The EV market continues to develop, not just in sales but in technology. A critical technology aspect for EVs is the thermal management of the various components to keep everything operating at the optimal temperature. Active cooling with water-glycol type coolants is the dominant thermal management strategy for the battery, but what about the other components in the drivetrain, specifically the motors and power electronics? These components are also critical to EV operation and have their own thermal requirements.
While the optimal operating temperature for the battery is similar to a human’s (~15-30°C), the motors and power electronics are higher, with operation often above 60°C. This generally means that the motors and inverter are on a separate cooling circuit to the battery, although these can interact to transfer heat between them for optimal vehicle efficiency. How the heat is dealt with within the motor varies between manufacturers, with the options generally being segmented into water-glycol and/or oil-cooled motors.
A water jacket is a commonly used method where water-glycol coolant flows in a jacket around the outside of the stator. This helps cool the copper windings in the stator that generate the electric fields used to drive the rotor. Some have adopted alternate water-cooling geometries; for example, Audi uses a water-cooled channel through the center of the rotor as well as the water jacket, enabling more effective thermal control of the rotor. The major limitation to water glycol is its electrical conductivity; this limits its use such that it cannot be used in direct contact with electrical components. This is where oil cooling comes in.
Traditional combustion vehicles are very accustomed to being lubricated with oils in the transmission. This can also be true in an EV, but the oil can also be used within the electric motor to directly cool the rotor or stator windings. This can be done in a few geometries and the water jacket may remain, but the overarching benefit is that the direct contact means that heat can be removed from the internal components of the motor more effectively, and the oil also provides lubrication. If the water jacket is eliminated, this can also lead to a smaller, and hence, more power-dense motor. In the first half of 2022, motors with oil cooling became the dominant form in the electric car market, taking 50% market share.
The downside of oil cooling is the addition of extra components and typically, the water-glycol circuit still exists to remove the heat from the oil and interact with the rest of the vehicle’s thermal system. Despite this, the performance benefits are outweighing the complexity. IDTechEx is predicting oil to gain an even greater market share with purely water jacket-cooled motors remaining in a significant way.
Directly Oil Cooling Inverters?
While oil cooling is now the dominant thermal strategy for electric motors, the inverter Si IGBTs or SiC MOSFETs are almost always cooled by water-glycol cold plates on one or both sides of the inverter modules. However, there has been some interest in directly oil cooling the inverter. As the inverter is typically packaged alongside the motor in a drive unit, one could imagine that removing the need for a water-glycol loop within the drive unit would simplify the drive system and still provide the benefits of direct oil cooling within the motor and inverter.
In fact, there has been a consortium formed investigating this strategy. The project is termed SingleOilCnL and is aimed at developing higher-density drive systems by eliminating the water-glycol system and cooling the motor and inverter directly with the lubrication oil. This project includes Dana, Diabatix, Lubrizol, Siemens, and Flanders Make. The project started in September 2020 and is set to run until February 2023.
IDTechEx thinks this is a promising approach, especially as the EV market continues to develop toward a more efficiently integrated drivetrain. While this approach has not been adopted, and IDTechEx does not expect it to become the dominant strategy in the near future, there is a promise for this approach.
