MAXQ3183 Maxim, MAXQ3183 Datasheet - Page 57

• Manufacturer Part Number: MAXQ3183
• Description: The MAXQ3183 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, power, energy, and many other metering and power-quality parameters of a polyphase load
• Manufacturer: Maxim

But current sensors and other external circuitry compo-
nents introduce a phase distortion to the current signal,
and this phase distortion may not be constant at all cur-
rent values. Consequently, for the most precise mea-
surements, the phase between the voltage and current
signals must be compensated. In the MAXQ3183, the
energy signals are compensated for phase offset by
performing a complex multiplication of the signal with
the contents of the appropriate phase offset register.
Determining which phase offset register is appropriate is
a matter of comparing the incoming RMS current for the
phase with the contents of the I1THR and I2THR regis-
ters. It is the responsibility of the administrative software
to ensure that I1THR is greater than or equal to I2THR. If
the raw RMS current is greater than or equal to the con-
tents of I1THR, then the angle expressed in PA0 is used
to compensate the phase angle. If the raw RMS current
is less than I2THR, then the angle expressed in PA2 is
used to compensate the phase angle. And if the raw
RMS current falls between I1THR and I2THR then PA1 is
used to compensate the phase angle. In this way, a
three-piece stepwise approximation of the phase
response of the current sensor is available.
To use a constant phase compensation, set I1THR and
I2THR to zero and insert the phase compensation value
into PA0.
The same processing can be performed to calculate
the reactive energy value. But reactive energy can be
calculated in another way: calculate apparent energy
by multiplying the raw RMS volts and raw RMS current,
Figure 11. Phase Compensation for Energy Calculations
PA
=
E
E
Low-Power, Multifunction, Polyphase AFE
P
Q
⎧
⎪
⎨
⎪
⎩
PA
PA I THR I
PA
______________________________________________________________________________________
1 1
0
2
,
,
, , I
I
RMS
RMS
≥
>
<
I THR
I THR
1
with Harmonics and Tamper Detect
2
RMS
COMPENSATION
≥
PHASE
I THR
PA0
PA1
PA2
2
)
⎫
⎪
⎬
⎪
⎭
LINEARIZATION
LINEARIZATION
OFFS_LO
OFFS_LO
GAIN_LO
GAIN_LO
OFFS_HI
OFFS_HI
square this value, then subtract the squared real
power. The square root of this value is the reactive
energy.
Similarly, apparent energy can be calculated in either
of two ways: either as the product of the raw RMS volts
and amps, or as the square root of the sum of the
squares of the real and reactive energy. Which of these
is selected depends on the value of the APPSEL bit in
the OPMODE2 register: if 0, then apparent energy is
the product of the raw RMS volts and amps and reac-
tive energy is calculated using the difference of
squares method; if 1, apparent energy is calculated
using the sum of squares method and reactive energy
is calculated directly from the complex energy.
Line Frequency and Phasor Angles: Line frequency
can be taken directly from the NS value. Recall that NS
is the number of frames in a DSP cycle. Since each
frame is 360μs, simply multiply NS by 360μs and divide
by CYCNT to obtain the line period. The reciprocal of
this is the line frequency.
To calculate phasor angles, the numbers of samples
between zero crossings on phase A and B and on phase
A and C are taken. Since NS is the number of samples
during a complete DSP cycle, it is easy to calculate the
fraction of a complete cycle. The software then converts
this value to degrees and adjusts it such that no negative
angles are reported. No calibration is required for line
frequency and phasor angle calculation.
Once real and reactive energy over the most recent
DSP cycle has been calculated, it is necessary to accu-
mulate the result.
For reactive energy, the result accumulated during any
DSP cycle may be positive (for an inductive load) or
E_GAIN
AVERAGE
AVG_C
Energy Accumulation
E
E_RAW
E_RAW
REAL
REAL
REACTIVE
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