71M6533G Maxim, 71M6533G Datasheet - Page 55

• Manufacturer Part Number: 71M6533G
• Description: The Teridian™ 71M6533 and 71M6534 are third-generation polyphase metering systems-on-chips (SoCs) with a 10MHz 8051-compatible MPU core, low-power RTC, flash, and LCD driver
• Manufacturer: Maxim

2 Functional Description
2.1 Theory of Operation
The energy delivered by a power source into a load can be expressed as:
Assuming phase angles are constant, the following formulae apply:
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 71M6533 and 71M6534 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.
Figure 18
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.
Rev 2



shows the shapes of V(t), I(t), the momentary power and the accumulated power, resulting from
P = Real Energy [Wh] = V * A * cos φ* t
Q = Reactive Energy [VARh] = V * A * sin φ * t
S = Apparent Energy [VAh] =
-100
-200
-300
-400
-500
500
400
300
200
100
Figure 18: Voltage, Current, Momentary and Accumulated Energy
0
0
Current [A]
Voltage [V]
Energy per Interval [Ws]
Accumulated Energy [Ws]
5
E
P +
2
=
∫
Q
0
t
V
2
10
) (
t
I
) (
t
dt
15
20
55