Technical Measures to Fulfill Practical and CISPR 25-Compliant EMC Tests on E-Axles

Electromagnetic compatibility (EMC) testing of e-drives and e-axles at the component level per CISPR 25 requires both technical measures and carefully developed implementation strategies to ensure that measurements accurately reflect real-world performance. Given the rising demands on performance classes, particularly for high-power electric axles, these tests require a setup that closely resembles the e-axle’s actual in-vehicle installation conditions. Achieving this realistic test setup presents unique challenges, as the scale and complexity of the test object directly impact the design and setup of the test bench.

For realistic testing, it is essential to account for the mechanical and spatial demands of larger e-axle systems. This often necessitates a significantly larger test bench than those typically used for conventional automotive components. Such an expanded setup results in increased metal structures within the test environment, specifically in the anechoic chamber. This added metal can influence the electromagnetic field distribution, potentially introducing reflections and resonances that affect measurement accuracy. Thus, ensuring that metallic elements do not interfere with the EMC results is a primary concern in the test bench design.

Another critical factor is the routing of cable harnesses. In many test benches, these cables must be routed near the metallic structures of the test setup. Ideally, they should be placed at the standardized length specified by CISPR 25 on the table’s ground plane. However, this proximity to metal surfaces can lead to unintended electromagnetic coupling and alter the interference profile, creating additional challenges for measurement precision.

The long wire method has typically been used to assess the impact of these factors on the interference emission range of 150 kHz to 1 GHz. This method allows for testing multiple test setup variations, providing insights into how different configurations influence emissions. While the long wire approach can identify general trends, it may not fully replicate the actual setup of a complex e-axle system. Nevertheless, it serves as a useful basis for studying how elements such as metallic proximity, cable routing, and structure size impact emissions across various test configurations.

The findings from our investigation highlight the complexities and potential variabilities involved in EMC testing for electric drives and axles, particularly when different test benches yield slightly different results. Consequently, this study aims to initiate a broader discussion on the standardization and reproducibility of measurement results for electric drives and axles across diverse test environments. By sharing these findings, we hope to encourage further research into optimizing EMC test setups and, ultimately, to foster more standardized and comparable testing practices for e-drive systems.

Normative Basics and Crucial Aspects for EMC Testing

When performing EMC testing on automotive components according to CISPR 25, there are several crucial aspects to consider:

• Test environment: The test environment should be controlled and free from external electromagnetic interference. CISPR 25 tests are typically performed in shielded rooms or anechoic chambers to allow compliant testing and meet the ALSE verification in the defined frequency range.
• Grounding and bonding: Proper grounding and bonding are essential to ensure accurate testing results. Test setups must replicate the actual grounding conditions of the vehicle to achieve realistic results, especially for devices that are connected to the vehicle chassis or grounded to it.
• Cable and harness layout: The layout of cables and wiring harnesses used during testing should be as close as possible to the actual installation in the vehicle. This includes considerations like spacing, routing, and orientation of the cables to replicate in-vehicle conditions.
• Frequency range and limits: CISPR 25 specifies emission limits for various frequency bands (typically from 150 kHz to 2.5 GHz), which are relevant for automotive systems. It’s crucial to know the frequency range required by the component and ensure compliance with specified limits.
• Measurement equipment and probes: Use CISPR 25-compliant measurement equipment, like antennas, line impedance stabilization networks (LISNs), and RF probes, to ensure consistent and accurate results. Each piece of equipment must be calibrated to meet CISPR standards.
• Radiated and conducted emissions: CISPR 25 covers both radiated and conducted emissions. Radiated emissions testing focuses on the electromagnetic field emitted from the device, while conducted emissions testing measures noise on power or signal lines. Ensure that the test setup properly isolates and measures each type of emission as required.
• Power supply considerations: Use a stable and regulated power supply to replicate the vehicle’s electrical environment. Automotive components are often tested at different voltages (e.g., 12V, 24V) to ensure they meet CISPR 25 standards under typical operating conditions.
• Component mode of operation: Test the component in all its possible modes of operation. The device should be tested in idle, active, and any special modes to ensure it meets standards across its entire functional range.
• Compliance with test levels: CISPR 25 defines various test levels for different types of devices and applications. Selecting the correct test level is crucial based on the component’s placement in the vehicle and its susceptibility to or generation of electromagnetic interference.
• Data logging and analysis: Comprehensive logging of results, including peak, average, and quasi-peak values, is essential. Accurate data analysis will determine compliance with CISPR 25 standards and help identify any specific issues needing mitigation.

Adaptation of CISPR 25 and Possible Set-Up Variants

Meeting the previously-referenced normative basics help to ensure that automotive components meet EMC requirements per CISPR 25, improving reliability and safety in the electromagnetic environment of a vehicle.

