Current Measurement

Course overview

In this course, you’ll explore how to measure electrical current accurately using DewesoftX and various sensor types. Beginning with Rogowski coils, you’ll learn to set up these integrative devices for non-intrusive AC measurement. You’ll configure integration filters or math modules in DewesoftX to convert induced voltage into true current values.

You’ll then be introduced to current clamps, based on Hall-effect sensors. The training shows you how to use them with SIRIUS modules, set correct scaling in-channel settings, and compensate for sensor phase shifts to ensure synchronized voltage and current readings.

For shunt resistor measurements, you’ll wire the resistor in series with the load and use DewesoftX’s scaling functions to convert voltage drop into current. This method is ideal for low-current applications and both DC/AC measurements.

The course also walks through software configuration: adjusting analog input ranges, applying transducer scaling (including reverse polarity if needed), and calculating offsets to ensure measurement integrity.

Lastly, the course demonstrates how to add these current channels into DewesoftX’s Power module, enabling full power analysis—including real, reactive, and apparent power, phase shift, total harmonic distortion, and interharmonic analysis  . You’ll learn to display results live using graphical meters, recorders, FFT plots, and export data for further analysis or reporting.

By the end of this course, you’ll be able to select the right current sensor, set it up correctly in DewesoftX, configure scaling and filtering, and integrate current data into full power analysis workflows—ready for electrical testing, inverter testing, and grid applications.

An electric current is defined as the rate at which electric charge flows past a point. Current exists whenever there is a movement of electric charge.

Now let’s return to the water analogy of electricity to explain what current is.

Electric current is a physical quantity driven by voltage and represents the flow of electrons between different electric potentials (typically from the positive to the negative pole in the case of direct current, though this differs for alternating current). In the water analogy, current corresponds to the actual flow rate of water moving from upstream to downstream. In concrete terms, current is the flow of electric charge between two poles.

As mentioned above, there are two types of current: direct current (DC) and alternating current (AC).

• DC is the simpler of the two—electrons flow in only one direction, and the flow remains constant.

• AC, on the other hand, involves electrons that continuously change direction and amplitude according to the supply frequency. For example, in Europe the grid frequency is 50 Hz, meaning electrons change direction (and amplitude) 50 times per second.

AC is harder to explain using the water analogy, since water in most cases flows only in one direction and does not typically reverse.

The cause of direct current is a constant voltage source, such as a battery.

To generate alternating current, an alternating voltage source is required—for example, an AC generator in a power plant.

The typical waveform of alternating current is a sine wave, where the positive half-cycle represents current flow in one direction, and the negative half-cycle represents current flow in the opposite direction.

Now let’s take a look at how current measurements are performed. The simplest method is by using an ammeter. To make the measurement, the circuit must be opened and the ammeter connected in series with it. To minimize the effect on the current flow, ammeters are designed with very low impedance.

Since many types of current transducers are available for DAQ instruments, current can be measured in a variety of ways. Current measurement is generally divided into two major groups:

1. Direct Measurement – The conductor must be disconnected, and a sensor is connected in series with the circuit.

2. Indirect Measurement – Specialized sensors allow current measurement without opening the circuit. These methods also provide galvanic isolation between the sensor and the conductor.

The direct measurement method of current works without any additional logic circuits. The most common technique is the use of a shunt resistor, which is connected in series with the measured electrical circuit.

What is a shunt resistor?

A shunt resistor is a resistor with a very low, precisely defined resistance predetermined by the manufacturer. It operates on the principle that, when connected in series with an electrical circuit, the current flowing through it produces a measurable voltage drop across the resistor.

According to Ohm’s Law, this voltage is directly proportional to the flowing current, since the exact resistance of the shunt is known. Selecting a shunt with high accuracy is essential, as it directly determines the precision of the measurement.

With this method, both AC and DC can be measured, but several factors must be considered. First, the declared current rating of the shunt must not be exceeded, as this may burn out the resistor. Second, the shunt will heat up and may eventually overheat if the maximum declared current flows through it for extended periods. As the shunt heats up, its resistance changes, and if it overheats, this change can become permanent. For this reason, a shunt is typically used only up to about 60% of its rated current.

Another important factor is the common-mode voltage, discussed earlier. This can cause complications, particularly in the early stages of current measurement. For example, when measuring the current flowing through a normal incandescent light bulb using a shunt resistor, the voltage difference at the amplifier is very small. Although the measurement points are still above ground, they can reach grid voltage levels. If such a grid voltage is applied to a 10 V range amplifier, the measuring instrument will be destroyed—leaving only sparks to "measure." To prevent this, it is strongly recommended to use an isolated measurement instrument.

