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Forum Index : Windmills : F&P startup

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dazz
Regular Member

Joined: 15/04/2008
Location:
Posts: 78

Posted: 01:12am 02 May 2008

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Hi All,
Could somebody tell me how fast you would expect the blades to spin(ie RPM) in a typical application with say 2m blades at the min wind speed to keep the blades rotating, like 15km/h?

I just have no concept of how fast they would spin. I can imagine them trundling around at about 20RPM but i can also imagine 100RPM. In other words, I have no clue.

Cheers, Daryl

Tinker

Guru

Joined: 07/11/2007
Location: Australia
Posts: 1904

Posted: 10:37am 02 May 2008

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Daryl, think about what LOAD there is on the blades.
Like, turn your bike upside down and spin a wheel. Then try the same with the brake on. Got it?

Tinker

Klaus

dazz
Regular Member

Joined: 15/04/2008
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Posts: 78

Posted: 11:57am 02 May 2008

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Hi Klaus,

Well I was assuming very low load. At the lowest speeds I would assume the voltage produced would be well below cut in and hence the load would be small. Am I wrong?

If you consider a slowly increasing wind starting from dead calm then presumably the force on the rotors would increase with no movement of the blades until the force was great enough to overcome cogging. Once it does so the cogging is reduced so the same force will drive the blades faster.
So at that point, roughly what is that speed. I'd be surprised if people hadn't noticed this sort of thing staring at their windwill on low wind days.

Daryl

PS. Just because it's a simple question, doesn't mean I'm simple, got it?

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