Measuring high frequency, large magnitude, sinusoidal currents. For example in rf transmitters, high frequency induction heating or plasma applications

PEM produce ac current probes with a very wide-bandwidth capable of measuring from sub 1Hz into the MHz range. However if the continuous (rms) product of current and frequency is very large, of the order of > 3000A/µs, then it may be  necessary to produce an optimised Rogowski probe. Typical applications where very  high frequency sinusoidal and/or large magnitude currents are encountered include  rf transmitters and amplifiers, certain induction heating systems and plasma  deposition systems.

Other forms of wide-bandwidth current sensor are limited in their ability to measure continuous high frequency sinusoidal currents;

  • those measurement technologies that use a magnetic core such as Current Transformers or Hall Effect devices have limited rms current capability even if they have sufficient high frequency bandwidth.
  • Co-axial shunts are not isolated and have limited power capability

PEM term the continuous product of frequency and current the ‘rms di/dt’. Our standard CWT and RCT Rogowski sensors have an rms di/dt limit beyond which it is possible to damage the coil. This value is listed on all our product datasheets. However this limitation can be overcome. There are a number of techniques dependent on the magnitude of the rms di/dt and the frequency of the current.For example in:

Induction heating

For certain high power or high frequency induction heating applications PEM have supplied modified versions of the CWT and RCT ranges to measure currents of 100  to 1000A at frequencies of between 100kHz and 1MHz. The only resultant change to our standard probes is a limitation of the low frequency performance, but this can still be as low as 10Hz in most cases. These Rogowski sensors have been used both for diagnostics in developing new induction heating inverters and even for control of industrial processes.

RF transmitters and amplifiers

PEM have supplied Rogowski coils using passive integration for permanent installation on a high power radio frequency (rf) transmitter. The Rogowski coil uses passive Lr (also known as self) integration. The Rogowski probe is required to measure a largely undistorted sinewave with an amplitude of up to 1kA at a frequency of up to 3MHz. The coil is located in a coaxial set-up so that the current is central in the Rogowski loop.

The benefit of using passive Lr integration in this application is that it enables a high frequency 3dB bandwidth that is significantly higher than either active or passive RC integration for a given coil length. Using self-integration in this application minimises power loss in the passive integrator enabling the integrator to fit conveniently in the coil clip-together mechanism, ideal if the coil has to fit into a limited space which is often the case.

Passive Rogowski CoilHowever there are limitations to Passive Lr integration which make it suitable for only certain applications. If there are significant harmonics in the sinewave or the conductor is located at the edge of the coil, not centrally, the response of the transducer can be oscillatory. This is because the Rogowski coil is not terminated with its characteristic impedance and thus unwanted resonant effects in the coil will distort the measurement. Additionally, with no active integrator, Lr integration has a severely limited low frequency (-3dB) bandwidth. This limitation is not a problem in applications where sinusoidal currents of greater than several hundred kHz need to be monitored, but makes it unsuitable for wide-band applications.