Background
The continuous improvement and price reduction of open-loop hall-effect magnetic field sensors has allowed us at Raztec to explore these benefits. Traditional current sensors use bulky magnetic cores to concentrate the magnetic flux that surrounds a current carrying conductor. Cores also offer some shielding against stray magnetic flux which can offset the true output signal but the shielding effect does get degraded particularly in the case with higher current open-loop current transducers which require a large air gap in the core which degrades it shielding ability.
There are several other issues with cores:
- Magnetic cores are bulky. The higher the current the greater the bulk in proportion to the current that is required to be measured. As the bulk increases, so does their cost.
- There is always some magnetic loss in cores due to flux oscillations. The higher the frequency and the depth of flux change, the greater the loss. This effect is alleviated somewhat may using very special magnetic material and very thin laminations. This adds more cost and still does not eliminate the heating issue.
- All magnetic materials have hysteresis. Some more than others. Again, the more expensive materials are superior, but again not perfect. There can be some 10s of milliamp error which can be significant around zero current.
- Another issue is that these low hysteresis cores have low saturation levels so are not practical for measuring high (>~300A) currents. Low current accuracy is a useful quality in battery management. Raztec coreless sensors have effectively zero hysteresis.
- Magnetic saturation will generate non-linearities in the output vs current.
Ferrite cores are an option. They are low cost and low loss but also have low saturation so are most economical below ~200A. Metal cores are reasonably economical below ~400A unless it is necessary to measure accurately currents around zero amps.
A more economical solution to the large cores
Cores can be eliminated with the substitution of an array of magnetic field sensors surrounding the aperture. The sum of their outputs effectively rejects uniform common mode stray fields (such as the earth’s magnetic field). The challenge here is that each sensor sees a weak magnetic field from the current carrying conductor.
(A magnetic core focusses the magnetic flux onto the hall sensor) The resulting small signal can be compromised by thermal drifts and noise. This is where new generation magnetic field sensors come into their own. They have excellent thermal stability even with high sensitivity and are low noise.
By eliminating cores, we can dodge the above core issues but like most engineering, there are compromises. Metal cores do offer a degree of electrostatic shielding against high rates of change of voltage. Effective shielding (and insulation) is introduced with Raztec coreless designs with custom faraday screens. Environmental protection is introduced with suitable coatings or housings. Insulation can be tailored for the requirements of the application. It is a strength of Raztec to customise product for a particular application in
order to optimise the cost/performance ratio.
Above ~500A Raztec coreless sensors really come into their own. This is particularly the case where low weight, small size and superb linearity are important. As currents go up, the benefits become more profound.
Coreless designs lend themselves to customisation such as neatly fitting a particular bus-bar. The customisation can easily extend to splitting the structure for clipping over a conductor without compromising sensor performance.
Splitting a core of a cored sensor introduces a second air gap which makes the assembly very sensitive to stray fields. Also, inconsistencies within the gaps can lead to performance change.
We at Raztec see coreless technology as the way of the future for measuring higher currents.
But wait, there is more!
Eliminating cores opens the possibility of measuring currents with frequencies of 1MHz and more. By integrating a very specially configured Rogowski coil into the current sensor structure we can substantially extend the frequency response whilst retaining the excellent hall-effect DC and low frequency performance.
High frequency current sensors are needed when it comes to measuring power levels that contain DC plus high frequency components which occur in variable speed motor drives and certain switch-mode power supplies.
Also, very usefully, for the very fast detection of overload. Additionally, there is practically no limit to the maximum current that can be sensed. A million amps is possible!
We at Raztec, have developed a construction method that allows fast and relatively low-cost method of manufacturing special shapes and sizes of coreless current sensors. If a customer has a specific requirement, we may well be able to meet that requirement. An example follows for a current sensor for an electroplating plant.
Appropriate coating & encapsulation can be added to protect the sensors to the expected environment. We at Raztec see coreless technology as the way of the future for measuring higher currents. Contact Raztec (New Zealand) Limited for a full Product Datasheet: sales@raztec.co.nz
