footer from 8 footer parts.

[Hank]

Thanks for the reply Countryboy,

My offset is about 5.5 inches now. I increased the air gap (de-tuned it) significantly when I first made it (about 3/8 inches on each side mags to stator) but this was not enough to run it in star without stalling with the 8 footer I believe the cut-in rpm was about 100 in star config.

Going to delta increased the cut-in speed and it's operating well.

I could get more power out of it by decreasing the air gap but I'll need a larger prop, hence my questions.

Currently it furls in about 30 mph winds producing about 45 amps which is fine for me. I would just like to get more low wind power.

The unit is an axial twin rotor 24 mags (12/rotor) 9 coil.

I have an additional stator w/fewer turns of heavier wire which I will probably end up using.

Just kind of figured that with a larger prop the tendency would be to furl earlier, everything else being equal, since the projected area is larger thereby having more force trying to swing the prop out of the wind.

[Flux]

Your offset would suit a 10 ft prop but may be too small for 12 ft.

You will most likely need to increase tail length for stability and you may need to increase weight if you furl too early.

I don't think your 12 magnets will cope with a 12ft prop unless they are much bigger than the 2 x 1 x 1/2. Either be content with 10 ft or rebuild.

At least you have learnt the effects of too low a cut in speed.

Flux

[Nando]

Hank:

Where are you located ?.

You have an alternative that does not require any mechanical change and it is the use of a controller that allows the generator to peak its RPM and converts the voltage ( power ) to the battery bank you have, at the same time if the controller has MPPT then about 20 - 30 % higher power can be harvested, all this depends on the generator voltage generation curve.

In addition the controller may need an ELC loop to keep the power up enough for proper furling once the battery bank is charged

Can you tell us the battery voltage and amp-hour ?.

Nando

[Hank]

Thanks Flux and Nando,

Here's the scoop if my notes are correct (it's been several years).

When I built it initially:

RPM/Volt = 6 in Star

Stator resistance 1.4 total in star

I detuned by increasing the air gap resulting in:

RPM/Volt = 8.8 in Star

With this config. using the 8 footer it would produce 5 amps and stall. Cut in speed was to low.

I changed over to Delta which brought the RPM/Volt to about 15 and it's been flying ever since.

I'm charging a 12 volt 1000 amp-hr battery system.

Charge controler is a Xantrax C40.

I'm not having a problem with it, just thought I could get more juice out of it at lower winds by increasing prop size and tuning it back up or going back to star. I also have the option of replacing the stator which I have made, 50 turns/coil as opposed to my current 80 turns/coil.

Nando, I live in Massachusetts, USA

Hank

[Nando]

Hank:

You could start with Star then change to Delta once certain STAR current has been reached indicating that the voltage as a Delta is high enough.

The STAR-DELTA change can be made with a 3PDT relay, or using TRIACS to do the same.

Nando

[Hank]

Nando,

My problem is that the unit will not start up in star as the cut-in rpm is to low (about 100 rpm).

It just sits there turning slowly and not producing much more then 5 amps regardless of wind speed.

by changing it over to delta It increased my cut-in speed to about 180 rpm and all is well.

It appears that the pma is to powerful for my prop size when set up in Star. All of this is after I detuned it significantly.

That's the reason I'm asking the question of going to a larger prop size and up-tuning my pma.

Been away from it for a couple of years and it looks like I'll have to crack some books again.

Thanks for your help,

Hank

[Flux]

Your idea to increase low wind output is sensible, but you will never use that stator star connected, its resistance is far too high.

With a reduction in air gap it may take a 10ft prop in delta.

I don't think you will have enough magnet to hold a 12ft prop down even if you change the stator. Also I don't think your offset is safe for 12v.

My advice try it cautiously with 10ft and think about a fresh start if you need to go up to 12 ft.

Flux

[Hank]

Thanks Flux,

I agree with you. I may change out the stator when I bring it down and do a better job in tuning it for my condition.

Hank

[Nando]

Hank:

Do You have a good data of Volts/RPM of the unit ?.

If it produces power at low RPM, then the possibility of using a charger controller to allow the wind mill to power peak and proper harvesting of the watts generated.

MPPT charger may be good here, without the need to change the Star stator .

Nando