How many Blades.

Bryan, Sunday 28 January 2007 - 21:36:43

How many blades.?
? This is a question most people ask when thinking of how to build a wind turbine.
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First you want to ask “what is it you want the wind turbine to do?”
? If it is pumping water you want a turbine that has a large amount of torque.? This is because pumping water takes a lot of start up force to get the pump started. This
is typically done with a turbine with a large numbers of blades.

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Wind generators are much different. Wind generators have what is called a cut in speed which is the rpm the generator or alternator needs to obtain a rpm that allows the cut in voltage to become high enough for the batteries to start charging. The wind generator will free spin (spin with no load) up to the point where it reaches the cut in voltage which is higher than the batteries that you are charging.

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? ? Alternators and generators vary a great deal when it comes to cut in voltages this all depends how they are built.? The thing you need to know when starting to build is what rpm the alternator will reach the desired cut in voltage.

Most low RPM turbines are from 150 rpm to 600 rpm.
? Because a wind generator needs to reach high rpm’s the need for more than three blades becomes inefficient. This is due to the lift drag ratio.?

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Now a single blade turbine is the most efficient, it has the highest lift to drag ratio. But it also is a very difficult to dynamically balance.

? A two blade turbine works well and is an improvement over the single blade and is only slightly less efficient. But the two blades also has its problem's
two blade turbine has a wobble problem caused by a couple different reasons.
? The two blade may receive higher wind speeds at the top than the bottom. This causes a vibration problem and vibrations in wind turbines are undesirable.
? The second problem is when the blade is in a vertical position it will YAW easily but when the blade becomes horizontal this changes.? ? The easiest way to explain this is an example Hugh Piggott uses in his book. “Windpower workshop”If you were to hold a broomstick in your hand extended away from your body
In a vertical position and then turn your whole body in circles the broomstick in your hand has very little resistance.
? Now turn that broomstick horizontal and try it again and it will to pull away from you. So the blade is going from vertical to horizontal at a rapid rpm and this develops a great deal of vibrations. And remember vibrations are a bad thing when it comes to wind turbines.
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So the only time I would recommend a two blade turbine is in an area where the wind has low turbulence and for a home built wind generator that is 8 foot and under.

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So we come to the three blade wind turbine which solves all the problems mentioned above.? A well balanced 3 blade will spin very smooth. Any thing more than a 3 blade will just add more drag. And drag is bad.?

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You can design the most efficient air foil in the world and you will not be able to increase the overall efficiently by adding more blades. You will just make them less efficient.

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Now in order to keep a blade efficient you need to have the blade moving through the air at least 3 times faster than the wind speed. Lower than this will cause a stall in the blade and then you are just pissing in the wind.

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One thing to remember is there is only so many watts in the wind and you can’t make more watts by adding more blades


some recommended reading on this subject
http://en.wikipedia.org/wiki/Wind_turbine_design#Angular_momentum_and_wake_rotation
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