To simplify measurements with Dewesoft instruments, two different DSI adapters with integrated shunt resistors are available. For example, the DSI 20 mA adapter contains a 50 Ω, 0.01%, 0.25 W shunt. Additional information about these shunt adapters is provided in the table below.

Measurements with these two adapters are straightforward, as no additional calculations are required. The adapters include built-in TEDS, which automatically recognize the sensors in the Dewesoft X software. This saves valuable time during sensor configuration. However, the conductor must still be split and connected to the sensor in order to perform the measurement.

Interrupting the conductor to attach an adapter for current measurement is sometimes not possible. In such cases, the flowing current can be measured with current sensors. This is possible because an electric current generates a magnetic field around the conductor, and current sensors measure the intensity of this magnetic field in various ways. These sensors are also galvanically isolated.

Current sensors provide an easier, faster, and safer method of measurement because they are isolated from the conductor. This isolation protects both the user and the measurement instrument, eliminating the risk of high common-mode voltage—a problem often encountered when measuring high-voltage currents with shunt resistors.

It is important to note, however, that these types of sensors introduce a phase shift in the output voltage relative to the measured current. The extent of this phase shift depends on the sensor type and the measured frequency. With high-accuracy current sensors, the phase shift is nearly zero, while cheaper sensors may exhibit a shift of more than 10° at the fundamental frequency, and even greater shifts at higher frequencies. Although phase shift can present challenges, if it is accounted for during measurement setup, it should not cause significant problems. In addition, Dewesoft offers software-based sensor calibration (via the Sensor Editor) that further improves both accuracy and phase response.

Current Sensors to be Described in the Following Sections:

• Rogowski coil

• Iron-core clamp

• Hall-compensated AC/DC clamp

• Zero-flux transducers

• Current transducers in public grids

Overview

The following table summarizes the main differences between these types of current transducers and the applications for which they are most suitable.

A Rogowski coil is a simple measurement device that enables AC current measurement without splitting the conductor. It consists of a helical coil of wire, with the lead from one end returning through the center of the coil to the other end, so that both terminals are located at the same end. The coil is wrapped around the conductor where the current measurement is to be taken, allowing measurement without cutting, disconnecting, or stripping the wire. The alternating current in the conductor induces a voltage in the coil.

Measurement with a Rogowski coil offers several advantages. Rogowski coils are available for measuring very small currents (around 100 mA) up to extremely high currents (>100 kA). The coil itself is flexible, thin, lightweight, and robust. Since no magnetic materials are used, Rogowski coils cannot saturate and therefore have a high overload withstand capability. They are highly linear and immune to DC currents, which allows accurate measurement of small AC currents even in the presence of a large DC component. The bandwidth of Rogowski coils depends on the type and price but can reach several MHz.

However, there are also disadvantages. The measurement principle is based on detecting the voltage induced by the current flowing through the conductor, which is proportional to the derivative of the current. For this reason, an integrator circuit must be used on the output side to make the voltage proportional to the actual current. Consequently, an external power supply is required. Rogowski coils cannot measure DC currents (with the exception of special designs that can).

The most significant drawback is phase shift. This shift depends heavily on the positioning of the coil (e.g., vertical or horizontal orientation). Such positioning errors cannot be compensated using the Dewesoft Sensor Editor. However, phase and amplitude errors caused by frequency response can be corrected with the Sensor Editor.

To measure AC current, simply use a Dewesoft current sensor that operates with a Rogowski coil. These sensors are integrated in a manner similar to the DSI shunt adapters, featuring built-in TEDS chips that store all configuration data. Note that TEDS support is available only on certain models.

Current clamps enable current measurement with galvanic isolation. They feature jaws that can be opened and clamped around the conductor, allowing non-intrusive measurement. The operation of current clamps is based on either Hall-effect technology or current transformer technology, both of which utilize the magnetic field generated by the flowing current to produce a corresponding voltage output.

The iron-core clamp works on the principle of a transformer. Depending on the number of windings on the primary side compared to the secondary side (the turns ratio), a specific current is induced in the secondary winding. Like any transformer, this method is limited to measuring AC current.

The advantages of current clamps are that they are inexpensive, require no external power supply, and are available for measuring currents ranging from very small to very high levels. The disadvantages are that they are heavy, inflexible, and cannot measure DC currents. In addition, their bandwidth is limited (maximum of 20 kHz).

