BL0952A/BL6513
Three-
Three-phase Active
Energy Meter
output pulse is given to the counter motor out of the chip, and then the counter value proportional
to the consumed energy is obtained.
Offset Effect
The dc offsets come from the input signals and the forepart analog circuitry.
Assume that the input dc offsets on the voltage channel and the current channel are
U
offset
and
I
offset
, and PF equals 1 (
ϕ
=
0
o
).
p
(
t
)
=
[
U
cos(
ω
t
)
+
U
offset
]
×
[
I
cos(
ω
t
+ Φ
)
+
I
offset
]
=
UI
UI
+
I
offset
U
cos(
ω
t
)
+
U
offset
I
cos(
ω
t
)
+
cos(2
ω
t
)
2
2
Fig.6
Effect of different offset cancellation methods
As can be seen, for each phase input, if there are simultaneous dc offsets on the voltage
channel and the current channel, these offsets contribute a dc component for the result of
multiplication. That is, the offsets bring the error of
U
offset
×
I
offset
to the final average real power.
Additionally, there exists the component of
U
offset
×
I
+
I
offset
×
U
at the frequency of
w
. The
dc error on the real power will result in measure error, and the component brought to the
frequency of
w
will also affect the output of the average active power when the next low-pass
filter can’t restrain the ac component very completely.
When the offset on the one of the voltage and the current channels is filtered, for instance, the
offset on the current channel is removed; the result of multiplication is improved greatly. There is
no dc error, and the additional component at the frequency of
w
is also decreased.
When the offsets on the voltage channel and the current channel are filtered respectively by
two high-pass filters, the component at the frequency of
w
(50Hz) is subdued, and the stability
of the output signal is advanced. Moreover, in this case, the phases of the voltage channel and the
current channel can be matched completely, and the performance when PF equal 0.5C or 0.5L is
improved. In BL0952A/BL6513/BL6511, this structure is selected. Though it is given in the
system specification that the ripple of the output signal is less than 0.1%, in real measure of
BL0952A/BL6513/BL6511, the calibration output is very stable, and the ripple of the typical
output signal is less than 0.05%.
http://www.belling.com.cn
-9-
4/21/2009
Total
9 Pages
BELLING [ SHANGHAI BELLING CO., LTD. ]
