A Rogowski coil is an electrical transducer used for measuring AC currents such as high-speed transients, pulsed currents of a power device, or power line sinusoidal currents at 50 or 60 Hz. The Rogowski coil has a flexible clip-around sensor coil that can easily be wrapped around the current-carrying conductor for measurement and can measure up to a couple thousand amperes of very large currents without an increase in transducer size.
 

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How does a Rogowski coil work?

The theory of operation behind the Rogowski coil is based on Faraday’s Law which states that the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit.

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The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The key difference is that the Rogowski coil has an air core as opposed to the current transformer, which relies on a high-permeability steel core to magnetically couple with a secondary winding. The air core design has a lower insertion impedance, which enables a faster signal response and a very linear signal voltage.

An air-cored coil is placed around the current-carrying conductor in a toroidal fashion and the magnetic field produced by the AC current induces a voltage in the coil. The Rogowski coil produces a voltage that is proportional to the rate of change (derivative) of the current enclosed by the coil-loop. The coil voltage is then integrated in order for the probe to provide an output voltage that is proportional to the input current signal.



Advantages

Rogowski coil current probes offer many advantages over different types of current transducers or sensing techniques.

• Large current measurement without core saturation --

  Rogowski coils have the capability to measure large currents (a very wide range from a few mA to more than a few hundred kA) without saturating the core because the probe employs non-magnetic “air” core. The upper range of the measurable current is limited by either the maximum input voltage of a measuring instrument or by the voltage breakdown limits of the coil or the integrator circuit elements. Unlike other current transducers, which get bulkier and heavier as the measurable current range grows, the Rogowski coil remains the same small size coil independent of the amplitude of current being measured. This makes the Rogowski coil the most effective measurement tool for making several hundreds or even thousands of amperes of large AC current measurements.

• Very flexible to use --

  The lightweight clip-around sensor coil is flexible and easy to wrap around a current-carrying conductor. It can easily be inserted into hard-to-reach components in the circuit. Most

  Rogowski coils are thin enough to fit between the legs of a T0-220 or TO-247 power semiconductor package without needing an additional loop of wire to connect the current probe. This also gives an advantage in achieving high signal integrity measurement.

• Wide bandwidth up to >30 MHz --

  This enables the Rogowski coil to measure the very rapidly changing current signal – e.g., several thousand A/usec.

  High bandwidth characteristic allows for analyzing high-order harmonics in systems operating at high switching frequencies, or accurately monitoring switching waveforms with rapid rise- or fall-times.

• Non-

  intrusive or lossless measurement -- The Rogowski coil draws extremely little current from the DUT because of low insertion impedance. The impedance injected into the DUT due to the probe is only a few pico-Henries, which enables a faster signal response and very linear signal voltage.

• Low cost

  Compared to a hall effect sensor/transformer current probe, the Rogowski coil typically comes in at lower price point.

Limitations

• AC only -- Rogowski cannot handle DC current. It is AC only.
• Sensitivity - Rogowski coil has a lower sensitivity compared to a current transformer due to the absence of a high permeability magnetic core.

Applications

Rogowski coil current probes have a large number of applications in broad power industries and power measurement applications. The following are some examples of Rogowski coil applications:

• Flexible current measurement of power devices such as MOSFET or IGBT device as small as TO-220 or TO-247 package or around the terminals of large power modules

• To measure power losses in power semiconductors

• To monitor currents in small inductors, capacitors, snubber circuits, etc.

• To measure small AC current on a conductor with high DC current or in the presence of a high DC magnetic field.

• To measure high frequency sinusoidal, pulsed, or transient currents from power line frequency to RF applications

• To measure current in motor drives and, in particular, power quality measurements in VSD, UPS or SMPS circuits

• To evaluate switching performance of power semiconductor switches (double pulse tester).

• Power distribution line monitoring or utilities pole probe monitoring

• Smart grid applications

• Plasma current measurement

Conclusion

There are a number of different ways of measuring electric current where each method has advantages and limitations.

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. However, Rogowski coils have the capability to measure large currents (very wide range from a few mA to more than a few kA) without saturation because of its non-magnetic “air” core. The air core design also has a lower insertion impedance to enable a faster signal response and a very linear signal voltage and is very cost effective compared to its hall effect sensor/current transformer counterpart. This makes the Rogowski coil the most effective measurement tool to make several hundreds or thousands of amperes of large AC current measurement.

