71M6515H
Energy Meter IC
DATA SHEET
MARCH 2008
Calibration with Three Measurements
The simplest calibration method is to make three measurements. Typically, a voltage measurement and two Watt-hour (Wh)
measurements are made.
If the voltage measurement has the error EV and the two Wh measurements have errors E0 and E60, where E0 is measured
with φL = 0 and E60 is measured with φL = 60. These values should be simple ratios—not percentage values. They should be
zero when the meter is accurate and negative when the meter runs slow. The fundamental frequency is f0. T is equal to 1/fS,
where fS is the sample frequency (2520.62Hz). Set all calibration factors to nominal: CAL_IA = 16384, CAL_VA = 16384,
PHADJ_A = 0.
From the voltage measurement, we determine that
1.Î AXV = EV +1
We use the other two measurements to determine φS and AXI.
IV AXV AXI cos(0 −φS )
2.
E0 =
−1 = AXV AXI cos(φS ) −1
IV cos(0)
E0 +1
cos(φS )
2a.
3.
AXV AXI
=
IV AXV AXI cos(60 −φS )
IV cos(60)
cos(60 −φS )
E60
E60
=
=
−1 = AXV AXI
−1
cos(60)
AXV AXI
[
cos(60)cos(φS ) + sin(60)sin(φS )
]
−1
3a.
cos(60)
= AXV AXI cos(φS ) + AXV AXI tan(60)sin(φS ) −1
Combining 2a and 3a:
4.
E60 = E0 + (E0 +1) tan(60) tan(φS )
E60 − E0
(E0 +1)tan(60)
5.
tan(φS ) =
φS = tan−1
⎛
⎞
E60 − E0
(E0 +1) tan(60)
6.Î
⎜
⎜
⎟
⎟
⎝
⎠
and from 2a:
E0 +1
AXV cos(φS )
7.Î
AXI
=
Now that we know the AXV, AXI, and φS errors, we calculate the new calibration voltage gain coefficient from the previous ones:
CAL _V
CAL _VNEW
=
AXV
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