Data Sheet 71M6531D/F-71M6532D/F
FDS 6531/6532 005
2
Functional Description
2.1
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
P2 + Q2
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 71M6531 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
0
5
10
15
20
-100
-200
-300
-400
-500
Current [A]
Voltage [V]
Energy per Interval [Ws]
Accumulated Energy [Ws]
Figure 18: Voltage, Current, Momentary and Accumulated Energy
Figure 18 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.
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