71M6541DSLASHFSLASHG datasheet(80/166 Pages) MAXIM

80 / 166 page

71M6541D/F/G and 71M6542F/G Data Sheet

Rev 5

Functional Description

3.1

Theory of Operation

The energy delivered by a power source into a load can be expressed as:

∫

)

(

)

(

Assuming phase angles are constant, the following formulae apply:

 P = Real Energy [Wh] = V * A * cos φ* t

 Q = Reactive Energy [VARh] = V * A * sin φ * t

 S = Apparent Energy [VAh] =

For a practical meter, not only voltage and current amplitudes, but also phase angles and harmonic content

may change constantly. Thus, simple RMS measurements are inherently inaccurate. A modern solid-state

electricity meter IC such as the 71M654x functions by emulating the integral operation above, i.e., it

processes current and voltage samples through an ADC at a constant frequency. As long as the ADC

resolution is high enough and the sample frequency is beyond the harmonic range of interest, the current

and voltage samples, multiplied with the time period of sampling yield an accurate quantity for the

momentary energy. Summing up the momentary energy quantities over time results in very accurate

results for accumulated energy.

Figure 28: Voltage, Current, Momentary and Accumulated Energy

Figure 28 shows the shapes of V(t), I(t), the momentary power and the accumulated power, resulting from

50 samples of the voltage and current signals over a period of 20 ms. The application of 240 VAC and

100 A results in an accumulation of 480 Ws (= 0.133 Wh) over the 20 ms period, as indicated by the

accumulated power curve. The described sampling method works reliably, even in the presence of dynamic

phase shift and harmonic distortion.

-500

-400

-300

-200

-100

100

200

300

400

500

Current [A]

Voltage [V]

Energy per Interval [Ws]

Accumulated Energy [Ws]

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