Active Power Measurement with the CR6250/60 Power Transducer
138
Applications
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current sense points.
L1 L2 L3 N
LOAD
1
2
3
4
5
6
7
8
9
H1
H1
H1
X1
X1
X1
R
R
R
I line
A
B
C
V
L-N
N
V
L-N
The CR6250 and CR6260 series of Power Transducers gives the designer a simple and effective way to measure and
record the instantaneous power usage for 3 – phase 4 – wire loads. These devices are also known as 3 element
power transducers. All 3 current inputs are needed because 4 – wire power systems have a neutral return path,
and differences in phase currents results in neutral currents, thus requiring all phase information. Hence, these
Wye connections must be measured with 3 current sense points.
Calculation of full scale output:
Using a CR6250-500-5 for example, the unit is calibrated to output the
fullscale reading when a balanced purely resistive load is being powered by 500 Vrms line to neutral, and the
amount of current draw on each leg is 5 Arms. Power in any one leg will be the volts across the leg times the
current through the leg times the power factor. For calibration, the value of the power factor is unity (1) for a
purely resistive load. For the above diagram, the voltage across the element is V L-N, or 500Vrms. The current
is Ip, which for balanced Wye loads flows through each element. Thus, the power in one of the legs for a full
scale reading using a 500-5 unit would be 500 X 5 = 2,500 Watts. There are then 3 legs on a 3-phase line, so
the total power, full scale would be 3 X 2,500 = 7,500 Watts. The 500-5 transducer will output fullscale (5VDC
or 20mA) when the power being used is 7,500 Watts. Using this same methodology, the following simplified
formula will give the full scale reading from any Wye transducer: Part number CR6250 – AAA - BB,
Full
scale in Watts = AAA X BB X 3
.
External current transformers and voltage transformers
can be used to extend the reading range of
any transducer. Voltage and current transformers are sized according to turns ratio. A turns ratio on a current
transformer of 100:5 represents a turns ratio of 100/5 or 20. Thus for every 1 amp from the current
transformer, this represents 20 Amps in the measured line. Since power = V X I X PF (power factor), the
amount of power measured when using external current and voltage transformers will be V X I X PF X current
ratio X voltage ratio. Using our same example as before, with a CR6250-500-5, if an external current
transformer with a ratio of 20 is used, as well as an external voltage transformer with a ratio of 2, then the
new full scale output of the transducer will equal 7,500 X 20 X 2 = 300,000 Watts!!! A simplified formula for
this situation is:
Full Scale Watts = AAA X BB X 3 X CT ratio X VT ratio.
Please refer to “
Selecting
ANSI Class Metering Current Transformers”
and “
Using External Current Transformers with Other CR Devices”
for more information on transformer ratios and applying external transformers to transducers and other
devices.