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this is a tough question for me.looking for your guys's help.This energy harvester use DC motor to collect the energy.And an external electrical load R is connected to the generator,i got an oscilloscope to to record the curve of voltage.Then the power of output is

enter image description here

is it right? and i tried the resistance of 200ohm,the curve likes this


enter image description here

but i tried the 1Mohm the curve is almost keep the same. If you can help me with this i wll be really really grateful for you kindness.

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  • \$\begingroup\$ What is the winding resistance of the DC motor? Useful search term : "maximum power transfer theorem". Try loads equal to, and either side of, the predicted load for maximum power. \$\endgroup\$ – Brian Drummond Jul 21 '17 at 10:36
  • \$\begingroup\$ the wingding resistance is about 2ohms.but is the load is too small the motor will be really hard to work(or turn) have a forve against the motor to work. \$\endgroup\$ – 章哲人 Jul 21 '17 at 11:23
  • \$\begingroup\$ Exactly - if you want more power out of it, you have to put more power in - of course. \$\endgroup\$ – Brian Drummond Jul 21 '17 at 11:29
  • \$\begingroup\$ thans.so it's normal phenomenon. \$\endgroup\$ – 章哲人 Jul 21 '17 at 11:31
  • \$\begingroup\$ A normal phenomenon called the Conservation of Energy. Decide how hard you want to work turning the motor, adjust the load to match that force (torque), and see ow much electrical power you get. \$\endgroup\$ – Brian Drummond Jul 21 '17 at 11:32
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Yes, the average power is as per your equation.

Possibly why the waveforms look similar between a 200 ohm load and a 1 Mohm load is that you are nowhere near the best maximum power transfer load of your generator i.e. 200 ohms and 1 Mohm both represent a light load and will hardly alter the output voltage shape at all.

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  • \$\begingroup\$ Thanks a lot.So if i wanna calculate the poewer of this harvester,what should i do.find a proper resistance load?And there is another thing i wanna know how can i calculate the resistance of the motor,because once the motor work the resistance will change immediately.Thanks. \$\endgroup\$ – 章哲人 Jul 21 '17 at 10:55
  • \$\begingroup\$ You need a few resistors of lower value to find the max power transfer point. It's hard to calculate motor resistance so why not try to measure it? \$\endgroup\$ – Andy aka Jul 21 '17 at 10:59
  • \$\begingroup\$ But if i tried a 10ohms the motor will be hard to work(or turn),so if the load are too small,the motor will work life having short-circuited? \$\endgroup\$ – 章哲人 Jul 21 '17 at 11:06
  • \$\begingroup\$ Energy harvesting is all about power transfer and power transfer means putting power into something and power in means the generator rotor becomes harder to turn. There will be an optimum resistor value where maximum power transfer is accomplished. Above this value the motor is easy to spin and below this value the motor is hard to spin. That is what power transfer in a generator is ALL about. \$\endgroup\$ – Andy aka Jul 21 '17 at 11:44
  • \$\begingroup\$ I can easily use this generator to run the 1W led for a flash,but i calculated the per power,it is about 20 mW.Why?Maybe the max power reached 1W? \$\endgroup\$ – 章哲人 Jul 23 '17 at 3:50

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