71M6511/71M6511H
Single-Phase Energy Meter IC
DATA SHEET
AUGUST 2007
FUNCTIONAL DESCRIPTION
Theory of Operation
The energy delivered by a power source into a load can be expressed as:
t
E = V (t)I(t)dt
∫
0
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] = P2 + Q2
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
71M6511/6511H 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.
500
400
300
200
100
0
-100
-200
Current [A]
Voltage [V]
-300
Energy per Interval [Ws]
Accumulated Energy [Ws]
-400
-500
time [ms]
0
5
10
15
20
Figure 12: Voltage. Current, Momentary and Accumulated Energy
Figure 12 shows the shapes of V(t), I(t), the momentary and the accumulated energy, 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 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.
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