generator rpm vs. output power curve

Discussion in 'Windtura Generators' started by WindyFAQ, Jun 28, 2011.

  1. WindyFAQ

    WindyFAQ WindyNation FAQ Staff Member

    Our power source is slow moving and we need generator rpm vs. output power curve or ratio between wind speed and generator rpm
  2. windyguru

    windyguru WindyNation Expert

    We do not have this data in its entirety but I think I can give you some useful information. The Windtura is designed to reach "cut-in" for a 24 volt battery bank in about 8 mph wind. Cut-in refers to the wind speed at which the generator begins to charge the battery bank. In order for a wind turbine or any generator to charge a battery bank, the voltage of the generator must be higher than the voltage of the battery bank. This is necessary to overcome the impedance of the battery.

    For this reason, the Windtura 750 must reach approximately 24-28 volts to charge a 24 volt battery bank. I can tell you that the Windtura 750 is designed to reach 28 volts at 200 rpm. So, we know that a 24 volt battery bank will begin charging in 8 mph and and we also know the rpm of the generator is at 200 rpm. If you watch the 24 volt battery charging video for the Windtura 750 which can be found here:

    you will see that the Windtura 750 does about 0.5 amps at 27 volts in 8 mph wind (200 rpm). This is 13.5 Watts at 200 rpm. Luckily, the power of the generator does increase exponentially. The Windtura 750 reaches 750 Watts (rated power) into a 24 volt battery bank at 700 rpm.

    I know this is not a graph of rpm vs. power but it is all the information we have at the moment:

    13.5 Watts @ 200 rpm
    750 Watts @ 700 rpm

    I hope this helps. If you have more questions with regards to this subject, you can post them in this forum thread.
  3. leamywind1

    leamywind1 WindyNation Engineer

    Good info for blade choices for the 750 PMA
    13.5 Watts @ 200 rpm runing a large 3 blade set
    750 Watts @ 700 rpm running a large 3 blade set

    Now I would assume the top end production will be lower using 5 blades due to getting lower RPM"s with the 5 blade 80" set. Trading off for more constant lower wind production. Does anyone know the rated RPM of the 5 blade 80" set?
  4. windyguru

    windyguru WindyNation Expert

    Hi Larry,

    I do not know. I have not done exhaustive tests with the five blades on the Windtura 750. I would think that the 5 blade will give better power production in extremely low winds (less than 10 mph) and that after 10 mph the three blade will begin to outperform the five blades. Thus, I do believe three blades is the better call for areas that have wind.
  5. murray2paddles

    murray2paddles WindyNation Engineer

    Larry , ask Windy if he has not done exhaustive tests on the 5 blade how do they know the three blade will out prform it ! lol
  6. TomT

    TomT WindyNation Engineer

    Like I said before and have seen it posted on other sites. That 5 blades will outperform to 20-25 mph then the 3 blades will outperform the 5 blades. But I am sure that it depends on blade construction.
  7. murray2paddles

    murray2paddles WindyNation Engineer

    Goodmornig Tom, actualy I was just trying to poke some fun at Windy, BUT for me Tom you are dead on !
    I agree with you on the blade construction being critical,,,,,, & if there is any truth to the 20-25 mph, well in some cases the furling is designed for 28 mph so maybe the 5 blades are at the top of the wind scale for some projects.

    Some of us have been saying for quite some time that we would prefer to take advantage of the low to medium winds ( that most of us have) and not be concerned about the big winds which might happen a handful of times per year.
    Unfortunately it is the big wind watts / volts / amps that catch all the attention and influence the sales.
    It seems to take a few years of trial and error until a DIY learns what gives them a more constant power return .
  8. windyguru

    windyguru WindyNation Expert

    Well, I did do some tests almost 2 years ago. I can tell you with 100% certainty that the data showed there was a noticeable/significant decrease in power in windspeeds above ~18 mph (can't remember exact number) for the 5-blade compared to the 3-blade. I did write down these numbers and could search for the notebook that has the data if I necessary.

    What I can say is that after we did the "quick" test with the 5-blade configuration, we all said "OK. no need to do more tests on the 5-blade system. Less power for more money (2 extra blades cost money) is not what people are looking for."

    That is the reason we did not do exhuastive tests. It only took 15 minutes to realize the 3 blades made more power. I do regret no checking the low end power in more detail because the 5-blades do indeed begin to spin in a lighter wind. But even the three blades will start spinning in constant ~4 mph wind.

