Tutorials > Power module tutorials > Single phase power measurement

Obviously we need to tell DEWESoft that we have a sensor which is not perfect and somehow enter the transfer curve of our sensor. A transfer curve gives information about amplitude and phase for sensors at certain frequencies, and from this information DEWESoft can compensate for these errors.

Let's choose the Settings → Sensor editor... menu item. We get the list of all possible sensors. Now let's Add one sensor and enter the Sensor type and Serial number. Choose the GENERAL tab, enter the Physical (input) unit, which is A (amperes) in our case and the Electrical (Output) unit, which is V (volts).

Sensor editor

Next let's enter the SCALING factor. Since the sensor is linear with amplitude, we only need to enter the scaling factor, which is 1 in our case (1A=1V). We also select that we can define Additional scale factor can be defined in channel setup to be able to reverse sensor polarity.

Now we came to the most important part - the definition of the transfer curve. In the table under the TRANSFER CURVE column we select Yes to signify that the transfer curve will be defined. Now we need to enter the points of the curve. We need to enter the a[dB] - amplitude deviation in dB and the fi[deg] - phase angle in degrees. The next question is: where do we to get this transfer curve? There are lots of transfer curves for the most common sensors, already measured, so it's worth asking if it already exists. A second option is to copy it from the calibration sheet of the sensor, if the cal sheet includes the transfer curve. The third option is to measure it with DeweFRF, but this requires some equipment. When we get this transfer, we need to enter it in the table. We see that at 50 Hz, the angle was around 10 deg, which could explain phase shift we saw in the measurement.

Save the sensors with the Save file button and close the sensor editor with Exit. Now let's go back to the analog setup and choose the sensor for the current channel. Open the Sensors tab and select the serial number of the sensor previously entered in the Sensor field of the editor. Nothing much happens, but note that we can't enter the normal scaling or sensitivity anymore. We only have a choice to enter a Transducer scale, which we can use for reversing the polarity of the sensor by entering a value of -1.

Sensor for current channel setup

That's it. For the next setup we don't have to define a sensor anymore, instead we can just select it from the sensors list. Now let's see what the effect on our measurement is. The results are much better. The phase angle is virtually eliminated and the power is calculated correctly.

Measurement with sensor correction

From this example we can clearly see how big the errors could be if the sensor characteristics are not taken into account for power measurements. By the way, amplifiers and multiplexed AD cards also have phase shifts, but DEWESoft automatically takes care of that. With these useful tricks we can reduce the error in power calculations to below 0.1%.