BASIC OPERATION - ROGOWSKI CURRENT TRANSDUCERS.

Rogowski coil and active integrator basic operation

A Rogowski coil is a closely and evenly wound coil of N turns/m on a non-magnetic former of constant cross sectional area A sq.m. For PEM coils, the free end is returned to the other end along the central axis of the former and the two ends are permanently connected to a co-axial cable.  The free end is normally inserted into a socket adjacent to the cable connection but can be unplugged to enable the coil to be looped around the conductor or device carrying the current to be measured.

Provided the coil constitutes a closed loop with no discontinuities, it may be shown that the voltage E induced in the coil is proportional to the rate of change of the encircled current I according to the relationship E=HdI/dt, where H the coil sensitivity in (Vs/A) is proportional to NA and is typically in the range 5 to 300 nVs/A depending on the design.

To obtain an output voltage Vout proportional to I it is necessary to integrate the coil voltage E; hence an electronic integrator is used to provide a bandwidth extending down to below 1Hz. The op-amp integrator, in its simplest form, with an input resistor R and feedback capacitor C has an output Vout=(1/CR)∫ Edt. The overall transducer gain is therefore given by, Vout=RshI, where Rsh= H/CR is the transducer sensitivity (V/A).

Rogowski coil and integrator frequency response

The relationship Vout proportional to I is valid throughout the transducer bandwidth. The bandwidth is defined as the range of frequencies from fL to fH for which sinusoidal currents can be measured to within 3dB of the specified sensitivity Rsh.

At low frequencies the integrator gain increases and in theory will become infinite as the frequency approaches zero. This would result in unacceptable dc drift and low frequency noise; hence the integrator gain has to be limited at low frequencies.  This limitation is achieved by placing a low pass filter in parallel with the integrating capacitor. The low pass filter sets the low frequency bandwidth fL, typically this is less than 1Hz.

Furthermore, due to the distributed inductance and capacitance of the Rogowski coil there is a high frequency bandwidth fH, (generally 1MHz or greater) above which the measurement is attenuated and significant phase delay occurs. The bandwidth of the electronic integrator and the length of co-axial cable connecting the integrator to the coil also influence this limit.

PEM has produced numerous technical publications on the operation of Rogowski current transducers, these can be viewed in Publications.