FDS 6531/6532 005
Data Sheet 71M6531D/F-71M6532D/F
2
2.1
Functional Description
Theory of Operation
t
The energy delivered by a power source into a load can be expressed as:
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] =
P
2
+
Q
2
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 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 accumu-
lated energy.
500
400
300
200
100
0
0
-100
-200
Current [A]
5
10
15
20
-300
-400
-500
Voltage [V]
Energy per Interval [Ws]
Accumulated Energy [Ws]
Figure 18: Voltage, Current, Momentary and Accumulated Energy
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.
v1.2
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53
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