Current Measurement Methods

There are two main ways to measure current – one is based on electromagnetics and is associated with the early moving coil (d’Arsonval) meter, and the other is based on the main theory of electricity, Ohm’s law.

D’Arsonval Meter/Galvanometer

A d’Arsonval meter is a type of ammeter, which is an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil.

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The d’Arsonval form used today is constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position.

When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current.

Other Ammeters

Essentially, most of today’s ammeters are based on the fundamental theory of electricity, Ohm’s law. Modern ammeters are essentially voltmeters with a precision resistor, and using Ohm’s law, an accurate yet cost-effective measurement can be made.

Ohm’s Law – Ohm’s law states that, in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference (in other words, voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.

The mathematical equation that describes this relationship is:

I = V/R

where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), called the resistance.

Ammeter Operation – Today’s ammeters have an internal resistance to measure the current across the particular signal. However, when the internal resistance is not enough to measure larger currents, an external configuration is needed.

To measure larger currents, you can place a precision resistor called a shunt in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure larger currents.

Any resistor is acceptable, as long as the maximum expected current multiplied by the resistance does not exceed the input range of the ammeter or data acquisition device.

When measuring current in this fashion, you should use the smallest value resistor possible because this creates the smallest interference with the existing circuit. However, smaller resistances create smaller voltage drops, so you must make a compromise between resolution and circuit interference.

Figure 2 shows a common schematic of current measurement across a shunt resistor.

Figure 2. Connecting a Shunt Resistor to a Measurement

Using this approach, the current is not actually directed into the ammeter/data acquisition board but instead through an external shunt resistor. The largest current you can measure is theoretically limitless, provided the voltage drop across the shunt resistor does not exceed the working voltage range of the ammeter/data acquisition board.

Current Conventions

Conventional Currents

Conventional currents are the current measurements common in today’s electronics, electrical circuitry, transmission lines, and so on. They do not conform to a transmission standard, and they can range from zero to large values of amperage.

Current Loops/4-20 mA Convention

Analog current loops are used for any purpose where a device needs to be either monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

“Four to 20 milliamp current loop,” or 4-20 mA, is an analog electrical transmission standard for industrial instrumentation and communication. The signal is a current loop where 4 mA represents the zero percent signal and 20 mA represents the 100 percent signal.[1] The “mA” stands for milliampere, or 1/1000 of an ampere.

The “live zero” at 4 mA allows the receiving instrumentation to distinguish between a zero signal and a broken wire or a dead instrument.[1] Developed in the 1950s, this standard is still widely used in industry today. Benefits of the 4-20 mA convention include wide use by manufacturers, relatively low implementation costs, and its ability to reject many forms of electrical noise. Also, with the live zero, you can directly power low-power instruments from the loop, saving the cost of extra wires.

Accuracy Considerations

Placement of the shunt resistor in the circuit is important. If the external circuit shares a common ground with the computer containing the ammeter/data acquisition board, then you should place the shunt resistor as close to the ground leg of the circuit as possible. If not, the common-mode voltage produced by the shunt resistor might be outside the specification for the ammeter/data acquisition board, which could lead to inaccurate readings or even damage the board. Figure 3 shows the correct and incorrect placements of the shunt resistor.

Figure 3. Shunt Resistor Placement

Data Acquisition Device Measurements

There are three different methods of measuring analog inputs. Please refer to the “How to Make a Voltage Measurement” article for additional information on each configuration.

As an example, consider the NI CompactDAQ USB data acquisition system. Figure 4 shows the NI cDAQ-9178 chassis and an NI 9203 analog current input module. The NI 9203 does not require an external shunt resistor due to the presence of an internal precision resistor.

Figure 4. NI cDAQ-9178 Chassis and NI 9203 Analog Current Input Module

Figure 5 shows the connection diagram for Reference Single-Ended (RSE) current measurements using an NI cDAQ-9178 chassis with an NI 9203 as well as the pin-outs for the module. In the figure, Pin 0 corresponds to the “Analog Input 0” channel and Pin 9 corresponds to the common ground.

Figure 5. Current Measurement in RSE Configuration

In addition to the NI 9203, general-purpose analog input modules, such as the NI 9205, can provide current input functionality using an external shunt resistor.

Getting to See Your Measurement: NI LabVIEW

Once you have connected the sensor to the measurement instrument, you can use LabVIEW graphical programming software to visualize and analyze data as needed.

Figure 6. LabVIEW Current Measurement

References
Bolton, William (2004). Instrumentation and Control Systems. Elsevier. ISBN 0750664320.

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