An 800V inverter that requires 24% less pumping power

Inverters in electric vehicles (EVs)

EVs use the power of rechargeable batteries to generate energy for the propulsion motors. The batteries of an EV are DC (direct current) devices, indicating it has a positive and negative terminal, with a steady voltage associated with them. A DC motor could be connected to these batteries; however, they do not operate strictly well for speed and torque. As an EV motor requires a high level of control in all conditions, the AC (alternating current) motor is the best solution since the voltage changes cyclically.

The role of the inverters is to convert the high-voltage, high-current DC provided by the batteries into an AC for the vehicle's traction motor. As an essential part of an EV,  the inverter's performance is always a target for innovative technologies.

A key development area is 800V inverters, prepared to receive 800V from the batteries. The combination of changes in current flow directly from DC to AC and continuously fluctuating voltages generate a high heat load within the system. An efficient cooling system can avoid damage to the vehicle, the batteries, and charging stations and provide users safety.

GLM is investing in the great potential of the 800V Silicon Carbide (SiC) inverter. Using SiC permits faster, more efficient, and lightweight drivetrains and, with ColdStream, a more efficient cooling system.

Cooling system for the 800V SiC Inverter

Having used conventional design cycles up to this point, GLM decided to invest in the power of generative design for their cooling system. To achieve this, they used the Custom Design package of ColdStream. The generative design through topology optimization pushes thermal performance to the limit, resulting in a completely customized design that meets design targets and manufacturing requirements.

GLM 800V inverter cooling system showcased here is displayed in Fig. 1.

The inverter's operating conditions provide the input for the design creation in ColdStream. The heat source is the electric components that deliver a heating power per surface of 667W at the top and the bottom of the cooling channel on three surfaces, as shown in Fig. 1. The coolant is water, with a flow rate of 9.5L/min; the figure also indicates the inlet and outlet.

For this cooling system, two distinct design regions (Fig.2) are defined. In each design region, ColdStream is allowed to create the optimized topology. The definition of the design regions requires specification of the manufacturing technique, in this case CNC milling. The material of the inverter cooling system is aluminum.

Constraints are set to guarantee the best performance of the cooling system. A temperature variance constraint keeps the temperature on the heat source within a pre-defined range. The variance is chosen as 1 K^2, meaning that, in a Gaussian distribution, the temperature variation related to 99.7% probability is 6K.

A second constraint is the pressure loss. This constraint guarantees that the static pressure drop in the flow through the cooling channel will be equal to or smaller than the prescribed value, set as 4400 Pa.

Finally, the design objective is temperature minimization. This goal aims to minimize the temperature across the heat source by minimizing the mean temperature. By combining this objective with the temperature variance constraint, decreasing the mean temperatures will automatically in a reduction of the maximal temperature along the heat source.

With the case ready on ColdStream, the algorithm processes and offers the best design for the given targets, as shown in Fig. 3 for the GLM inverter. Note that top and bottom design regions have a different optimized geometry.

ColdStream design is compared with GLM's original cooling channel to check the improvement. Table 1 exhibits the maximum, mean and minimum temperature of each one of the design's heat sources. ColdStream could achieve the same temperature level with a better temperature distribution, as shown in Fig. 4 and 5. The custom design has 97.4% of the temperature spread within 6°C.

The most significant improvement of the ColdStream design is the decrease in pressure loss along the cooling channel (Fig. 6). The custom design has a pressure drop of 4.36 kPa, representing 24% less pressure than the original GLM cooling system design (5.6 kPa). The custom design reached the same performance temperature as the original one with a remarkably lower pressure drop. The lower pressure drop decreases the required pumping power without sacrificing thermal performance.

Conclusion

Taking advantage of the cloud-based platform ColdStream, GLM could generate a more efficient cooling system for their 800V SiC inverter. The result of reducing the system's pressure loss downsized the necessary pumping power to circulate the coolant by 24%. Additionally, ColdStream autonomously generated the design, requiring at most 30 minutes of engineering time to submit the case. With the custom generative design, GLM significantly reduced their engineering time compared to the traditional design methodology and eliminated the HPC-related costs for this design due to ColdStream's ability to host simulations on the cloud.