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I placed a normal cooling fan 45cm away from a small turbine generator. i had 3 turbines, one with 4 blades, 8 blades, and 12 blades.

The one with 12 blades always rotated the fastest, even after changing any possible variables such as height or distance from the fan.

I did some research and apparently turbines with higher number of blades rotate more slowly.

So why did my experiment show that more blades on a turbine gives more RPMs? Any ideas?

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  • $\begingroup$ I'm not sure why you'd expect anything different. As long as each blade contributes more torque than drag, more blades implies more torque and more torque implies higher RPM. $\endgroup$
    – Dave Tweed
    Apr 19 '16 at 21:49
  • $\begingroup$ But i did a lot of research, and the higher number of blades, the more solidity you will have. Meaning more torque, but less speed and less efficiency. $\endgroup$
    – Mohdak
    Apr 19 '16 at 22:30
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    $\begingroup$ Maybe the 12-blade unit has the lowest-friction bearing system? Maybe the blades on the various turbines have different pitches? Without knowing a lot more, it's hard to suggest a reason. $\endgroup$ Apr 20 '16 at 12:11
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    $\begingroup$ Without some drawings or pictures of the complete set up, it is impossible to guess what results you "should have" got. We have no idea if your turbines are similar to the ones where "apparently turbines with higher number of blades rotate more slowly," or not. By definition, the results you actually got are the correct results for your experimental set-up, but the design of your experiment might be different from what you were trying to measure. $\endgroup$
    – alephzero
    Apr 21 '16 at 0:41
  • $\begingroup$ Can you add some data and a diagram of your setup for testing? $\endgroup$
    – Diesel
    Oct 5 '17 at 21:51
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In the following, I will assume that the only thing that changed, was the number of blades of your test turbines. Thus, all other parameter of your turbines such as swap area, blade geometry, height, generator, etc. will stay constant.

Blades of wind turbines use the same aerodynamic principle as airplane wings do: By generating a pressure difference on both side, lift is caused. Since the blades are forced to move on a circular path, lift is transformed into torque. Hence, incrementing the number of blades, also increments the torque $M$ and the power $P$ since: $$ P = 2 \pi n M \quad,$$ where $n$ denotes the number of revolutions. As stated in the comments above, the more blade a wind turbine has, the less is its efficiency, but this may be neglected for small numbers of blades.

The resistance of the model turbines (As the only difference is the number of blades) will only increase with an increase in revolutions until equilibrium is reached, explaining the increase in revolutions as it was observed.

The same applies for real wind turbines. However, for wind turbines it may be favorable to keep its number of revolutions constant or even reduce them as excitation frequencies are couple to them by $$ f_{1P} = n \qquad \text{and} \qquad f_{3P} = 3n $$ due to imbalances in the rotor ($f_{1P}$) and tower shading ($f_{3P}$). Eigenfrequencies of wind turbines are typically at 0.2 Hz to 0.3 Hz, what exactly is between those two frequencies for three-bladed wind turbines with a rated rotational speed of 10 rpm. In order to avoid resonance, the number of revolutions should stay roughly the same or even decrease. Said that, on increasing the number of blades you may also increase the generators torque in order to keep the number of revolutions constant.

An other possibility could be the fact, that by keeping the power constant (for various reasons) you can reduce the number of revolutions by the same amount you increase the torque. Despite the effect of reducing the excitation frequency with the number of revolutions, noise pollution (blade tip speed) and load on mechanical parts such as bearings and gears is reduced as well.

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