71M6521BE
Energy Meter IC
DATA SHEET
JANUARY 2008
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
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 71M6521BE 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 15: Voltage. Current, Momentary and Accumulated Energy
Figure 15 shows the shapes of V(t), I(t), the momentary power 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
(= 0.133Wh) over the 20ms period, as indicated by the Accumulated Energy curve.
The described sampling method works reliably, even in the presence of dynamic phase shift and harmonic distortion.
V1.0
© 2005-2008 TERIDIAN Semiconductor Corporation
Page: 47 of 97
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
