FDS_6533_6534_004
71M6533/71M6534 Data Sheet
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 con-
tent may change constantly. Thus, simple RMS measurements are inherently inaccurate. A modern sol-
id-state electricity meter IC such as the Teridian 71M6533 and 71M6534 functions by emulating the
integral operation above, i.e. it processes current and voltage samples through an ADC at a constant fre-
quency. 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 17: Voltage, Current, Momentary and Accumulated Energy
Figure 17 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 ac-
cumulated power curve. The described sampling method works reliably, even in the presence of dynamic
phase shift and harmonic distortion.
v1.1
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