High-precision transducers are becoming more in demand in high-performance industrial applications, specifically for medical equipment, precision motor controllers, metering, measurement accessories and test equipment.
Most of these applications use high-precision transducers but with a limited temperature range of 10 to 50°C, whereas many new applications require a wider temperature range while still remaining highly accurate. A good example of this would be for automotive tests benches.
It was a challenge for LEM to build a new high-performance family, the Ultrastab IT xx5 series, for a wider temperature range, where performance and reliability are guaranteed.
The accuracy of the measurement will not only depend on the accuracy of the measuring resistor but also on the sensitivity of the flux detector. However, in spite of the DC measurement function accuracy, there are some drawbacks to this DC measurement system (see Fig. 1).
Fig. 1: Ultrastab IT xx5 series technology.
As the winding D of the flux detector is coupled with the compensation winding S, the applied square wave voltage is re-injected into the compensation winding and creates a parasitic current in the measurement resistor. However, the square wave voltage induced in the S winding by this flux may be practically cancelled out when a second D’ winding (identical to D) is mounted on a second detector core inside the compensation winding S. The residual flux (the sum of the opposed fluxes in D and D’) will create very small voltage peaks that cause the remaining signal correlated with the Fluxgate excitation.
The magnetic part of the transducer is represented schematically by three cores. A fourth winding W is wound in between the compensation winding S on the main core to extend the frequency range of the transformer effect to lower frequencies. It is connected to an integrator which modifies the output current via the power amplifier to compensate for the too-small induced voltage in a frequency range too high for the Fluxgate detector.
Fig. 2: Electrical offset drift from -40 to 85°C – IT 605-S model measured in ppm of IPN.
The Ultrastab IT xx5 series has been characterised over its new operating temperature range of -40 to 85°C, producing the results shown in Figs. 2 and 3.
These show a very low drift of offset and almost no drift of linearity over the whole temperature range, resulting in a global accuracy lower than 30 ppm from -40 to 85°C ambient temperature (see Fig. 4).
Fig. 3: Electrical linearity error from -40 to 85°C(IT 605-S model) measured in ppm of IPN.
All these performances have been measured during type tests with high enough quantities for ±3σ value calculations to guarantee the values given in the datasheets.
Thanks to the use of an iron-nickel magnetic core and perfect homogeneity of the winding process, accuracy in AC is very good. Fig. 5 gives the IT 605-S overall accuracy in AC at room temperature.
Fig. 4: Overall accuracy from -40 to 85°C(IT 605-S model) in ppm of IPN.
This material and dedicated winding process are helpful to reach a large bandwidth and a very low phase shift, as shown in Fig. 6.
These frequency performances can be reached through a combination of three different techniques (see Fig. 8):
Fluxgate for DC up to low frequencies (a few Hz).
Pick-up coil, which works like a Rogowsky coil, starting at very low frequencies and compensating for the small error created by the current transformer.
The current transformer working up to few hundred kHz.
Fig. 5: Overall accuracy in AC (IT 605-S model) in ppm of IPN at room temperature.
Test and measurement market
Test and measurement equipment is an applications requiring a wider operating temperature range. When qualifying the efficiency of power electronics-based equipment such as inverters for hybrid and electric vehicles, wind turbines or solar systems or industrial inverters and motors, this not only provides the efficiency at ambient temperature – which can be proven by the test benches – but also efficiency throughout the operating temperature range in real use cases.
To achieve the best efficiency, the design of the power electronics-based equipment must be such that all the components used are optimised according to their losses. Efficiency measurements for power electronics and drives components need a highly-accurate power measurement system.
The new current transducers can ensure the same functionality but over the wider operating range of -40 to 85°C. Active power is calculated from measured current and voltage values. The accuracy of the power value depends mainly on two parameters:
Fig. 6: Amplitude and phase responses (left) versus frequency (right).
The accuracy of the measured current and voltage (amplitude error).
The phase error coming from the phase shift between the voltage and the current.
For current measurement above a few amp, high accuracy current transducers are needed as interfaces connected to the power analyser. Phase error (phase shift) is a factor that cannot be ignored in this application. Indeed, the influence of a phase error increases as the power factor decreases.
At power factor 1,0, there is no phase shift between current and voltage (power factor = cos σ where I and U are sinusoidal functions over time); σ is the phase shift between I and U. A phase shift of only 1° would result in a power factor of 0,9998 at a small power error of only 0,2%. At power factor 0,1, the phase shift between voltage and current is already of 84°. An additional phase error caused by an instrument or a transducer of 1° would lead to a huge power error of 17,4%.
Fig. 7: Three techniques for a large bandwidth and a low-phase shift.
This explains the need for high-accuracy, low phase-shift current measurement tools.
Meanwhile, power meters must be highly accurate to measure power at the input and output of the equipment under test as it is not possible to measure losses directly. The losses are then calculated from both values. In the worst case, the errors of both measurements are opposite. This problem increases with the efficiency of the load. Electrical drives have an efficiency of around 95%, and inverters up to 99%. Only high-precision instruments and current transducers coupled with power analysers can deliver reliable and acceptable results.
Conclusion
Optimum current transducers for measuring power over a wide operating temperature range of -40 to 85˚C combine all the requirements for power measurement current transducers. Offset and linearity over the temperature range are in the 36 to 400 ppm range for offset in temperature and from 8 to 12 ppm for linearity in temperature; the values depend on the model used.
An accuracy of 1 ppm is equivalent to 0,0001%. Since the offset is so small, the transducers can be used from a few amps and just one model can cover the entire required current measurement range. Other transducers using different technologies would require the use of several transducers to cover the same range to keep this accuracy level all across the range. This brings a non-negligible cost advantage.
Contact Mervyn Stocks, Denver Technical Products, Tel 011 626-2023, denvertech@pixie.co.za
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