Three key questions to ask when choosing a current transducer:
- How accurate does the measurement need to be?
- What current range needs to be covered?
- What is the operating temperature range?
Higher accuracy, a wider current range and greater stability over temperature are three major factors that will increase the cost of a transducer.
Current transducers in order of decreasing performance and cost:
- High Precision e.g. ITx range
- Fluxgate e.g. LFxx10 range
- Closed-Loop e.g. LA/LT/LF05
- Open-Loop e.g. HA/HT/HX/H ranges
Accuracy and Sources of Error
The total error of a current transducer is given by the sum of the ratio error, the thermal drift, the DC offset and the linearity. All four sources of error are related to the technology. Ratio Error (aka Gain Error) can be considered to be fixed since this is an error resulting from the number of turns in the transducer. The other three sources of error can vary during measurement.
The error equation
Total Error = Ratio Error + Thermal Drift + DC Offset + Linearity
Approximate Sources Of Error By Technology
|Technology||Ratio Error (aka gain error)||Thermal Drift||DC Offset||Linearity|
Most current transducers offer reasonable accuracy at room temperature. However, the performance of different technologies vary over a temperature range. High Precision and Fluxgate current transducers deliver temperature stability which can be quoted in terms of ppm/degC. Whereas an open loop transducer will typically vary by 0.1%/degC, which over a range up to 85degC amounts to a sizeable error. If measurements are to be performed at room temperature then an open or closed loop transducer will probably be adequate. However, accurate measurements over a wide temperature range mean that a more costly Fluxgate or High Precision transducer is more appropriate.
Generally, high levels of accuracy rely on more expensive materials and manufacturing processes. It is therefore advisable to consider a transducer that delivers adequate performance over one that delivers a higher degree of accuracy if cost is an issue.
All four technologies discussed here support current transducers up to 2kA, with high precision solutions available up to 26kA. For Open-Loop, Closed-Loop and Fluxgate transducers the cost penalty for increasing current range between 1kA and 2kA is much less than the cost per amp relationship below 1kA.
At very low currents, High Precision transducers are orders of magnitude more expensive than Fluxgate, Open-Loop or Closed-Loop because of the nature of the design. High Precision is therefore an expensive solution below 1kA, above which the difference is less marked.
Unlike Open-Loop and Closed-Loop transducers, Fluxgate transducers are not based on the Hall effect. Accurate measurement of DC current is based on compensating the current linkage created by the current to be measured by creating an opposing current linkage flowing through a known number of turns. To obtain an accurate measurement, it is necessary to have a highly accurate device to measure the condition precisely.
Fluxgate detectors rely on the property of magnetic materials to exhibit a non-linear relationship between the magnetic field strength and the flux density. The detection of the zero flux condition is based on observing the variation of the magnetic field strength and the flux density. This phenomenon allows a Fluxgate transducer to be very sensitive to small values of a residual magnetic flux created by the current linkage and therefore maximise the level of the detector output signal.
Closed-Loop Hall Effect
Similar to Fluxgate transducers, the magnetic flux created by the primary current in a Closed Loop hall effect transducer is balanced by a complementary flux produced by driving a current through the secondary windings. A hall device (rather than a fluxgate) and associated electronic circuit are used to generate the secondary (compensating) current. The secondary current is therefore an image of the primary current. Closed loop transducers offer a wide frequency range, low temperature drift, good overall accuracy, a fast response time and excellent linearity.
Open-Loop Hall Effect
An Open-Loop Hall Effect transducer works because the magnetic flux created by the primary current is concentrated by a magnetic circuit into an air gap which is then measured using a Hall device. The output signal from the Hall device is then conditioned to provide an exact representation of the primary current at the output. Open-Loop Hall Effect transducers are small, light, offer low power consumption and a wide measuring range.