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Custom Design
PEM has been at the forefront of current measurement for over 20 years. Our engineering team understand the fundamentals of the technology and its capabilities and have published papers on the subject in leading international journals and conferences. We can offer consultation on the appropriateness of Rogowski technology in your application and offer custom designs for OEM and turnkey projects where necessary.
Our custom designs solutions can range from simple changes to the standard CWT and RCT products (e.g. longer coil lengths, modified frequency responses or adaptation to specific environments), to major new developments for industrial and automotive markets.
We can provide custom designs for one off applications or volume manufacture.
The following case studies provide examples of some of the solutions that PEM have created to meet the diverse needs of our customers:
Quantifying the harmonic content of interference currents in traction applications resulting from VSDs
A common problem in electrical engineering is the measurement of small high frequency interference currents or low strength higher order harmonics in the presence of a large fundamental current.
With traditional methods of current measurement the primary transducer, often a Current Transformer or Hall Effect device, has to be of sufficiently large rating to not saturate in the presence of the fundamental component. Thus the primary transducer is expensive, bulky and in combination with the power analyser or oscilloscope often without the resolution necessary to measure these small higher frequency currents accurately.
The Rogowski current transducer (either RCTi or CWT) offers a solution to this problem:
- the coil size can be specified independently of measured current
- the fundamental current component cannot damage the transducer and the Rogowski coil does not suffer from magnetic saturation
- it is a simple modification to tune the CWT or RCTi bandwidth to attenuate the fundamental components and enhance the sensitivity to provide a large signal to noise ratio at those frequencies of interest.
- and the RCTi has a sufficiently high (-3dB) cut-off to measure accurately into the MHz range
PEM Ltd has supplied a number of such transducers applications in the traction industry. For example to facilitate EMC assessment of a new national railway PEM has supplied a customised RCTi transducer. The railway signalling system passes coded messages between a trackside cable and the train antenna at 30-60kHz. The EMC issue concerns the traction harmonic currents flowing in the third rail and running rails which can couple inductively into the signalling cable and also cause interference at the antenna. It is necessary to provide a certain signal to noise ratio for the transmitted data. Therefore an optimised RCTi transducer which is flexible and clip-around to easily wrap around the rail track is required. The low frequency cut-off is optimised to give a cut-off of 4.2kHz with a flat response at 10kHz but -40dB of attenuation at 1kHz. Thus the RCTi can measure small interference currents in the rail of the order of a few mA’s in the range 30-60 kHz yet reject much larger components (of the order of A’s) from power frequency sources.
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.
However 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.
We have also recently completed the following projects; more detail will be available shortly:
- Monitoring stray currents in machine shafts that can lead to bearing failure.
- Permanently installed lightning measurement.
- Measuring power frequency earth leakage currents in large structures or in high power applications.
- Fast transient fault current detection.
- A wide-band low cost automotive current sensor.
- RCT1A in protection applications driving inductive loads such as relays.
If you want advice or think that your application lies outside the capabilities of our standard ranges, please Contact Us giving as much information about your requirement as possible and we will be happy to discuss your application.
