Electronic – Why do Rogowski coils work for measuring current


Rogowski coils measure current by completing a loop of coils around the conductor seen below.

enter image description here

This makes sense, but question 1: This is a current transformer, and transformers couple, so in this system when the Rogoski coil couples with the measured wire, does this not affect/damage the measured wires current (we are measuring) as we are drawing/collapsing its magnetic field in order the induce a voltage in the coil?

Question 2: Why can the circuit diagram below not do the same job? The Rogowski coil only works when you form a loop around the conductor. Why does the Rogowski coil only work when its a closed loop? Why can a coil no just be in parallel with a wire and measure current?


simulate this circuit – Schematic created using CircuitLab

The right hand rules says that in the coils are wound in the correct direction, in the circuit diagram would couple the magnetic field the current is inducing.

Image source: http://www.electrotechnik.net/2009/09/what-is-rogowskis-coil.html

Best Answer

A Rogowski coil is not a current transformer.

The current in the main conductor generates a magnetic field round itself. The Rogowski coil samples the field, generating a voltage as the field changes. It is weakly coupled to the main conductor. As little, ideally no, current flows in the Rogowski coil, there is no effect back on the main conductor.

Any loops couple. The sampling coil would generate a voltage whether it formed a closed loop around the main conductor or not. However, that's not terribly useful, as the coupling, and hence the generated voltage, would change if the conductors changed position. The sampling coil would also respond to any current changes anywhere in the vicinity.

The reason the Rogowski coil takes the form of a uniform toroidal winding is because due to the symmetric situation, it couples to all the current flowing within the toroid, regardless of the position of the conductor. There is zero coupling to any current that does not pass through the hole. An incomplete circle, or a non-uniform winding, would not have these two desirable properties.