What Does That Sentence in Bold Mean?

In order to slow down such a turbine, it might be necessary to backward-pitch the blades momentarily, and then rotate them to 90 degrees to keep the turbine from spinning in either direction.

Individually controlling each blade seems like a recipe for inducing destructive vibrations (imbalance) in the rotor assembly.

So what makes pitching the blades to control the speed so new? That technology was standard issue on wind generators back in the late 1920's.

90+ years later doesn't seem too new to me!

I agree the individually pitching the blades will cause force imbalances between one blade and the next. Its just asking for trouble.

Being able to do it is not the same as needing to do it.

Not sure what the objection is. Pitching the blades is standard practice for wind turbines.. There is no issue with it and it is the most effective means for controlling produced power where wind speeds vary.

Braking is accomplished with a caliper style brake, but by pitching the blades, most of the the rotational force is removed making the breaking system more effective.

The following Wiki has good info, but also googling wind turbine blade pitch control also brings up lots of info...

http://en.wikipedia.org/wiki/Wind_turbine_design#Pitch_control

Pitching the blades is also standard practice on many propeller-driven aircraft and ships.

http://en.wikipedia.org/wiki/Controllable_pitch_propeller

Pitching all of the blades the same amount at the same time will keep the forces balanced equally between them.

However the statement was that they can pitch each blade individually. To me that would be a bad thing to attempt because the forces from each blade will no longer be equal.

One blade may still be trying to drive the turbine while one is neutral and the third is in a braking position. The wind load and thrust forces on all three are now very different. Centrifugally they are still in balance but power and wind loading wise they are not.

Ever drive a vehicle with a tire thats got shifted belting in one spot? It can be still balanced but its not actually round any more.

Hi wingman,

You wrote:

Applying the full braking effect to the rotor requires pitching all three blades to a 90-degree position the wind. And why is this in bold?

Perhaps this is to emphasize that in the writer opinion, it is necessary to have all three blades alter pitch at the same time. Not perhaps the possibly disastrous effect of the 90°, but perhaps 50°, but whatever it is it must happen to all three blades, not just one.

That would be my guess anyway.

Good luck

The usual tower reduces airflow over each blade in turn, the generator torque fluctuation resulting might be reduced by the affected blade's pitch adjustment. It seems likely that the mtbf loss and benefit will not be worth while, particularly with rectifier/inverter power handling.

Maybe the pitch changes required for control can take advantage of the fluctuation thereby reducing the required blade-incidence actuator's torque by optomised timing. This would improve mtbf and save energy.

What is the source of this paragraph?

Individually controlling the blades might be necessary to allow for minor variations between blades. This type of scheme would improve the balance of forces. Anti-lock braking on cars works this way.

Hi,

turbulence and steady variations in air velocity (with time and rotation angle) may very well motivate the individual setting of the blades to an optimum, not only at braking conditions.

The sentence in bold is wrong as this 90° position will not generate any torque except from displacing air.

Going from a positive angle of attack to negative will generate reverse torque.!

RHABE

In the light of further info, I want to modify my earlier idea that all blades should be pitched in sync. As each blade occludes the tower, the slipstream varies a bit, suggesting a slight alteration in blade trim at that point. I haven't found out yet if the best way to do this is with independent pitch control for each blade, versus identical pitch with some other trim adjustment such as a small aileron on each blade.

I agree with the others that have posted here that the blades must be pitched simultaneously for "aero-braking" of the turbine blades, as well as to avoid an unbalanced condition and resultant vibrations which will eventually rip apart the entire propeller hub assembly. This is known as "feathering" the prop and is routinely done in the aviation world with piston-powered powerplants and turbo prop engines....all blades pivot simultaneously about the hub by a single mechanism like a worm gear, something that's been around since the early 1920's I believe. Nothing new about that technology and it's been proven to be very effective when the pilot decides to shut down the engine due to engine failure or fire......feathering the prop blades 90 degrees into the slipstream will reduce the drag tremendously.

Feathering of the propeller blades INDIVIDUALLY WILL RESULT IN AN UNBALANCED LOADING CONDITION, and hence, the destruction of the propeller hub, blades and turbine transmission. Also, IMHO feathering of the props by separate or individual mechanisms instead of a single central electrically or hydraulically driven/operated gear system is inviting diaster...the FAA would never approve of such a design for feathering of props on aircraft!

Just my 80 Cents worth....

Have a great sunny day!

Hi CapMoosie,

Re: post #11, it is worth at least a Dollar!

Hey There babybear!

How the heck are you doing?

I've been meaning to write back to you, but it's been so crazy around here the past few weeks. I have a ton of emails to respond to and very little time to do it all....my Bad!

Been helping the Mrs. Moosie with her Fall Cleaning around the house the past 2 1/2 weeks...both inside and outside then house. When she's on a roll look out! *LOL* Wel, at least we got the basement and garage all cleaned out of the old junk and brought it to the County Waster Transfer Station nearby....a whopping 2.65 Tons worth of old junk! I couldn't believe my flaming ears when the Attendant at the tipping scales told me the weigh!!! Now I have some room to finally set up my steel fabrication shop and start fabricating the steel trusses for the toolshed/storage building as well as start to put together the solar thermal collector tube guts and heat transfer manifolds!!!! About time!!

