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A Rogowski coil is a toroid of wire used to measure an alternating current through a cable encircled by the toroid. The picture shows a Rogowski coil encircling a current-carrying cable. The output of the coil, , is connected to a lossy integrator circuit to obtain a voltage that is proportional to .]]

A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring alternating current (AC) or high-speed current pulses. It sometimes consists of a helical coil of wire with the lead from one end returning through the centre of the coil to the other end so that both terminals are at the same end of the coil. This approach is sometimes referred to as a counter-wound Rogowski.

Other approaches use a full toroid geometry that has the advantage of a central excitation not exciting standing waves in the coil. The whole assembly is then wrapped around the straight conductor whose current is to be measured. There is no metal (iron) core. The winding density, the diameter of the coil and the rigidity of the winding are critical for preserving immunity to external fields and low sensitivity to the positioning of the measured conductor.

Since the voltage that is induced in the coil is proportional to the rate of change (derivative) of current in the straight conductor, the output of the Rogowski coil is usually connected to an electrical (or electronic) integrator circuit to provide an output signal that is proportional to the current. Single-chip signal processors with built-in analog to digital converters are often used for this purpose.

  • Because it has no iron core to saturate, it is highly linear even when subjected to large currents, such as those used in electric power transmission, welding, or pulsed power applications.

Disadvantages

This type of coil also has some disadvantages over other types of current transformers.

  • The output of the coil must be passed through an integrator circuit to obtain the current waveform. The integrator circuit requires power, typically , and many commercial sensors obtain this from batteries.
  • Traditional split-core current transformers do not require integrator circuits. The integrator is lossy, so the Rogowski coil does not have a response down to DC; neither does a conventional current transformer (see Néel effect coils for DC). However, they can measure very slow changing currents with frequency components down to and less.

Applications

Rogowski coils are used for current monitoring in precision welding systems, arc melting furnaces, or electromagnetic launchers. They are also used in short-circuit testing of electric generators and as sensors in protection systems of electrical plants. Another field of usage is the measurement of harmonic current content, due to their high linearity.

<math display=block>L = \mu_0 N^2 \left(R - \sqrt{R^2 - r^2}\right).</math>

Similar devices

A device similar to the Rogowski coil was described by Arthur Prince Chattock of Bristol University in 1887. Chattock used it to measure magnetic fields rather than currents. The definitive description was given by Walter Rogowski and W. Steinhaus in 1912.

More recently, low-cost current sensors based on the principle of a Rogowski coil have been developed. These sensors share the principles of a Rogowski coil, measuring the rate of change of current using a transformer with no magnetic core. The difference from the traditional Rogowski coil is that the sensor can be manufactured using a planar coil rather than a toroidal coil. In order to reject the influence of conductors outside the sensor's measurement region, these planar Rogowski current sensors use a concentric coil geometry instead of a toroidal geometry to limit the response to external fields. The main advantage of the planar Rogowski current sensor is that the coil winding precision that is a requirement for accuracy can be achieved using low-cost printed circuit board manufacturing.

See also

  • Balisor, a device that gets energy from the electric field, instead of the magnetic field
  • Current
  • Current transformer
  • Index of electronics articles
  • Pulsed power
  • Toroidal inductors and transformers
  • Current sensing

References

  • Rogowski Coils , Using Rogowski Coils for Transient Current Measurements , Rocoil Ltd Operating Principle
  • Rogowski Coil Designs , PAC World, Autumn 2007, protection relaying applications
  • Miniature Wideband Current Probe sensor using this principle
  • PEM UK Rogowski current transducer theory
  • An Overview of Rogowski Coil Current Sensing Technology