Coil loss measurement. Accurate. In seconds.
WPT – Electric cars will soon be charged wirelessly
Increasingly higher switching frequencies are a general trend in many areas of the electrical industry. Especially in electromobility. One example is the development of SiC-based inverters. Another is research in the field of wireless power transmission (WPT). In the future, the batteries of electric vehicles will be charged just like an electric toothbrush or a cell phone, conveniently via charging coils installed in parking lots or even in the streets. Such wireless charging technologies will further increase the convenience and acceptance of electric cars. This can be made possible by inductive charging technologies based on magnetic resonance or magnetic induction. The development efforts of more efficient wireless charging systems are running at full speed at car and EV charger manufacturers. One crucial area to achieve higher efficiency is to analyze and reduce the losses in the coils of these WPT systems.
Coil loss measurement in seconds
To date, the focus of development engineers in the design of WPT has been on minimizing the switching and conduction losses of the semiconductors in the system. To increase system efficiency even further, it is also important to use the optimal coils in the system. It is therefore important to analyze the coil losses under operating conditions. A commonly used method to measure coil losses is the Calorimeter. This method is accurate, but it has one major disadvantage: the test can take up to half an hour.
Alternatively, a Power Analyzer can be used to determine coil losses in just seconds. But this is easier said than done, as it is a challenging measurement. Below is the basic electrical diagram of an EV WPT system shown with the system coils circled in red (figure 3). Using the PW8001 Power Analyzer combined with HIOKI current sensors and voltage dividers, overall system efficiency and coil losses can be accurately determined thanks to several unique features.
The Two-Coil loss measurement with a power analyzer
To determine the loss of a coil the so called “Two Coil Measurement” is used. With this measurement the total loss of the coil and the core loss can be determined by measuring the current flowing through the coil and the voltage over both the primary and the secondary coil. The measured values for voltages, current and phase angle etc., are the basis for the loss calculation. The coil loss calculation is done using the standard UDFs (User Defined Calculation Functions) inside the PW8001, based on which the total loss and core loss can be calculated. The copper loss is then easily determined by subtracting the coil loss from the total loss.
Accurate coil loss measurement
It is quite a challenge to accurately measure the loss of a coil accurately. To ensure useful results, it is recommended to measure the losses in actual working conditions, so for WPT this means at currents up to 500 A and a voltage at the receiving coil of 3 kV. The use of a high voltage divider is necessary to handle these high voltages. The power factor of the coils is extremely low. Due to this characteristic the influence of the phase error on the measurement result is extremely high, as is shown in figure 5 below.
Beyond 10 kHz, conventional power analyzers measure inaccurately
The test results of traditional power analyzers with an internal shunt resistor, become unreliable for coil loss measurement beyond the 10 kHz threshold. Phase errors have a minimal impact up to switching frequencies of approximately 10 kHz. However, beyond this threshold, many power analyzers yield inaccurate loss results due to inaccurate determination of the phase angle between the voltage and the current. For higher current measurements, they also utilize third-party current sensors that were never specifically designed for coil loss measurement, adding to the poor accuracy and repeatability.
Precise measurements of parameters such as voltage, current, power factor and harmonic distortion, enable engineers to better understand the performance of transmitting and receiving coils in a WPT system to ensure efficient and reliable energy transfer. Accurate, speedy assessment of input and output power as well as losses during contactless transmission increases the system performance and speeds up the development process.
Genaue, sekundenschnelle Leistungsanalyse ist der Schlüssel
Hier sind genaue Messungen mit Leistungsanalysatoren der Schlüssel essenziell. Diese messen Parameter wie Spannung, Strom, Leistungsfaktor und Oberwellenverzerrung. Mit diesen Daten entwickeln Ingenieure sowohl die Effizienz als auch die Zuverlässigkeit der Energieübertragung zwischen den Sende- und Empfangsspulen ständig weiter. Die genaue, schnelle und richtige Bewertung der Verluste bei der Eingangs- und Ausgangsleistung der kontaktlosen Übertragung ermöglicht eine höhere Systemleistung und beschleunigt maßgeblich den Entwicklungsprozess.
Bisher lag der Schwerpunkt der Entwicklungsingenieure auf der Reduzierung von Schalt- und Leitungsverlusten der Halbleiter. Um die Systemeffizienz weiter zu steigern, konzentrieren sie sich nun verstärkt auf die Spulen im WPT-System und analysieren deren Verluste unter Betriebsbedingungen. Dafür werden landläufig Kalorimeter verwendet. Diese Methode ist sehr genau, hat aber einen großen Nachteil: Der Test dauert bis zu 30 Minuten. Ein Leistungsanalysator dagegen bestimmt alle Parameter in Sekundenschnelle.
Mit dem Leistungsanalysator PW8001 in Kombination mit den HIOKI-Stromsensoren und Spannungsteilern können der Gesamtsystemwirkungsgrad und die Spulenverluste dank einzigartiger Funktionen genau bestimmt werden. Nachfolgend ist das elektrische Diagramm eines WPT-Systems für elektrische Autos dargestellt. Die Systemspulen sind rot eingekreist (Bild 5).
Präzise Ergebnisse auch bei hohen Frequenzen
Die Messergebnisse herkömmlicher Leistungsanalysatoren mit internem Shunt-Widerstand sind bei der Messung von Spulenverlusten ab 10 kHz extrem unzuverlässig. Unterhalb dieser Marke wirken sich Phasenfehler nur minimal aus. Überschreiten die Frequenzen jedoch diese Schwelle, liefern übliche Leistungsanalysatoren ungenaue Werte, da der Phasenwinkel zwischen Spannung und Strom ungenau bestimmt wird. Für Messungen höherer Ströme werden stets Stromsensoren von Drittanbietern verwendet. Diese wurden ursprünglich nicht für die Messung von Spulenverlusten konzipiert. Dies beeinträchtigt die Genauigkeit und Reproduzierbarkeit bei der Spulenverlustmessung.
Um diese Herausforderung bei hohen Frequenzen zu meistern, kompensiert der Leistungsanalysator PW8001 den bekannten Phasenfehler von HIOKI Stromsensoren und des Spannungsteilers VT1005 (Bild 6). Das geschieht mit dem PW8001 und den abgestimmten Stromsensoren von HIOKI sogar automatisch mit der „Automatischen Phasenverschiebungskorrektur“.
The Automatic Phase Shift Correction minimizes phase errors
To correct measurement errors in the phase difference at high switching frequencies, HIOKI has developed a particularly effective phase shift correction for the PW8001 power analyzer. Two conditions need to be met for this to function reliably:
- a power analyzer that performs phase correction correctly,
- a zero-flux current sensor with a known time delay.
To overcome this challenge, the PW8001 compensates for the phase error of the current sensors and voltage dividers. For the current sensors the phase error is compensated even automatically by the PW8001, we call this unique feature Automatic Phase Shift Correction.
Important to know: A delay of 100 ns at 100 Hz does not have the same effect as a delay of 100 ns at 1 MHz. This becomes clear when the time delay is converted into the phase delay (expressed in degrees) as above.
The ideal route to maximum precision
HIOKI has developed its zero-flux current sensors together with its Power Analyzers for a good reason: It’s the solution to precision and reliability. In order to efficiently compensate for phase delay, the time delay of the current sensor must remain constant regardless of the frequency. In addition, the most important key figures of the current sensors are automatically transmitted to the Power Analyzer as soon as current sensor is connected and the phase error of the current sensor is automatically compensated. And there you have it: Coil loss measurement. Accurate. In seconds.
