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A Rogowski coil is a toroid of wire used to measure a current along its axis. The above picture shows the leads on opposite ends of the torus. Often, one lead is wound back along the toroid so that both are gathered at the same point. This reduces interference from magnetic fields and allows the coil to be wrapped around a cable without disconnecting it.

A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring alternating current (AC) or high speed current pulses. It 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. The whole assembly is then wrapped around the straight conductor whose current is to be measured. 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 in order to provide an output signal that is proportional to current.

One advantage of a Rogowski coil over other types of current transformers is that it can be made open-ended and flexible, allowing it to be wrapped around a live conductor without disturbing it. Since a Rogowski coil has an air core rather than an iron core, it has a low inductance and can respond to fast-changing currents. Also, 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. A correctly formed Rogowski coil, with equally spaced windings, is largely immune to electromagnetic interference.

A device similar to the Rogowski coil was described by A.P. Chattock of Bristol University, ("On a magnetic potentiometer", Philosophical Magazine and Journal of Science, vol. XXIV, no. 5th Series, pp. 94–96, Jul-Dec 1887). Chattock used it to measure magnetic fields rather than currents. The definitive description was given by Walter Rogowski and W. Steinhaus in "Die Messung der magnetischen Spannung", Archiv für Elektrotechnik, 1912, 1, Pt.4, pp.141-150.

More recently, low cost current sensors based on the principle of a Rogowski coil have been developed.¹ 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 sensors 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.

Formulae

The voltage produced by a Rogowski coil is

,

where A = πa² is the area of one of the small loops, N is the number of turns, and l = 2πR is the length of the winding. is the rate of change of the current threading the loop. This formula assumes the turns are evenly spaced and that these turns are small relative to the radius of the coil itself. At high frequencies, the Rogowski coil's inductance will decrease its output. The inductance of a toroid is

is the magnetic constant, R is the major radius of the toroid, and a is its minor radius.

See also

• Current
• Current transformer
• List of electronics topics
• Pulsed power

External links

• Miniature Wideband Current Probe sensor using this principle

References

Wikipedia content modification information:

• This page was last modified on 31 October 2008, at 17:15.

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