power measurement with variable frequencies


I'm new to this forum and to the topic of OEM's as well. I've spend some time now looking in this forum for an answer to my question but didn't find anything. So I make up a new topic.

Is it possible to measure electric power with the EmonPi or EmonTX if the frequency in the circuit is alternating (e.g. in an off-grid system).

Thank you for your help.

Greetings KuchenMitKaese

Robert Wall's picture

Re: power measurement with variable frequencies

Yes it is possible.

What is the range of frequencies over which your off-grid system works?

The OpenEnergyMonitor system - except for MartinR's "PLL" sketch - works on both 50 Hz and 60 Hz systems, and there is no reason why it cannot work at any reasonable frequency. The emonPi and the emonTx both use almost exactly the same circuit to make the measurements.

The factors that will give inaccurate measurements are:
1. The voltage and current transformers - these are designed for operation at 50/60 Hz and if your operating frequency is significantly different, they will be inaccurate.
2. At very low frequency (< 25 Hz), the capacitors inside the emonTx will not be big enough to work correctly. Below 50 Hz, the emonTx will need a separate 5 V power supply - the internal supply from the ac adapter will not work.
3. At very high frequencies, there will not be enough samples in each cycle to make the calculations accurately.
4. If you only measure current and not voltage, then there will be an "end effect" because the rms calculation will not have an exact integer number of whole cycles over which to calculate, therefore the average will be wrong.

KuchenMitKaese's picture

Re: power measurement with variable frequencies

Thank you very much Robert! Do you know if there is any limit how fast the frequency may change?

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