60 capacity and how long it needs to be delivered are important to specify, as they affect cooling requirements. Knowing the maximum fundamental frequency of the motor is also crucial. This depends on parameters such as maximum rpm, and the number of magnetic poles. Knowing the maximum fundamental frequency determines the control frequency needed to calculate the required duty cycles and therefore create the current waveform at that speed, he explains. Our traction inverter and battery management system (BMS) developer argues that the crucial item in an inverter or any power device that goes into a vehicle is the microcontroller because of the increasing importance of safety requirements and cyber security norms. “If the micro is not ready to implement those functions, the vehicle might be compromised at some point,” he says. The firm uses the latest-generation Aurix microcontroller, which is certified to ASIL D level under ISO 26262 regulations. “Vehicle integration engineers should understand the safety goals at vehicle level, and define the technical safety concept to achieve them. Engineers also need to understand the safety mechanisms the inverter has available, and learn from the supplier what safety strategy they use in the product. For some applications, it is not common to find ISO 26262-compliant products, and even less so to find suppliers who are open to sharing documentation with the customer to provide evidence of safety.” Considering efficiency Efficiency is also important, he adds. While virtually every inverter is said to have peak efficiencies in the high 90s in percentage terms, that peak is less relevant than the efficiency map as a whole. Even with large vehicles, the benefits of the compactness from high power density are important for vehicle integration and power management, he says. In some cases, numerous inverters for traction applications and auxiliaries may be required, garbage trucks being a good example, and finding space for as many as four or five inverters can be challenging. At the other end of the scale, he points to the debate over whether light vehicles, such as two-wheelers, should run at low voltage, defined as under 60 V, or jump to high voltage. “The higher the voltage, the lower the currents for the same power, and that will make the electrical system cheaper, more efficient and enable features such as faster charging.” The trade-off here is the cost of the isolation and connectors, etc that are required above 60 V, where the electrical safety requirements change significantly. The efficiency of the EDU is always the first consideration before the inverter, according to the global automotive technology company, whose expert says: “The best-case efficiency for the inverter will be very low switching frequencies and minimised current levels through the power stage. However, these choices can be detrimental to overall system efficiency. The largest contributor to losses in the EDU is the traction motor. Therefore, a key objective for the inverter is enabling loss reduction through motor downsizing, as well as advanced control methods to generate more useful magnetic flux.” Beyond motor considerations, inverter efficiency is increased by high-voltage architectures because lower currents reduce losses. Also, component and module integration within the inverter makes the overall package smaller and lighter, and the lowest possible resistance between the dies in the power stage and the cooling channel eases thermal management. However, there may be higher switching losses and EMI, and possibly higher resistance from highvoltage transistors, which also need more space around them for isolation. Another aspect to consider is material availability, the auto company expert argues. “With supply chains already squeezed, the fewer materials needed for manufacture, the better. By reducing the copper usage in an inverter, the engineer can make significant savings in terms of cost, weight and sustainability, while still improving power and efficiency.” There is still considerable scope for improvement, particularly in reducing switching and conduction losses, according to our advanced semiconductors and control algorithms company. “Advancements in semiconductor materials, switching techniques and control algorithms are areas where significant gains can be made,” its expert says. Efficiency maps are important in judging such improvements because electric motors usually move around the entire map during operation, and it is vital to know the efficiency at all operating points before choosing an inverter or a core switching technology, our traction inverter and BMS developer expert says. Its expert notes that state-of-the-art inverter technology can vary in efficiency from 80% in low-speed applications to 98% in high-speed, high-torque ones. A more efficient semiconductor is no more important than a good thermal design or a good control algorithm, the expert adds, noting they are twice as expensive as IGBTs in some cases and can bring electromagnetic compatibility issues. In response, the company has developed an advanced switching March/April 2024 | E-Mobility Engineering Focus | Inverters EPowerlabs’ R350 inverter has a nominal rating of 350 kW and an operational DC link voltage range of 48-800 V (Image courtesy of EPowerlabs)
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