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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 to provide an output signal that is proportional to the current.
This type of coil has many advantages over other types of current transformers:
It is not a closed loop, because the second terminal is passed back through the center of the toroid core (commonly a plastic or rubber tube) and connected along the first terminal. This allows the coil to be open-ended and flexible, allowing it to be wrapped around a live conductor without disturbing it.
It has an air core rather than an iron core, it has a low inductance and can respond to fast-changing currents.
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.
Below you may see schematic drawing of a Rogowski coil used as an AC current sensor, and connected to an integrator circuit to obtain an output voltage proportional to the instantaneous value of the current i(t)