# Measuring AC power output from harvester

In order to characterise an energy harvester, I want to measure its power output. It is a rotary harvester (basically like a regular generator) with three phases.

I do not have fancy measuring equipment, just an oscilloscope and a handheld multimeter, neither with interfaces to a PC.

My idea would be to rectify the voltage with a B6U bridge and put a load resistor behind it, then measure the voltage across R_load over time. The oscilloscope can log the measured voltage every 2 µs in a csv and I would calculate the integral below numerically for each point measured.

$P_{avr}=\frac{1}{T}\int_{0}^{T}\frac{v(t)^{2}}{R_{load}}dt$

Is this a suitable way to do so, or do you have another suggestion?

I could also charge a capacitor for a certain amount of time and measure its voltage and then maybe calculate the stored energy.

• Be careful that using the oscilloscope at 2us sampling time as a data logger you will flood any conceivable memory space. Use the oscilloscope for sanity check of waveforms, use the multimeter as a data logger (e.g. 1 sample/sec.). (otherwise set the oscilloscope equivalently to bout 1 Sa/s). Be careful that rectified voltage will have ripple, so getting the most representative instantaneous value would be troublesome. If you include a smoothing capacitor, take care of the impulsive current it will take from the generator (that might work out of spec or not as desired). Commented Jun 9, 2021 at 8:51
• The idea of a large capacitor is not bad, provided again impulsive inrush current is not an issue. You could measure the capacitor voltage every X seconds or minutes (no info on ratings) and then calculate the energy steps. At least it is precise if energy is what you are interested in; if you have sudden changes or fluctuations in the gen operation, then they will be flattened out. Commented Jun 9, 2021 at 8:55
• The harvester usually runs at 1000 rpm with a signal-period of 10 ms. I dont think inrush current would be an issue, so I might give the cap charging a try. Commented Jun 11, 2021 at 14:44

I'd tend towards your first method. But: especially for high-impedance sources like your generator, it matters what the load is, so your measurements won't mean much unless your load is similar to what you want to do with it in the end.

Therefore, I'd probably so something like having the bridge rectifier, a stabilizing capacitor, and then do a sweep over different resistive loads from 1 Ohm to 100 kOhm and see where given a fixed excitation of the generator, you can extract the most power.

If applicable, I'd repeat that for different levels of excitation.

• So you suggest to search for the maximum power point instead rather than trying to calculate the power of the output voltage? Commented Jun 11, 2021 at 14:54
• An output voltage itself has no power. Power is voltage times current, and when you connect a load to let a current flow, the voltage supplied by your harvester will greatly decline compared to the open-circuit voltage measurement. Theregore, only looking at the voltage is meaningless.
– mmmm
Commented Jun 12, 2021 at 1:55
• Ofc I would not just look for the voltage or OCV, my idea was to measure the voltage across the optimal load and then calculate the average power with the formula in the opening post. Before I would have to look for the optimal load via a sweep. Commented Jun 13, 2021 at 16:28
• But the optimal load changes with excitation
– mmmm
Commented Jun 14, 2021 at 22:32
• Do you mean the inductive reactance? Commented Jun 15, 2021 at 16:04