However, considering those crucial aspects of CISPR 25, testing can present a difficult challenge, especially when accounting for the component mode of operations that are required for e-motors or e-axles, as well as its correct and impact-free implementation into an EMC environment.

Furthermore, according to CISPR 25, a setup for e-axles is normatively not defined. Unlike conventional automotive components, e-axles can combine an electric motor, inverter, and transmission, which often create complex interactions between these subsystems in terms of electromagnetic emissions. The lack of a dedicated normative setup in CISPR 25 means that engineers must pay close attention to how the test bench is designed, as the setup itself can significantly influence test results.

Influencing parameters in the design of test benches for e-axle systems include the arrangement of high-voltage cables, grounding and bonding practices, motor positioning, and cooling systems, each of which can affect EMC performance. For instance, cable layouts and connections must realistically mimic in-vehicle conditions to ensure accurate emissions measurements, as real-world installations are influenced by the vehicle’s metallic structure and shielding effects. Proper grounding and bonding of the e-axle and all associated components are also essential, as this can mitigate or amplify emissions depending on the test bench’s design.

Considering these issues, e-axle EMC tests at the component level aim to replicate installation conditions as closely as possible. Yet, they often fall short of simulating the entire electromagnetic environment of a vehicle. To address this, system-level tests based on CISPR 12 (applicable to whole vehicles) may provide more realistic insights into how an e-axle will perform once installed. CISPR 12 considers radiated emissions for the entire vehicle and can reveal potential integration issues that might not be evident in isolated component testing. This broader testing approach may also highlight interactions between the e-axle and other on-board systems, such as power electronics, battery systems, and auxiliary electronics, each of which can contribute to the vehicle’s overall electromagnetic emissions.

While conducting CISPR 12 tests at the vehicle level offers valuable insight into the real-world EMC behavior of e-axles, these tests are complex and resource-intensive. Therefore, engineers must carefully balance the thoroughness of the EMC testing process with the time and cost constraints typical of automotive development cycles.

Investigation of Test Setups and Results

In line with the requirements set forth in CISPR 25, we investigated several practical design variations to evaluate the setup, using the long wire method as a measurement basis. These variations included:

• Routing the cable harness close to metallic structures
• Using ferrites to minimize signal reflections
• Positioning the antenna near metallic structures
• Injecting a 120 dBµV signal within the 150 kHz to 1 GHz frequency range
• Measuring emissions, including consideration of the antenna factor
• Comparing the results to the CISPR 25 reference level to determine any deviations
• Ensuring compliance by achieving 90% of all measured points within a ±6 dB tolerance band

Case 1: Change in the Metal Structure Above the Table Ground Plane

Our testing verified that measurement results are negatively impacted when metal structures are placed directly on the test table, indicating that the presence of metal above the ground plane significantly affects these results. Metal structures introduce changes in the electromagnetic field distribution, which can create unwanted reflections and resonances, thereby influencing the accuracy and consistency of measurements.

Case 2: Influencing the Built-Up Metal Structure by Using Isolation Material and Ferrites

When the metal structures are positioned in isolation from the table’s ground plane, the measurement results improve notably, especially below 130 MHz, with a potential improvement of up to 6 dB. However, this adjustment leads to a distinct resonance peak at 160 MHz, which must be considered in the overall assessment. The use of ferrites positioned in front of metal structures shows minimal improvement below 30 MHz but proves effective in reducing interference between 30 MHz and 170 MHz. However, beyond 170 MHz, the emission results increase, indicating that ferrites may have limitations in high-frequency ranges.

Case 3: Comparing a Regular Test Table to a Real EMC Load Machine (EMC-BlueBox)

In our third test scenario, we performed initial measurements on a real component setup utilizing our company’s EMC-BlueBox, a specialized emission-free and mobile load machine capable of handling up to 120 kW. This setup aimed to approximate in-vehicle conditions more closely and yielded results that show comparability to those obtained with the long wire method. Importantly, the results suggest that metallic structures located below the ground plane are less impactful; however, metal above the ground plane remains a significant influence on measurement accuracy, especially in certain frequency ranges.

Requirements for a Proper Set-Up and Realistic Measurement

We conducted our investigations using the long wire method, which provides valuable insights but which cannot fully replicate the actual setup of a real component and its wiring harness. The long wire method is useful for identifying general trends but lacks the specificity needed for accurate component-level assessments.

These findings emphasize the need for additional investigations into the effects of metallic structures on EMC testing environments, particularly for e-drive and e-axle systems. It would be valuable to conduct broader studies across various EMC test benches used for e-drives and e-axles, considering that such systems often operate under high-power conditions and involve complex electromagnetic interactions. Such studies could lead to optimized test setups that better reflect real-world performance, improve measurement reliability, and support more accurate compliance assessments for automotive EMC standards.

