Now some current measurements will be done using DEWESoft equipment. The measurement will be done on two light bulbs, to determine the current consummation of a classic 40 W and an energy saving 11 W light bulb. In this measurement two approaches will be used, the first being a direct voltage measurement using a shunt resistor and the second using current clamps.
Before the measurement can commence some calculations need to be done. These calculations will determine which SIRIUS amplifier and the range of the amplifier needs to be used as well as the type of current clamps.
When both light bulbs are switched on simultaneously the declared power will be 51 W and the RMS value of the grid is 230 V, these are the variables that we need to do the calculation. Below is the calculation.
After the rough calculations have been made the current RMS will be approximately 0.22 A. But we need to consider that the max value of the sine wave signal is √2 times the RMS value, but since the energy saving light bulb doesn't use the current in sine waveform we should have some reserve in our measurement ranges due to the higher crest factor of the energy saving bulb. This taken into consideration a 10 A range is chosen on the current clamps and the shunt an MSI SHUNT 5 A adapter. The shunt has a resistance of 0.01 Ω, meaning that 1 A of current will cause a voltage drop of 10 mV in the shunt. This information is necessary for the measurement channel set-up that the voltage drop of the shunt will be measured on. The DEWESoft MSI adapters are already equipped with the channel set-up information (integrated TEDS), so the software can configure the channel set-up automatically. This is just one less thing to keep in mind when DEWESoft MSI adapters are used for measurements.
The measurement can now be started. For this measurement two different SIRIUS amplifiers, a HV module and an ACC module will be used. Below on the image is shown what the measurement set-up looks like when all the components have been connected. The current clamps are directly connected to the HV module and the MSI SHUNT 5A is directly connected to the ACC module.
As the image illustrates the wires must be split for the shunt installation. Please be careful when doing this as it can be dangerous due to the grid voltage. Next the channel configuration of channel 1 (shunt channel) is done. It is recommended to rename the channel to keep a clear overview of which components are connected to which channels (to do this simply click where it says channel name and type the chosen name in), in this example the name shunt current was chosen for the channel name.
Secondly the physical quantity will be set to current, the unit which is A (Ampere) is set automatically by the DEWESoft X software. Once these settings have been changed it is recommended that the sensor be “calibrated”. In this example a 2-point calibration will be chosen as it is already known that 1 V will equal 10 A. These two values are simply typed into the supplied space on the bottom right of the screen. If the parameters were set correctly and the classic 40 W light bulb is switched on, the sine wave from the current will be displayed in the lower left corner of the set-up screen on the scope. Please refer to the image on the next page.
For channel 8 where the current clamps are connected, the settings will differ a little from the shunt (Channel 1) set-up that was done before, this is mainly due to the fact that the current clamps are connected to the HV part of the SIRIUS instrument. The current clamps are set to a range of 10 A which means that they yield 1mV/1mA output (the scaling factor is 1). This means that that an output of more than 10 V is not possible, therefore that amplifier will be set to have a range of 50 V this is to ensure that the resolution is sufficient for the measurement. The physical quantity must be set to current again and the unit will again automatically switch to ampere (A).
The image below illustrates what the combined waveform of the energy saving light bulb and the classic light bulb would look like when they are switched on simultaneously. The wave form changes due to the non-sinusoidal wave form and the high crest factor of the energy saver light bulb.
When switched to the measure mode screen, the phase shift of the current clamps compared to the shunt resistor can be seen, as illustrated in the image below. The phase shift does not seem too big at first glance (it is at around 10°), but with applications such a power measurement the phase shift is a critical component in order to yield the correct results for measurements.
This means that the 10° phase shift that is present in the measurement at the moment can have a significant influence on the measurement results especially when doing a detailed power analysis (especially for reactive and apparent power). Again, this phase shift of the current can be compensated using the sensor editor, as was explained earlier.
Displaying the AC RMS value
There are two possible ways of displaying AC RMS in the power module
Using the basic statistic method
To see the RMS value of the current signal, add a Basic statistic math function. Select the math symbol on the main dashboard, that will open the math module where there are four choices, add math which has a selection of predefined math function that can be chosen form. Then there is the formula module where the user can enter any math formula imaginable. The next box is the Infinite impulse response (IIR) filter here an array of filters can be chosen from. Lastly is the basic statistic box is this will yield the set-up screen as seen below. Select the input channel (current signal) in this case I1 and the RMS as the output channel. I can be selected to display one value per measurement or it can display new values for each defined block. (For more information on the functionalities of the basic statistics tool please press F1 on the keyboard while the basic statistics tool is open and it will automatically open a web page with useful information and tips )
Using the Recorder
The second option that is available to display the RMS value of the signal is to do so directly in the measurement screen using the recorder. As seen in the image below there are two scopes, one displaying the real value of the AC current and one displaying the RMS value. (Tip: under the recorder options there are four options, one of them which is Unified properties, this is selected as a default and means that the when real value is selected that will be the case for both the recorders, when deselected one recorder can display the real value and one recorder can display the RMS value as is the case in the image below). To display the RMS value, navigate to under the the Y Axis options and select Display type, in the drop-down list select RMS (Bottom left of the image).