The Hall effect is commonly used to measure both AC and DC currents across a wide amplitude and frequency range (up to 100 kHz) with high sensitivity. For this reason, Hall-effect clamps are recommended for measuring DC currents.

Advantages of Hall-Compensated AC/DC Current Clamps:

• High accuracy (0.5%)

• Wide bandwidth (up to 100 kHz)

• Ability to measure both AC and DC currents

• No need to open the circuit for measurement

Dewesoft offers a variety of clamps that use the Hall effect for current measurement. These are listed in the table below.

The voltage output of such clamps is directly proportional to the measured current. However, current clamps also introduce a phase shift, which can be as high as ~10°. High-quality clamps can reduce this phase shift to less than 1°. Since the phase shift of current sensors varies with frequency, it is important to consider this effect when performing any power-related measurements.

Fluxgate current sensors use a high-permeability magnetic core to detect the magnetic field produced as current flows through a conductor. Fluxgate technology relies on a negative feedback circuit, which includes a magnetic circuit, as shown in the schematic image. This works by feeding a current back through the feedback coil to cancel out the magnetic flux generated by the measured current in the DUT. In this way, the effect of the material’s magnetic non-linearity is compensated and kept low.

Fluxgate technology can also detect DC current, which provides the inherent advantage of not requiring semiconductors. As a result, fluxgate current clamps feature excellent long-term stability, a wide temperature stability range, and a very low offset voltage.

Most clamp-type transducers have lower accuracy than their uni-body counterparts, primarily because of the split in the magnetic core needed to open the clamp. However, fluxgate current clamps are industry-leading in terms of linearity, bandwidth, and temperature drift, second only to high-quality uni-body zero-flux transducers.

Overview

Current transducers measure current flows with galvanic isolation by reducing high currents to much lower values. The conductor carrying the measured current must pass through the sensor’s loop, as current transducers operate on the principle of a transformer. This means they provide a current output signal, which can then be measured with the DAQ system.

Zero-flux current transducers are more advanced than simple transformers, featuring sophisticated designs and integrated electronics. They include two windings operated in saturation to measure DC current, one winding for AC current, and an additional winding for compensation. This type of current measurement is highly precise due to the zero-flux compensation principle.

This is important because the magnetic core of a transformer typically retains residual magnetic flux, which reduces measurement accuracy. In zero-flux transducers, this parasitic flux is perfectly compensated, ensuring high precision. For this reason, zero-flux current transducers are used where high accuracy is required. However, they are not as suitable for simple or fast measurements compared to iron-core clamps or Rogowski coils.

Zero-flux transducers are used to measure currents with the highest accuracy for both AC and DC. They also offer high bandwidth capabilities (up to 1 MHz). These transducers are highly linear and exhibit very low phase and offset errors.

Connecting a zero-flux transducer to a PWR-MCTS & a SIRIUSi DAQ

For demonstration purposes, an IT-400-S transducer will be connected to both a Sirius 4xHV 4xLV module and a PWR-MCTS2.

The connection requires the following components:

Step 1: Connect the zero-flux transducer IT-400-S to the SIRIUSi-PWR-MCTS slice at the Sensor 1 input using the D9m-D9f-5M-MCTS cable. This cable is a simple extension cable and can be used with all zero-flux transducers (from 60 A up to 1000 A).

Step 2: Connect the DSI-MCTS-400-03M cable to Output 1 of the SIRIUSi-PWR-MCTS2, and then connect it to the first LV input of the Sirius PWR amplifier.

Repeat Step 1 and Step 2 for all zero-flux transducers that need to be connected to the system. For measuring a three-phase system with a star connection, the setup will look as shown in the image below:

Instructions on how to connect voltage and current transducers for various wiring configurations—such as DC, single-phase, two-phase, three-phase (delta, star, Aron, V, etc.)—can be found in a later chapter of the Pro Training material.

Software configuration

Most Dewesoft current transducers include an integrated TEDS chip, which stores data such as scaling and calibration of the transducer. When the shunt cable is connected to the Low-Voltage input of the Sirius amplifier, all these configurations are applied automatically.

Therefore, the DSI adapters and TEDS sensors must be activated. Please check Device Settings and ensure that this option is enabled (see screenshot below).

After connecting the sensor (e.g., MCTS 400), the software will recognize the TEDS and automatically fill in the Ampl. name column with the current transducer description (e.g., DSI-MCTS-400). The type of measurement, physical quantity, and measurement unit will also be updated to Current.

Finally, a suitable measurement range should be set, and a low-pass filter applied if necessary, as shown in the image.