For accurate and compliant EMC testing, certain critical factors should be considered:

• Adherence to normative standards: Ensure all relevant normative requirements are met to achieve international compliance and recognition.
• Minimization of metallic structures: Avoid large metallic structures, especially above the ground plane, as they can affect measurement accuracy.
• Optimal cable harness placement: Maximize the distance between the cable harness and metallic structures and avoid parallel routing to reduce electromagnetic interference.
• Multi-directional measurements: Conduct measurements from multiple directions around the test object to obtain a comprehensive understanding of emissions.

Setup of a Dual Load Machine within an EMC Chamber for Testing E-Axles

With reference to the investigation and with the need to test e-axles, a setup based on CISPR 25 with external load machines as a dual arrangement with 90° angle gears inside the EMC test environment is recommended. This configuration offers multiple advantages:

• Minimal metal structure: Keeping metal structures within the EMC system as compact as possible to reduce interference.
• Shielded connections: Using shielded shafts for connections between the external load machines and the anechoic chamber.
• Optimized shaft length: Keeping the shaft connection to the 90° angle gear short, while maintaining the required distances from absorbers per CISPR 25.
• Flexible test table positioning: Allowing the test table to be positioned laterally for e-axle testing or perpendicular to focus on the periphery and cable harness.
• Integrated load machines: Where possible, embedding load machines into the floor can further reduce structural interference.

Key Findings and Suggestions for an Adapted EMC Test Bench for E-Axle Testing

The findings of our investigation highlight the factors critical to developing a realistic and accurate EMC test bench for e-axle systems:

• Impact of metallic structures on cable harnesses: Metallic structures near the cable harness strongly influence emission results and can introduce errors.
• Limited effectiveness of absorptive materials: Using ferrite materials provides only partial compensation for metallic influence and is effective only up to 1 GHz.
• Criticality of structures above the ground plane: Metallic structures above the ground plane are particularly influential, creating resonance points that significantly alter results.
• Reproducibility challenges: Variations in size and placement of metallic structures above the ground plane can lead to major, often non-reproducible changes in results, highlighting the need for further investigation.

These insights are essential for refining EMC test setups, particularly for e-axle testing, to achieve reliable and repeatable measurements that accurately reflect real-world conditions. However, further investigations are required.

Although metallic structures are necessary in an EMC test site for setting up effective e-axle test benches, a CISPR 25-compliant environment can still be achieved by closely adhering to the considerations highlighted in this study. By strategically managing these metallic elements, the impact on electromagnetic measurements can be minimized. Test setups with high-performance capabilities have already been implemented in alignment with this approach. For instance, setups using dual 250 kW load machines, running at 3,000 RPM and capable of producing 3,000 Nm, reflect the high-performance requirements of electric axles designed by leading automotive manufacturers.

However, there is a need for further investigation and standardization. We advocate for updates to the CISPR 25 standard to address the unique challenges of testing electric axles, ensuring that consistent and reproducible testing methodologies are available to all OEMs and service providers globally. Establishing a unified approach to test procedures, equipment, and environmental conditions would improve test reliability and comparability, creating a standard that supports the rapid evolution of electric mobility.

Several additional factors should be considered for adapting CISPR 25 to better support e-axle testing. For example, the specific positioning of antennas is critical for accurately capturing emissions, while simulating dynamic driving conditions could provide more realistic insights into how these systems will behave under real-world operating conditions. Increased automation in test setups, both in terms of measurement and equipment handling, could improve efficiency and reproducibility.

Integrating e-axle test benches into an EMC testing environment requires detailed preparation of the anechoic chamber and careful selection of the medium used to simulate passive or active load on the test object. Depending on the setup, this could involve electrical, hydraulic, or pneumatic loading methods. Additionally, the spatial needs of the test setup, both within and outside of the EMC testing environment, should be considered, as e-axle testing equipment may require substantial space due to load machines, auxiliary systems, and ventilation requirements.

Conclusion

Achieving a CISPR 25-compliant environment for e-axle testing is feasible with careful design and planning. But a revision of the standard to accommodate these unique needs is essential to ensure repeatable, accurate, and internationally comparable EMC test results for electric axles.

Our investigation is based on the needs of OEMs or service providers who specialize in e-motor and e-axle EMC tests. We, in partnership with other industry colleagues, advocate for finding a practical investigation of a solution for testing electric axles, which can be implemented with various suppliers of load machines. The focus of the solution is on the correct implementation of the knowledge gained within an EMC test environment and on the reproducibility of EMC tests. The renderings presented in this article offer different expansion options. But, most importantly, they represent a realistic technical conception and offer practical implementation strategies.