I have decent electronic understanding, but I've not much idea about motors.

I plan to make a small wind mill which should be enough to power a couple of LED lights for like an hour every day. I was thinking of buying a motor, and these are the options I have:

My idea is to buy a 24 volt DC motor, and then with decent wind, I'm assuming it would give at least 12 volts, so I'll attach a charge controller and 12 volt battery.

The thought process is this: I'll probably be able to generate 10-14 volts on a windy day. So I'm assuming my battery would be charged nicely when that happens.

Questions :

  • Can I use a 24 volt motor to charge 12V battery with above assumptions? What can go wrong?
  • Is a geared motor better suited than a normal e-bike for the purpose?
  • Is this experiment worth pursuing?
  • 1
    $\begingroup$ First step: evaluate the wind and then power available from the turbine. Then you can consider a motor. $\endgroup$
    – Solar Mike
    Jul 3, 2020 at 7:29
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    $\begingroup$ So, now the predicted power for the blades you have? $\endgroup$
    – Solar Mike
    Jul 3, 2020 at 7:45
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    $\begingroup$ So hawt or vawt? Any idea of diameter? Have you done any research on google as there are many sources available. $\endgroup$
    – Solar Mike
    Jul 3, 2020 at 7:55
  • 1
    $\begingroup$ Does this answer your question? Is this a viable mini wind turbine setup? $\endgroup$ Jul 3, 2020 at 11:36
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    $\begingroup$ "Is this experiment worth pursuing?" Geared motors suffer from high friction start speed, No you need better rotor designs to start then sustain power mps vs RPM with no-load and with load $\endgroup$ Jul 3, 2020 at 13:21

1 Answer 1

  • DC Motors are low electrical impedance when they start with just DCR winding resistance.

  • Typical motors start/stall at 10x max rated current at rated voltage

    • thus become 10xDCR at rated power at rated RPM. High eff. ones are 12x DCR.
  • with no-load at rated RPM, Motors use about 10% max. rated current thus is now 100x DCR impedance. The same for generators.

  • thus generator impedance will be a complex function of RPM and load but with constant rated load rise from DCR to 10*DCR and with no-load rise from DCR to 100 * DCR.

    • Then with PWM, Motor/gen impedance rises inverse with duty cycle.
  • you also have stored energy with the inertia of the wind and rotor/gen to help sustain the idle moments and it is possible to use a battery to overcome stall speed, that is ...

  • if there is sufficient wind to overcome the start losses and you have a small anemometer to prove that.

  • So you see motor/generator impedance starts at DCR and rises to 100xDCR with no power being generated up to rated RPM.

  • in other words motor/generator impedance can increase to 10x DCR at max power to 100xDCR with no load (from reducing the electrical load current)

  • Fans, Rotors or wind turbines are just the opposite. They start from high impedance at 0 RPM wind and then impedance can drop with rising wind speed and depends on the blade design.

  • Some wind rotors may be inverse slightly to wind speed due to Eddy current losses, others more constant.

  • Others may have starter blades then higher RPM power blades so the rotor impedance can depend on RPM but generally OPPOSITE to MOTOR Impedance.

This opposes the law of maximum power transfer or MPPT where impedances must be MATCHED.

Then there may be safety concerns to avoid breakup at unsafe high wind speeds if the centripetal forces cause self-destruction.

You will want to design a system with specs first for rotor, motor and charger impedance. DCDC converters use PWM to raise duty cycle and impedance for Buck converters and use PFM to lower impedance with rising pulse rate for Boost converters as an over-simplification.

Impedance vs power vs RPM

  • If you can figure out how to match source to load impedances, then you can begin to understand how to make it work at max power transfer ( but expect 50% losses)

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