Current transducers in public grids

Current transducers are used to monitor current flow in the public grid and protect equipment from overloading. A current transducer can be described as a transformer operated in short-circuit mode on the secondary side (or with a small load). On the output (secondary) side, the transducer provides a low current signal directly proportional to the current on the primary side. In public grid operation, the secondary current is standardized to either 1 A or 5 A.

There are different measurement classes of current transducers, which define their accuracy and phase shift. The classes range from 0.1 to 5.

• Class 0.1: accuracy of 0.1% for the measured amplitude, with a phase shift of ±5 minutes.

• Class 5: accuracy of 5%, with a phase shift of ±120 minutes.

The description of a current transducer also specifies the overload factor, rated power (load), and intended application (e.g., protection or measurement). The load (input resistance of the measurement device) is important, as it affects the overload capability of the transducer. If the load exceeds the rated value, the transducer will enter saturation prematurely and lose its overload capability.

As an example, a classic 40 W light bulb was measured. It is noticeable that the load on the grid is proportional to the voltage. The measured power is exactly 40 W, as expected. However, in the vector scope shown in the image below, the results are not quite as they should be. Since a light bulb is a purely ohmic load on the grid, the voltage and current should be perfectly aligned (no phase shift). So, what could be causing this discrepancy?

In previous chapters, the differences between current transducers and shunt resistors were explained. It was mentioned that these devices may introduce a phase shift during measurement (the extent of which depends greatly on the quality of the sensor used). This is precisely what can be seen in the image below: the current transducer used for the measurement is causing a phase shift in the vector scope, which negatively affects the accuracy of the measurement.

Luckily, in the DewesoftX software there is an option to correct phase shift, ensuring it is compensated and reducing the likelihood of measurement errors. Here is how it works:

As mentioned earlier, all current transducers exhibit frequency-dependent behavior with respect to amplitude and phase. In DewesoftX, compensation is performed in the Sensor Editor, which makes sensor-based measurements much more accurate. This is achieved by applying the specifications provided by the sensor manufacturer. This feature is unique among DAQ systems, as the compensation math is carried out automatically in the software.

To use this feature, navigate to the Settings section and open the Sensor Editor menu. A list of known sensors will be available, and users also have the option to add sensors that are not on the list. For this example, a new sensor is added by entering the sensor type and serial number. Then, define the Physical (input) unit—in this case A (amperes)—and the Electrical (output) unit, which is V (volts).

The next step is to add a scaling factor (usually provided by the manufacturer). In this example, the sensor is linear with respect to amplitude, so only the scaling factor needs to be added. In this case, it is set to 1 (1 A = 1 V). The polarity at this stage is not critical, as it can be adjusted or reversed later in the Channel Setup.

This is the most important step → defining the transfer curve. In the table under the Transfer Curve column, select YES to indicate that a transfer curve will be defined. Next, add the curve points:

• a [dB] → amplitude deviation in decibels

• phi [deg] → phase angle in degrees

Where can such transfer curves be obtained?

1. Many transfer curves for common sensors have already been measured, so it is worth checking whether one already exists.

2. The transfer curve may also be copied from the calibration sheet, if provided.

3. Another option is to measure it directly using the FRF (Frequency Response Function) feature, although this requires additional equipment.

Once the transfer curve is obtained, it must be entered into the table. As shown in the image below, at 50 Hz the angle is approximately 10°, which explains the phase shift observed in the vectorscope.

When the sensor setup is complete, save it using the Save File button and exit the Sensor Editor by selecting Exit. Then return to the Analog Setup and assign the sensor to the current channel. Open the Sensors tab and select the serial number of the sensor that was previously entered in the Sensor field of the editor.

At this point, there will be no visible changes, but note that manual scaling or sensitivity adjustments can no longer be made. To reverse the polarity of the sensor, go to Scaling by Function, choose Sensitivity, and click the ± button to invert the polarity.

The sensor has now been set up, and the software should automatically compensate for the phase shift. This sensor setup does not need to be repeated, as it has been saved to the sensor list (database). If needed, simply select it from the list and the setup will be ready to use.

The image on the next page shows how the sensor correction affected the measurement. The results are now accurate: the phase angle has been virtually eliminated, and the power is calculated correctly.

Now, some current measurements will be performed using Dewesoft equipment. The test will be conducted on two light bulbs to determine the current consumption of a classic 40 W bulb and an energy-saving 11 W bulb. Two approaches will be used: the first is a direct current measurement with a shunt resistor, and the second uses current clamps.

Before the measurement can begin, some calculations must be carried out. These calculations will determine which SIRIUS amplifier should be used, the appropriate amplifier range, and the type of current clamps required.

