This is a science fair question (yeah, and I'm the dad.) Our measurements came out the inverse and I am trying to figure out why.
Proposed study is to measure the change in power output of a simulated wind turbine propeller (fabricated from a 12" brass strip) as the diameter is reduced (by trimming the ends of the propeller.)
Propeller drives a small permanent magnet DC moter used as a generator, a box fan provides a steady wind. The load is a 10K ohm resistor and a solid state multimeter is used in parallel with the load. I had thought that as the prop was shortened, the power output would drop, indicated voltage would be lower, graph the results and you are done.
Except that the measured voltage increased as the prop got shorter until the prop lacked sufficient torque to carry the load. The measured voltages were in the neighborhood of 0.20 volts.
I have read in the tutorials here that the meter used this way (parallel with the load) participates in the circuit - my manual says my meter has 1 M ohm impedance but I'll be darned if I can figure out how to reconcile the measurements nor how to get a meaningful set of measurements. After my ten-year-old fabricates a new prop, that is.
Any help in sorting this out would be greatly appreciated. My only theory is that since resistance remained constant, as current dropped the voltage would have to increase but I can't get that out of the power equations.
Thanks in advance,
John
Proposed study is to measure the change in power output of a simulated wind turbine propeller (fabricated from a 12" brass strip) as the diameter is reduced (by trimming the ends of the propeller.)
Propeller drives a small permanent magnet DC moter used as a generator, a box fan provides a steady wind. The load is a 10K ohm resistor and a solid state multimeter is used in parallel with the load. I had thought that as the prop was shortened, the power output would drop, indicated voltage would be lower, graph the results and you are done.
Except that the measured voltage increased as the prop got shorter until the prop lacked sufficient torque to carry the load. The measured voltages were in the neighborhood of 0.20 volts.
I have read in the tutorials here that the meter used this way (parallel with the load) participates in the circuit - my manual says my meter has 1 M ohm impedance but I'll be darned if I can figure out how to reconcile the measurements nor how to get a meaningful set of measurements. After my ten-year-old fabricates a new prop, that is.
Any help in sorting this out would be greatly appreciated. My only theory is that since resistance remained constant, as current dropped the voltage would have to increase but I can't get that out of the power equations.
Thanks in advance,
John