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Practical Induction -- What Provides The Most Power?

  1. Dec 29, 2016 #1
    I'm having a hard time finding a starting point for this question. I'm looking for all the variables that can effect the amount generated by induction, and then hopefully ordering them from the most important to least important. Any references to a good starting point for someone who hasn't worked on physics in a long time would be great. But a simple, detailed explanation would also be nice.

    How do the following variables effect the power output?
    • Velocity
    • Force Input
    • Size and/or mass
    • Materials
    If the same materials are used, does power scale pretty directly (linearly) with the size of a generator? Will a lower velocity, but higher mass that inputs more force output more power? And do the materials basically convert the input force to output electricity at different efficiencies?

    For something like the Windbelt (https://en.wikipedia.org/wiki/Windbelt), is it more useful for the "rotor" to move back and forth faster, or is it more useful to add more force to it? I'm assuming velocity is the main factor, but for electric-generating bikes, I suppose it is the gear system that allows greater force to be turned into greater velocity. In that case, if you could modify force into velocity, could you use something like a giant pendulum and seismic waves to produce large amounts of power due to the sheer force between the two?
     
  2. jcsd
  3. Dec 29, 2016 #2

    CWatters

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    Perhaps remember energy is conserved. So if the system can't store energy and there are no losses then power out = power in.

    Work = force * displacement
    And so
    Power = force * velocity

    So in theory it doesn't matter which you increase. If you increase force or velocity by 10% the power increases by 10%.

    In practice there are losses so you have to look at those to see under what conditions they are minimised.

    I might not be able to get back to this thread for the next few days, sorry.
     
  4. Dec 29, 2016 #3
    The energy is conserved, but that doesn't mean it's harnessed in the way we want it. The arrangement of the molecules to transfer electrons in a specific way seems the most important for electrical transfer. So you still need the underlying electrical generator, and then some ways to add force to it, correct? We can't just roll a boulder down a hill and call it electricity.

    And are work and power both useful in generation? Would a force over a distance like a long, linear movement over a large stator provide as much as a rotor quickly shifting in and out of the magnetic field?
     
  5. Dec 30, 2016 #4

    CWatters

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    If both generators are ideal and the input power is the same then the output power will be the same. The problem is that in the real world you have to look at the losses as these define the efficiency (power out/power in).

    For example..

    Consider two 12V motors, both permanant magnet motors, both designed to drive the same load at same speed (eg same output power). However one uses ferrite magnets and the other rare earth magnets. The stronger field produced by the rare earth magnets means that fewer turns of wire are needed to produce the required back emf (which determins the speed). That means losses in the copper windings are reduced making it more efficient. So for the same output power the rare earth motor needs less input power. The efficiency of the ferrite motor might be 80% and the rare earth magnet motor >90%. The rare earth motor could be redesigned to produce more output power for the same input power as the ferrite motor. The key is that the extra power comes from reducing losses.

    So back to your question..

    Does the faster rotor have more losses due to windage (air resistance)?
    Does the slower rotor need more copper in the windings to generate the required output voltage and therefore have higher resistive losses?

    Depending on the answers to these and other questions either could be more efficient than the other. In short it's virtually impossible to answer the question without engineering a complete system and I'm probably not the best person to do that.
     
  6. Jan 1, 2017 #5

    berkeman

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    Interesting link, thanks for posting it.

    In general, airfoils/propellers will be more efficient at converting airflow energy compared to flat surfaces. That's one of the efficiency advantages you want to look for.

    Recent advances in DC-DC converters have helped to maximize the efficiency of power generation systems like solar panels and other installations. They allow you to run the conversion at the "Maximum Power Point" where the output power of the source is matched to the load via the DC-DC converter's operating point(s). Let me know if you want help finding links to reading about those key terms. :smile:
     
  7. Jan 8, 2017 #6

    CWatters

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    Back in the 1980's I was in Switzerland for an electric powered model aircraft competition. One of the classes was for solar powered models aircraft without any batteries. Power was marginal for sustained flight. One or two models had variable pitch propellers that varied the load on the motor so the solar panels were kept operating as near as possible to their maximum power point. You could demonstrate this on the ground by making a shadow with your hand fall on the panels where upon the pitch would vary. Solar powered models and indeed full size aircraft have come a long way since then.

    http://www.atlantiksolar.ethz.ch/
     
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