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Re: [AWES] Re: Airfoil "Curvature Control" as a Makani Design Driver

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  • Robert Copcutt
    ... That is only true for a generator at low speed driving a passive load. In that situation doubling the speed doubles the voltage which also doubles the
    Message 1 of 43 , May 17, 2012
      On Thu, 2012-05-17 at 02:25 +0000, Doug wrote:
      >
      > Power is proportional to RPM squared for any electric motor or
      > generator.

      That is only true for a generator at low speed driving a passive load.
      In that situation doubling the speed doubles the voltage which also
      doubles the current. However, as speed increases the heat in the coils
      increases until the insulation starts to degrade. At that point you need
      to limit the current by some means. The main options are;
      1) Limit generator speed.
      2) Use a feature of steel called magnetic saturation and design the
      machine so that saturation prevents coil burn-out.
      3) Add some current limiting electronics such as a maximum power point
      tracking controller (MPPT).

      If you use magnetic limitation the hysteresis increases as speed
      increases so power increase is a bit worse than linear with speed
      increase. If electronics are used the cooling improves with speed so
      power increase is a bit better than linear with speed.

      If you add an MPPT controller then it can step up the voltage from a low
      speed generator. Power then becomes proportional to revs throughout the
      rev range.

      So there are many ways to design a generator, but when the goal is
      maximum power to weight ratio then power is roughly proportional to
      revs. Bearing wear then comes into the equation so increasing air gap
      radius becomes more important. It also helps cooling. That is why I am
      still trying to find out more about why pancake machines are not
      universal. Windage is maybe one reason. Another thought is that maybe an
      economical way to wind the windings has not been found. (Windage is an
      unfortunate name in this context - too easily confused with windings.)

      The power supply for a motor can never be passive. The power of motors
      is therefore nearly always roughly proportional to revs.

      Robert.
    • Doug
      Hugh tells me his generators will never make as much power as mine since he uses no iron. His approach is all about smooth operation and home-buildability.
      Message 43 of 43 , May 22, 2012
        Hugh tells me his generators will never make as much power as mine since he uses no iron. His approach is all about smooth operation and home-buildability. He's looking for consistent charging and reliability whereas I'm building hot-rods. Grandpa isn't going to burn out the engine in his econo-box sedan - if you wanna see a lot of blown engines, go to a race track! Racing technology filters eventually to better performance in Grandpa's sedan.

        --- In AirborneWindEnergy@yahoogroups.com, Robert Copcutt <r@...> wrote:
        >
        > Doug,
        >
        > My experience is that when you hit a persistent problem
        > you can either apply brute force to it (eg. build bigger generators) or
        > you can swot up more on the theory and find an optimal solution. The
        > fact that you have had so many generators burn out tells me that you
        > never got around to developing a comprehensive computer model telling
        > you what is going on with them. To burn out 1 is a lesson. To burn out
        > several shows that there is a missing link in your chain of logic.
        >
        > It is magnetic hysteresis that creates the heat. If the iron is sent
        > beyond saturation then the hysteresis rapidly increases. However, it is
        > always there which is why Hugh Piggot went for air-core generators so
        > that the turbines could spin up under minimal resistance to start
        > catching the wind. Air-core machines can be way more efficient (98% has
        > been demonstrated) than iron-core ones (usually <90% unless very big). I
        > am in the process of working out if Visventis would be better of with
        > air or iron cores; or pancake or a more square format.
        >
        > Robert.
        >
        >
        >
        > On Fri, 2012-05-18 at 02:17 +0000, Doug wrote:
        > >
        > > Where do you get all this? Sound in error to me and I make generators
        > > and run them and measure them all day every day.
        > > How could you profess to have such a comprehensive knowledge of
        > > motors, and yet you say you wonder why pancake motors are not
        > > universal?
        > > Isn't that like Dave S., the world's latest leading authority in wind
        > > energy, asking why turbines don't use kites? Like motors used to be
        > > pancake-style, turbines used to use kites as blades. Then progress
        > > happened.
        > > Pancake motors were universal when the concept of a motor first came
        > > about, then it took a few weeks til they figured out how to do it
        > > better using less materials.
        > > Oh, and another thing,
        > > Magnetic saturation in your laminations can cause heat and help burn
        > > out your coils. Take it from the king of burned out generators!
        > > Umm, listen, I'm really sorry to tell you this but, again, everyone is
        > > entitled to their own opinion but not our own facts.
        > > Motors and generators produce power in proportion to the square of
        > > their RPM and that is just a simple well-known rule of thumb.
        >
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