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More accurate readings of total energy promised with metering ICs

11/20/2003 03:07 AM EST
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re: More accurate readings of total energy promised with metering ICs
kendallcp   10/18/2010 1:12:34 PM
Actually, the data sheet for the ADE7753 doesn't mention total energy anywhere! These devices, like most other electrical energy measurement chips, give an apparent energy reading that's rms-computed from the real energy measurement and a 'reactive' energy measurement obtained through a 90deg shift of one of the input variables. This was fine in the old days when the main non-resistive contribution was a pure reactance leading to a phase-shifted fundamental current. In the modern environment of non-PFC'd power supplies and triac-controlled loads, the main contribution to non-resistive load current is harmonics. These are just as much trouble as reactive currents (sometimes more) but are not correctly measured by most energy measurement chips. As a result, such chips display an incorrect, overoptimistic power factor reading on a range of common loads, if the standard apparent rms calculation is used. It's easy to show just with a spreadsheet that a standard power supply with a narrow conduction angle has a close-to-perfect reactively measured power factor but a terrible PF based on a proper calculation of the apparent load implied by the spiky harmonic-rich current. As more and more non-expert new companies pour into the 'smart meter' space and rely on the chip suppliers for the accuracy of their results, we could be sleepwalking into widespread inaccuracy in the installed meter base.

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