When both light bulbs are switched on simultaneously, the declared power is 51 W, and the RMS value of the grid voltage is 230 V. These are the variables used for the calculation shown below.

After the rough calculations, the RMS current is estimated to be approximately 0.22 A. However, we must consider that the maximum value of a sine wave signal is about twice the RMS value. In addition, since the energy-saving light bulb does not draw current in a perfect sine waveform, we should allow some reserve in the measurement range due to its higher crest factor.

Taking this into account, a 10 A range was selected for the current clamps, and a DSI SHUNT 5 A adapter was used for the shunt. The shunt has a resistance of 0.01 Ω, meaning that 1 A of current will cause a voltage drop of 10 mV across it. This information is necessary for setting up the measurement channel, as the voltage drop across the shunt will be measured.

The Dewesoft DSI adapters are already equipped with channel setup information via integrated TEDS, allowing the software to configure the measurement channel automatically. This makes setup easier, as there is no need to enter the information manually when using Dewesoft DSI adapters.

The measurement can now begin. For this test, two different Sirius amplifiers will be used: an LV module and an ACC module. The image below shows the measurement setup after all components have been connected. The current clamps are connected directly to the LV module, while the DSI SHUNT 5 A is connected directly to the ACC module.

As the image illustrates, the wires must be split for the shunt installation. Please exercise caution when doing this, as it can be dangerous due to the grid voltage. Next, configure Channel 1 (the shunt channel). It is recommended to rename the channel to maintain a clear overview of which components are connected to which channels. To do this, simply click on the channel name field and enter the desired name. In this example, the channel was renamed Shunt Current.

Next, set the physical quantity to Current. The unit (A – Ampere) is automatically assigned by the DewesoftX software. After these settings are applied, it is recommended to calibrate the sensor. In this example, a 2-point calibration was used, as it is already known that 1 V corresponds to 10 A. These two values are entered directly into the fields provided in the bottom-right corner of the screen.

If the parameters are set correctly and the classic 40 W light bulb is switched on, the sine wave of the current will be displayed in the lower-left corner of the setup screen on the scope. Please refer to the image on the next page.

For Channel 8, where the current clamps are connected, the settings differ slightly from the shunt setup on Channel 1. This is mainly because 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 corresponds to an output of 1 mV per 1 mA (scaling factor = 1). As a result, the maximum output cannot exceed 10 V. Therefore, the amplifier should be set to a range of 50 V to ensure sufficient resolution for the measurement. The physical quantity must again be set to Current, and the unit will automatically switch to Ampere (A).

The image below illustrates the combined waveform of the energy-saving light bulb and the classic light bulb when they are switched on simultaneously. The waveform changes due to the non-sinusoidal waveform and the high crest factor of the energy-saving bulb.

When switched to the Measure Mode screen, the phase shift of the current clamps compared to the shunt resistor can be observed, as shown in the image below. At first glance, the shift does not appear very large (approximately 10°), but in applications such as power measurement, even a small phase shift is critical for achieving accurate results.

A 10° phase shift can significantly influence the outcome, especially when performing detailed power analysis (e.g., for reactive and apparent power). Fortunately, this phase shift can be corrected using the Sensor Editor, as explained earlier.

Displaying the AC RMS value

There are two possible ways of displaying AC RMS in the Power Module.

The Basic Statistics Method

To see the RMS value of the current signal, add a Basic Statistics math function. Click the math symbol on the main dashboard to open the math module, which provides four options:

1. Add Math – contains a selection of predefined math functions.

2. Formula Module – allows the user to enter any custom math formula.

3. Infinite Impulse Response (IIR) Filter – provides a variety of filters to choose from.

4. Basic Statistics – opens the setup screen shown below.

In the setup, select the input channel (the current signal, in this case I1) and choose RMS as the output channel. The output can either display a single value per measurement or provide new values for each defined block.

For more information on the functionalities of the Basic Statistics tool, press F1 while the tool is open. This will automatically open a web page with useful information and tips.

Using the Recorder 

The second option for displaying the RMS value of a signal is to do so directly on the measurement screen using the recorder. As shown in the image below, there are two scopes: one displaying the real value of the AC current and the other showing the RMS value.

Tip: Under the recorder options, there are four settings. One of them is Unified Properties, which is selected by default. This means that when the real value is selected, it will apply to both recorders. When deselected, one recorder can display the real value while the other can display the RMS value, as illustrated in the image.

To display the RMS value, go to the Y-Axis options, select Display Type, and from the drop-down menu choose RMS (see the bottom left of the image).