I still have to reinsulate the basement ceiling and add the ARMA-FOIL Radiant Barrier foil. After that is done, I will be painting the entire interior of the house...it needs it badly and hasn't been done in about 8 years all told. Bought a bulk of HyTech's ceramic insulating paint additive (a powder to add to the interior latex paint) to apply to the house ceilings and the interior surfaces of the exterior walls...it'll bolster the reduction of heat conductivity through the exterior walls and ceilings....must likely it'll pay for itself in terms of heating oil use reduction in a matter of a few months, and the nice thing is that I get to take a Federal Energy Tax Credit for using it!!! Ditto with the Radiant barrier Foil I'll be installing this week!!!! hehehehehe! All of this stuff is incredible and is NASA spinoff technology.

So, whatcha been up to buddy???

Well, gotta go work on the exhaust system of my car. I've been putting it off for too long and must get it done before the snow flies within the next few weeks!!!

Have a great sunny day over there across the Pond!

Isn't this interesting?!

Reason saying is I was discussing this topic a while ago here and was blasted down soo bad for stating this same thing that the blades can be optimized for max power at all times when the wind is barely blowing they are set to max out what is available.

And when the wind picks up speed the props stay at max transfer rate till the system gets close to max RPM. then as wind speed increases the pitch is adjusted to keep the RPM's below the self destruct speed, regardless of wind speed be it 20 MPh or 60 MPh.

The system is made th get every watt it can at low speeds and when the wind is too fast it self protects bu adjusting the pitch.

When I said this I was literally attacked for not knowing what I'm talking about by some self proclaimed wind turbine guru!

It's amazing now how my thoughts are accepted as gospel now. intereesting

Hi NSS,

Yeah its a funny old world, there is always someone who knows, whatever you say, it is bound to be wrong!

There is an awful lot of stress on those blades and even more if they are feathered one by one and then put back to 'optimum'? It would mean the whole rotating mass is constantly out of kilter, and in my opinion can only kill or shorten the working life of the mill.

Good luck.

The original article is here.

I'd guess that they are saying that the braking effect comes from aerodynamic drag (which is, by definition, perpendicular to lift). The apparent wind is the only wind that acts in the blade, and the apparent wind direction is, when the blade is "flying" always within 15 degrees of the chord of the blade. The chord of the blade moves along a helix through a mass of air which moves past the blades, and if the blade is perpendicular to this helix, then maximum drag occurs (the blade being fully stalled).

In aircraft with controllable props, part of the run up checks is to test that "feathering" works. Feathering is used, (in flight) when one engine quits, to stop the engine from turning and to reduce drag on the side with the failed engine. Once the engine has stopped, the blade chord is roughly aligned the apparent wind seen by the aircraft as a whole. But during the deceleration, the propeller blade spirals though its apparent wind (which is from a different direction) with the flat side of the prop presented to that apparent wind (the helix). When you do the check on the ground, the braking effect on the engine is dramatic, so you only do the test very briefly. If you've been on ground when such a run-up is being done, if makes a distinctive sound (with increasing noise from the prop, but a dramatic slowing of the engine).

So I think the "wind" the the writer wrote about was the apparent wind (that seen by the blade) not the true wind (that seen by an observer in the ground).

An advantage of individual blade control is that differences in true wind speed near the base of the tower and near the uppermost blade tip can be accommodated, and the blade angle optimized, to keep the blade operating at it best L/D.

so, with this "incredible new idea" of pitching the blades to shed unwanted air force to keep the rotor from overspinning, we bring up braking to eliminate the excess winds usefullness.Has anyone ever thought about adding a second or third generator head to the system,to keep the blades from going too fast. That way, you capture and use the excess to create more electricity.Kind of the same principle used in regenerative braking systems on electric cars.When your generator reaches maximum output, a second load,generator could switch on and begin charging and the excess drag from the other genheads would control the speed of the blades.jees-kiss already. no brakes to burn out, no computer to flip a chip,and more total power output

I already use a similar approach. I just over size my generator to take the additional output power in stride. Theres a reason I have 2 KW peak capacity direct drive generators on 7 foot blades that are normally rated for 500 watts or less!

My big 21 foot system I am building has a blade rating of 5 -10 Kw. The generator I am going to use will easily take 40 Kw peaks for several minutes with no ill effects.

Sure they are much heavier and cost more to build this way but whats it worth to not have to rebuild one every time the wind gets a bit rough? Crash and burns greatly cut into cost avoidance numbers while harvesting that 5X+ wind power just helps those numbers!

I learned about the need for robustly designing wind generators years ago. Forget what the books say about generator capacity. Just use their blade size numbers and then take their recommendations for everything else and build those parts 5 times stronger.