Benchmark accuracy moves into new markets
Precision current monitoring
Power electronics has a crucial role to play in increasing the efficiency of products as diverse as hybrid and electric vehicles, wind turbines, solar panels, industrial inverters and high-performance motors. Each of these products must be optimised according to their power losses and precise current measurement is the basis for analyzing and optimising these losses.
Power Measurement System
Active electrical power is defined by integrating the multiplication of the voltage and current over one signal period as shown in the following formula:
The accuracy of the power measurement is dependent on precise synchronisation on the fundamental signal period, in addition to the amplitude error and phase error. The amplitude error determines the precision with which the voltage, u (t), and current, i (t), are measured, whilst the phase error determines the amount of time, or phase shift, between the sampling of the voltage, u (t), and the current, i (t).
Fig. 1: The power signal is calculated from u (t) and i (t)
(Power = blue. Voltage = yellow. Current = green)
Voltages up to 1000V can be measured directly with a power meter, whereas accurate measurement of current signals above a few amps requires current transducers of the highest precision.
The importance of phase error increases as the power factor decreases, as shown in Figure 2. At power factor 1, there is no phase shift between voltage and current. Even an additional phase shift of 1°, caused by a current transducer, would only result in a small power error of 0.2%. At power factor 0.1, the phase shift between voltage and current is already 84° and an additional transducer phase error of 1° would lead to a huge power error of 17.4%.
Fig. 2: Influence of power factor
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.
Fig. 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 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.
Fig.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, such as LEM’s IT series, 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.
Fig. 5: Six-channel power measurement of a KEB inverter
Often, power needs to be measured across three, four or even six channels when testing a multi-channel product such as an inverter for example. For these multi-channel tests, LEM’s multi-channel measurement system includes a power supply and transducer connection cables to enable a complete measurement system to be setup within minutes.
Precision motion control
The semiconductor manufacturing process is another application in which current transducers with ultra-high accuracy can be used.
The manufacturing process uses complex photolithographic techniques to image and create the nanoscale structures that form the integrated circuit components used in a chip. The use of a very short wavelength is crucial because the resolution of the process is directly proportional to the wavelength.
Fig. 6: A photolithographic scanning stepper
During the photolithographic process, a scanning stepper moves the silicon wafer through a series of positions so that an identical die or circuit can be etched onto each wafer. Each time an individual die is illuminated, the mask, wafer and light-source must be stationary in relation to each other.
To achieve the nanometer-scale geometries that produce the highest integration of components onto a die, and dice onto a wafer, precise positioning and motion control are essential. Positioning is split over two stages: stepping positioning in which the wafer is moved to a specific die position, and scanning positioning, which controls the movement of the wafer and the photo-mask.
The motion of the scanning stepper is controlled by measuring the drive current in the coil of the linear actuator. As near-perfect synchronisation between the two movements is imperative, a high-precision current measurement with extremely high differential linearity is essential.
An ultra-high precision DC current transducer, such as LEM’s PCB-mounted ITN 12-P, offers the precision and differential linearity that are essential for this type of application. The only valid alternative with the same level of linearity is a simple shunt resistor. However, as the drive currents typically range from 5 to15A, power loss and temperature-induced drift can both affect the accuracy of the shunt resistor. The output from a shunt resistor also has some common-mode signals which are not present with a DC current transducer in which the primary and secondary signals are galvanically isolated. An ultra-high precision closed-loop DC current transducer would therefore be the preferred choice for this type of application.
Current transducers with the highest levels of accuracy, drift and response times were once used mainly by test laboratories. Now, with energy efficiency becoming a prime objective in virtually every market, the latest transducer technologies are extending the use of ultra-high precision transducers beyond high-end test and measurement applications and in the industrial and medical markets.
Fig. 7: LEM Danfysik current transducers