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Benchmark accuracy finds its way into new markets

14 Nov 2014  | Stephane Rollier, Horst Bezold

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Differential measurement and high-efficiency
The biggest challenge to calculating efficiency is that direct measurement of losses is not able to provide sufficient accuracy. Even the most precise power meters only offer a basic accuracy of 0.02 to 0.1% and can only measure the input and output power. The losses therefore must be calculated from these power values although the measurement errors of the input and the output may be opposite and the challenge only increases with the efficiency of the load. Typically, electric drives operate at an efficiency of around 95% whilst inverters can achieve up to 99% efficiency. Comparing an actual loss of 5 W against a worst-case power calculation error of 0.195 W, as shown in figure 3, is equal to an error of 3.9%. This means that the only way to ensure reliable measurement is with instruments and current transducers of the highest precision.

Figure 3: Deviations of input and output power measured with a 0.1% accuracy power meter.

Using current transducers to measure power
For power measurement expectations, high accuracy current transducers must be considered. Take, for example, LEM's Danfysik IT series of ULTRASTAB current transducers which are to touted to achieve much higher accuracy than direct current measurement techniques. They achieve offset and linearity in the ppm range, where 1 ppm is equal to 0.0001%. Since the offset is so small, one sensor can be used to measure current from a few Amps right up to the kiloAmps range. The transducers can also measure signals from DC up to several hundred kilohertz as bandwidth, depending on the signal amplitude. The phase error of all transducer types is well below 1' (1/60 degree). The sensor is also galvanically isolated, eliminating common-mode signals which may influence the result.

Figure 4: Even at a low range of 50 A the accuracy of a 2000 A transducer is better than 0.005% and the phase error below 0.05'.

Current transducer technologies
Current transducers are typically manufactured using one of two distinct technologies: an open-loop configuration based mainly on Hall generators, or a closed-loop configuration. LEM's IT series of current transducers uses closed-loop Fluxgate technology to achieve accuracy which is measured in parts per million (ppm) of the nominal value.

The range covers nominal currents from 12.5 A to 24 kA with an overall accuracy of a few ppm at +25°C. Thermal offset drifts are extremely low, at just 0.1 to 6.7 ppm/K. For mounting onto a PCB, IT series transducers are rated from 12.5 A to 60 A nominal, whilst panel- or rack-mounted versions are rated from 60 A to 24 kA.

Current transducers in test & measurement
In addition to being used in test labs for DC and AC calibration, ultra-high accuracy transducers are also being used across other applications. For example, the ITZ series of transducers, rated for 2 kA and 5 kA, can be used in renewable energy applications for development of wind generators and solar inverters, whilst the IT and ITN families, rated from 60 A to 1000 A, can be used in tests applications for small solar inverters, small to mid-sized motors, industrial inverters and power electronics components for automotive applications.

The wide current rating offered by the IT series and their high accuracy all along their current range mean that these transducers can eliminate the need to switch to a different transducer when measuring across a complete current range.

Figure 5: Six-channel power measurement of a KEB inverter.

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