Note: All of the calculations are very similar when testing VA 2 1/2 Element
transducers. The primary difference is no COS or SIN functions. For the example above,
120 * 4.167 + 120 * 4.167 + 120 * 4.167 = 1500.12 VA
3.8.10.7.6 Watt/VAR 3 Element
This transducer is normally used in three-phase, four wire Wye applications, which
requires 3 voltages and 3 currents to test. The three voltages and three currents are all
refernced to ground. The MPRT will automatically select three voltage and three current
channels, V1, V2, V3, I1, I2 and I3. The test will initially start at the default voltage value,
that is set in the Default Setting Screen, for each Voltage output. Thus, for a default
voltage setting of 120 volts, V1 will be set to 120 Volts at an angle of 0°, V2 will be 120
Volts at 120° lagging and V3 will be 120 Volts at 240° lagging. This assumes that the
default phase angle is 0 - 360 Degrees lag, and not +/- 180 degrees. If the +/- 180 degree
phase angle option is used, then V2 will be at -120° and V3 will be at +120°. I1, I2 and I3
will be in-phase with their respective voltages. When the user inputs the MAX. Watts
value in the Transducer Setting Screen, the firmware can calculate the required test
currents for full scale value.
The formula required to calculate Watts for 3 Element transducer is,
V1 * I1 * (COS ∅) + V2 * I2 * (COS ∅) + V3 * I3 * (COS ∅) = Total Watts
Where ∅ is the incremental angular change between V1 and I1, V2 and I2, V3 and I3.
Example: The default voltage is 120.00 Volts AC, and the user inputs 1500 Watts as
Max. Value. The current required for full scale output from the transducer is,
120 * I1 * COS 0° + 120 * I2 * COS 0+ 120 * I3 * COS 0° = 1500 Watts
I1 = 500 Watts/(120 Volts * COS 0° ) , or I1 = 500/120
then I1, I2 and I3 will be 4.1667 Amperes each
Therefore, when the user inputs 1500 Watts in the MAX. value window, in the Test
Screen the test set should automatically show a test current value for I1 of 4.167
Amperes at an angle of 0°, I2 will be 4.167 at 120° (-120°) and I3 will be 4.167 Amperes
at an angle of 240° (+120°). Note that the current is rounded up to 7 in the last digit
displayed.
When the test is Started, the measured voltage and current outputs are displayed and the
calculated Watts is based on the measured voltage and current outputs. This is the value
that gets displayed in the Transducer Test Screen under the MPRT Output, next to the
Label of Watt 3 Element Another value of Watts gets calculated using the measured dc
Volts or dc milliampere output as displayed in the Transducer Output section. Let us
assume that in our example transducer, the output is in dc milliamperes. In this example,
let 20 milliamps of dc current be equal to the full scale output of 1500 Watts. For this
example, let's say that the measured output voltages are 120.01 Volts (V1, V2 and V3)
and the measured output currents are 4.167, Amperes. The measured output Watts
would be,
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