    Now, if you still want to add five blade to the Windtura 750, the first problem is going to be is the the wind turbine is going to be front heavy which will make it off balance. The second problem is the dynamics of the furling are going to change due to the increased weight from the two extra blades and larger hub, which is not a good thing.
  9. murray2paddles

    murray2paddles WindyNation Engineer

    I appreciate all the points you have made and from a $$ point of view , yes the three blade makes sense. You are also correct when you said the three blade will start spinning in 4 mph winds, I suppose some of us were looking beyond just spinning in low winds and we started to wonder about the different loads on the turbine for instance a 12 v, 24 v, 48 v banks and then the grid ties. All would be creating a different power curve for the same turbine in the same wind conditions.

    Not knowing the math many of us then moved to 5 blades hoping to create more torque / power in the low to mid wind mph.
    Most of us do not really have the knowledg base to test properly nor have the correct equipment to log information for good on site data. Chart information collected from theory is of no interest to me, but charted information collected from an onsite turbine then becomes very real to me.
    In your opinion were we wrong ? Good honest information saves years of testing !

    Out of curiosity how did the 5 blade hub become available as a sale item, was it just due to demand from the consumer?
  10. windyguru

    windyguru WindyNation Expert

    We made the five blades because they do offer the advantage of higher start-up torque. Some wind turbines, like a few of the car alternator type PMA's, have significant cogging which requires a lot of torque just to get them spinning. With cogging PMA's, I think 5 of our blades becomes a necessity. I have spoken with many customers who switched to five of our Hyperspin blades on Delco PMA's and finally started making power in wind speeds under 10 mph.

    I do not think it is wrong to use five blades. I just think that for the case of the Windtura 750, if you take into account all wind speeds, you are better off with three WindGrabber blades than five WindGrabber blades.
  11. windyguru

    windyguru WindyNation Expert

    I would add that this is for clean, non-turbulent wind. Maybe if you are mounted low to the ground in turbulent wind, the five blades could become advantagous?
  12. leamywind1

    leamywind1 WindyNation Engineer

    LOL. Well guy's you have all brought good pro's and con's to the subject at hand. I must say great answers and questions. So what is the answer? Perfect test conditions result in perfect test results. 99% of the people which is mostly DIY's, do not have the perfect site or can not go high eneough for what ever reason. For me to sum it all up. The higher you go and the less obstructions the more a 3 blade system will work better thus yeild closer to the perfect conditon test. The lower you are and the more obstructions you have the more a 5 blade will be more sucessfull. Going from a 3 blade to a 5 blade is a different mind set with considerations and sacrafices many do not relize. These are the results i have found at my site. I am a big believer in yeilding more constant watts then getting full power. Getting full power requires 30MPH winds and i have yet to meet some one who gets those types of constant winds on a daily or weekly basis. As an installer i am very causious about selling a turbne to a customer considering those reuirements. In all fairness to any wind turbine seller if the consumer thinks of buying a turbine and mounting 30 ft up in front of a 50ft tree will change his electric bill, well shame on him. Bottom line is the actual site will dictate how much sucess a person will have with 3 or 5 blades let alone a windturbine.

    My question on the RPM of the blades was so we/i can figure out how much top end the 750 will be loosing. We know what rpm is needed to make power from the 750 PMA with 3 blades, at a 30MPH winds. 750 watts
    I will have the answer but it will take me months to provide.
    Anyone else up for the challange?:)
  13. TomT

    TomT WindyNation Engineer

    I do not have a 750 so I am out of this one.
    But for the 500 and all of my changes.
    I will not be able to say anything till this weather pattern changes later this year.
    At present being 99 percent of all the wind and very little of it is coming from the southwest right thru a tree.:( And all my readings are anywhere from below 12v to way above the 24v range. There is no clear curve I can get.:(
    As of now the tree to the southwest has been removed.:)
    Last edited: Dec 10, 2019
  14. windyguru

    windyguru WindyNation Expert

    Well, my wife is 8 months pregnant so I am not going to commit to anything at the moment!
  15. Minnesota

    Minnesota WindyNation Engineer

    At 80" and in 13mph conditions, a turbine operating at a Cp of 30% can yield 100w to the grid. A true low-wind PMA and blade set that could operate with one wind 12-28V 300w GTI without clamping would be killer. How about a delta wired thinner gauge stator on the 750 PMA?

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