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Threshold speed in wind turbine. Is it dependent on frictional resistance?

  1. Jul 4, 2011 #1
    Wind turbines have got certain threshold speed of wind (of about 6-7 m/s) which acts as a mechanical resistance to its rotation.
    Is this mechanical resistance a result of frictional resistance in bearings alone, or it is also a result of its own weight?

    If yes, approximately how much of it is contributed by both of them??
  2. jcsd
  3. Jul 8, 2011 #2
    Not that I know anything about wind turbines, but I feel that I still should understand your questions...so, maybe you could be clearer?

    In any case, I will take a shot at making a few commentaries, maybe they will help in some way.

    I am not sure what you are thinking when you wonder if wind turbines have some kind of threshold....I mean, they were designed with a certain functionality in mind, given the wind availability, i.e., wind speeds...so, clearly, the designers were trying to maximize power out of them (torque-speed combination)

    As far as weight limit, I presume that you are talking about centrifugal force...yes, blades are optimized taking this into account. Given the maximum speeds that they expect to see, designers make things as big as they can get away with for as long as the stresses at the root of the blade are not larger that some threshold that depends on the material used (modulus of elasticity).

    Then, there is fatigue...this happens when a part is subjected to compression and then tension, and the compression and then tension...so, I don't know at what speeds wind turbine rotate and how heavy they are but chances are that a blade is under tension when is at the bottom and in compression when is at the top.

    You mention friction, too...I would hope that friction in the bearings is not the main reason of resistance to rotation...the main resistance to rotation comes from energy conversion from mechanical rotation to electricity...you see, up there where the shaft of the wind turbine is, there is an electrical generator being driven by it...the shaft drives a moving (rotating) field that induce currents on the stationary part...this is where electricity comes out of...and if this 'new' currents and their corresponding field did not oppose the one that created them, then you would have a perpetual machine...remember, you cannot create energy, just transform it.

    Hope this helps
  4. Aug 4, 2011 #3
    Thanks for your response.

    Sorry for not being clear. Let me reframe my question.
    Is there a minimum speed of wind required for the operation of wind turbines? If yes then i suppose you are saying that it is not because of frictional resistance, but the resistance from generator.
  5. Aug 5, 2011 #4
    Because the incident wind power is proportional to the wind velocity cubed, the wind power available is negligible at low wind speeds. At very low wind speeds, there is not enough power available to overcome friction.
  6. Aug 6, 2011 #5
    Well, if you start with a wind turbine that is not moving at all and with wind at speed zero...and THEN, you start increasing the speed of the wind....sure, there has to be some minimum wind speed that needs to overcome not only whatever friction there is in the system, but also the inertia of the turbine...recall Newton's 1st Law...I know, I know, it talks about rectilinear motion staying rectliniar motion unless external force..but I like to also look at when something is stationary, it tends to stay stationary unless an external force acts on it...
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