71M6521FE-IMSLASHF datasheet(48/107 Pages) MAXIM

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71M6521DE/DH/FE Data Sheet

Page: 48 of 107

Rev 2

FUNCTIONAL DESCRIPTION

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 Teridian 71M6521DE/DH/FE 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 will yield an accurate quantity for the momentary energy. Summing up the momentary

energy quantities over time will result in accumulated energy.

Figure 15: Voltage. Current, Momentary and Accumulated Energy

Figure 15 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 20ms. The application of 240VAC and 100A results in an

accumulation of 480Ws (= 0.133Wh) over the 20ms 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]

71M6521FE-IM/F 数据表(PDF) 48 Page - Maxim Integrated Products

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