Using an AC-AC adapter with the emonTx to measure the AC voltage as well as a CT to measure current is is optional but it does increase accuracy by enabling real power measurements to be made. Using an emonTx with only a CT assumes RMS AC voltage to be fixed at a certain level (240V UK) and power factor to be equal to 1.
Real power measurements take into account the power factor of the AC circuit, inductive loads such as motors reduce the power factor thus giving an artificially higher apparent power reading. It is real power which you are billed for by the utility company. For more information on the differance between real and apparent power see AC power theory 1 and theory 2.
Using an AC-AC adapter also enables the direction of the current flow to be determined by providing a relative point of reference. Knowledge of the direction of current is crucial in monitoring a domestic solar PV systems when due to how the generated power from the solar PV is connected into the fuse box consumption cannot be monitored separately to generation. More on this to come...
Our tests have shown that typically the apparent power reading of in a domestic home is 5-20% higher than a real power reading. Although this seems like quite a difference in reality the purpose of domestic energy monitoring is to increase understanding of how energy is consumed in the home, apparent power readings are accurate enough for this. Also installing an AC-AC with the emonTx next to the utility meter can be tricky since there are often no power sockets to hand. This offset can also be calibrated out by calibrating the apparent power kWh/d readings against the readings from the utility meter.
|Instantaneous real and apparent power readings of a domestic house AC circuit during the night, notice the fridge (compressor motor) reduces the PF thus reducing the real-power reading. Power in W is on the left axis, PF is on the right axis|
The blue trace is the new adapter (pictures below) as you can see it's slightly better than the old one were were using (yellow trace), it is more like a perfect sine wave without the slight 'kink' in the middle that the old (yellow trace) adapter exhibits. Also the new model does not have the +2V offset, switching to the new model will require re-calibration of the emonTx in software.
|The new AC-AC adapters. We have plenty in stock|
Testing these AC-AC adapters also gave us the chance to test out our new DSO Quad mini-digital scope. Very nice!
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