Electronics > Beginners
Efficiency of a small generator
software:
I have a small device that I would like to power with harvested mechanical energy. There is not much force available.
The plan is to have a rectifier and connect it it directly to a few AA NiMh cells.
I am trying to pick the most efficient geared motor to charge these AA cell batteries. For the time being I discharge cells, and count how many spins it takes take to up the battery voltage by 0.1. Very primitive and not scalable method.
I am looking for a way to measure the motor amperage, in mA. I will do my best to produce a constant rotation of the gear shaft.
Thanks in advance.
Zero999:
A brushless motor will be best efficiency wise.
Use a Schottky rectifier for low on voltage losses. If it's a brushed motor, a single diode would be more efficient than a bridge rectifier, but it would only work when the shaft is turning in one direction.
Generally, the smaller the motor/generator, the less efficient it will be and the smaller it is, the higher speed it needs to run at, to get the best efficiency.
software:
@Hero999, thank you for your advice. I have that. Now I need to measure mA it produces. When I connect the multimeter from the rectifier into the battery the results are all over the place, I'm not quite sure this is the correct way to measure the current the motor produces.
MrW0lf:
If you want so squeeze absolute max out of it experiment with impedance matching. To get the general idea you may connect variable resistor to driven brushless motor (=generator) and observe effects of changing resistance at given fixed RPM. Forget the multimeters. For energy consumption you use oscilloscope. Low value inline shunt resistor (<=0.1ohm for example) for amperage and voltage between lines on second channel. Use math function to calculate power. When you get proper hang of it may move to optimizing actual complex load.
From https://meettechniek.info/measuring/power.html
--- Quote ---With shunt resistor
circuit power measurement with an oscilloscope and shunt resistor
Fig. 10: Power measurement with an oscilloscope and shunt resistor.
The probe for the voltage measurement is connected directly to the load. The current is measured indirectly via a shunt resistor. Because the grounds of both scope channels are internally connected, the ground terminals of the probes must be connected to the same potential. Because of this the current is measured with inverted polarity. It's therefore necessary to invert the signal coming from the shunt.
By multiplying the voltage and current signal a third signal is created that represents the power as function of time. To take the average of this power signal over one full period the average power is determinate.
--- End quote ---
Zero999:
--- Quote from: software on September 05, 2018, 05:03:34 pm ---@Hero999, thank you for your advice. I have that. Now I need to measure mA it produces. When I connect the multimeter from the rectifier into the battery the results are all over the place, I'm not quite sure this is the correct way to measure the current the motor produces.
--- End quote ---
The current into what load?
If you just set the meter to current, then it will be the meter's current shunt resistance, which will be very low, possible much lower than that of the winding resistance, so can be treated as a dead short. You'll probably also notice that the motor's shaft becomes more difficult to turn, with the output shorted. This is because the current flowing is trying to turn the motor in the opposite direction.
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