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IEC TR 63021: Electric Vehicle Wireless Power Transfer — System Efficiency Measurement
Technical guidelines for measuring and evaluating the end-to-end efficiency of wireless EV charging systems
1. Introduction to IEC TR 63021 and WPT Efficiency Measurement
IEC TR 63021 addresses a critical gap in the electric vehicle (EV) wireless power transfer (WPT) ecosystem: how to accurately and repeatably measure system efficiency. While IEC 61980-1 defines the general requirements for EV WPT systems, and ISO 19363 specifies interoperability and safety aspects, neither standard provides a detailed methodology for measuring end-to-end efficiency from the grid connection to the vehicle battery terminals.
The efficiency of a WPT system depends on multiple interacting factors: coil alignment, air gap distance, operating frequency (typically 81–90 kHz for light-duty EVs), impedance matching network tuning, and the power level of the secondary-side rectifier and battery charger. Even a well-designed WPT system can exhibit efficiency variations of 5–10 percentage points depending on these conditions. TR 63021 provides the measurement framework to quantify these variations and enables fair comparison between different WPT implementations.
IEC TR 63021 defines efficiency at three levels: grid-to-battery (end-to-end, including all power conversion stages), coil-to-coil (magnetic link only), and power-stage-to-battery (rectifier plus DC-DC converter). Grid-to-battery is the metric that matters most for end users, as it determines the actual energy cost per kilometre.
2. Measurement Methodology and Key Challenges
2.1 Efficiency Definitions and Reference Points
The TR establishes clear measurement reference points. The primary-side measurement point (P1) is at the grid input to the WPT ground assembly (including the power factor correction stage). The secondary-side measurement point (P2) is at the output of the vehicle-side controller, after the rectifier and any on-board DC-DC conversion stage, measured at the battery terminals. The total system efficiency ηtotal = P2 / P1.
For coil-to-coil efficiency, the TR defines P1′ at the output of the primary-side resonant tank (after the inverter) and P2′ at the input of the secondary-side rectifier (before the rectifier). This allows engineers to isolate magnetic coupling losses from power electronic conversion losses.
| Efficiency Type | Measurement Points | Typical Range | Primary Loss Contributors |
|---|---|---|---|
| Grid-to-battery | P1 (grid) → P2 (battery) | 85 % – 92 % | PFC stage, inverter, coil, rectifier, DC-DC |
| Coil-to-coil | P1′ (tank out) → P2′ (rectifier in) | 93 % – 97 % | Copper losses, core losses, fringing field |
| Power-stage-to-battery | Rectifier in → battery out | 94 % – 98 % | Diode conduction, switching losses, magnetics |
2.2 Alignment and Air Gap Sensitivity
One of the most important contributions of TR 63021 is its treatment of misalignment. WPT efficiency is highly sensitive to lateral and vertical displacements between the ground pad and vehicle pad. The TR specifies that efficiency shall be measured at the nominal alignment position (typical tolerance ±10 mm) and at worst-case misalignment positions defined by the vehicle manufacturer (commonly ±75 mm lateral, ±20 mm longitudinal). The efficiency at the worst-case position must remain within 5 percentage points (absolute) of the nominal efficiency for the system to be considered “alignment robust.”
Misalignment is the single largest cause of real-world efficiency degradation in WPT systems. In practice, drivers rarely achieve perfect parking alignment. Engineers should design coil geometries with a “sweet spot” region — a flat efficiency plateau over ±50 mm lateral offset — rather than optimising solely for peak efficiency at perfect alignment. The TR explicitly recommends this design philosophy.
3. Engineering Design Insights for High-Efficiency WPT Systems
The TR provides several practical guidelines for optimising system design:
Operating frequency management. WPT systems operate in the 81–90 kHz band (aligned with the industrial, scientific, and medical (ISM) band allocation). Efficiency peaks at the resonant frequency of the coupled coil pair, but this resonant frequency shifts with alignment, ground clearance (typically 100–250 mm for passenger cars), and the reflected impedance of the battery load. The TR recommends adaptive frequency tracking — a control loop that dithers the inverter switching frequency by ±2 kHz around the nominal to maintain zero-voltage switching (ZVS) and minimise reactive power circulation.
Shielding and stray field losses. Ferrite shielding on both the ground and vehicle pads concentrates magnetic flux and improves coupling factor k (typically 0.15–0.30 for passenger car WPT). However, ferrite losses increase nonlinearly with flux density. Aluminium shielding plates on the vehicle underside prevent magnetic flux from coupling with the vehicle chassis, but eddy currents in the aluminium introduce additional losses. TR 63021 recommends that the combined stray loss (ferrite hysteresis + eddy current + chassis coupling) shall not exceed 3 % of the transferred power.
A key recommendation from TR 63021: for grid-to-battery efficiency measurements, the battery emulator used in the test setup must have an output impedance matching the actual battery impedance at the nominal state of charge (typically 50 % SoC). A mismatch of more than 10 % in impedance can skew efficiency measurements by up to 1.5 percentage points — enough to mask real design improvements or create false positives during type approval.
4. Frequently Asked Questions
Q1: What is the minimum acceptable grid-to-battery efficiency for a light-duty EV WPT system under TR 63021?
A: The TR does not set a mandatory minimum — that is left to regulatory frameworks. However, recommended practice suggests ηtotal ≥ 85 % at nominal alignment and ≥ 80 % at worst-case misalignment for a commercially viable system. State-of-the-art systems achieve 90–92 %.
A: The TR does not set a mandatory minimum — that is left to regulatory frameworks. However, recommended practice suggests ηtotal ≥ 85 % at nominal alignment and ≥ 80 % at worst-case misalignment for a commercially viable system. State-of-the-art systems achieve 90–92 %.
Q2: How does WPT efficiency compare to conductive (cable) charging?
A: Conductive charging (IEC 61851) achieves 94–97 % grid-to-battery efficiency. WPT is typically 5–10 percentage points lower due to the additional AC-DC-AC-DC conversion stages and air gap coupling losses. However, the convenience of automated charging often offsets this efficiency gap in real-world usage.
A: Conductive charging (IEC 61851) achieves 94–97 % grid-to-battery efficiency. WPT is typically 5–10 percentage points lower due to the additional AC-DC-AC-DC conversion stages and air gap coupling losses. However, the convenience of automated charging often offsets this efficiency gap in real-world usage.
Q3: Does the TR cover bi-directional (V2G) WPT efficiency measurement?
A: The current version focuses on unidirectional grid-to-vehicle (G2V) charging. Bi-directional (V2G) WPT efficiency measurement is more complex due to the bidirectional power flow through all stages and the need for grid-synchronised inverter control. The TR notes this as a topic for future revision.
A: The current version focuses on unidirectional grid-to-vehicle (G2V) charging. Bi-directional (V2G) WPT efficiency measurement is more complex due to the bidirectional power flow through all stages and the need for grid-synchronised inverter control. The TR notes this as a topic for future revision.
Q4: What instrumentation accuracy is required for valid efficiency measurements?
A: The TR specifies that power analysers used for efficiency measurement shall have accuracy better than ±0.5 % of reading for voltage and current. Phase angle accuracy between voltage and current channels must be ±0.1° or better at the operating frequency (81–90 kHz) to avoid large errors in reactive power calculation.
A: The TR specifies that power analysers used for efficiency measurement shall have accuracy better than ±0.5 % of reading for voltage and current. Phase angle accuracy between voltage and current channels must be ±0.1° or better at the operating frequency (81–90 kHz) to avoid large errors in reactive power calculation.
