Direct Cooled EV Motors with Polymer Casing

FREMONT, CA - Fraunhofer Institute of Chemical Technology collaborated with Karlsruhe Institute of Technology to work on a new cooling concept for electric vehicle (EV) motors. It will enable the utilization of polymers as housing materials for the motor, thus reducing the weight of the motor as well as the EV. Integration of the new cooling concept in EVs can also increase motor efficiency.

The primary factors that define the motor are its size, power density, and efficiency. An electric motor consists of a moving rotor and a fixed stator. Most of the losses occur in the stator, which contains copper windings to facilitate the flow of electric current. To reduce the losses, the researchers from the two institutes are developing a direct-cooled electric motor with lightweight polymer housing to enable the direct cooling of the stator and rotor.

Electric motors possess high efficiency and can convert over 90 percent of the electrical energy into mechanical energy. The other 10 percent is lost as heat. In conventional motors, the heat is conducted through a water-filled sleeve in the metal housing to prevent the stator from overheating.

The researchers replaced the round wire with a rectangular wire to reduce the space taken by the winding and create more spacious cooling passages beside the winding phases. The additional cooling passages can facilitate optimum heat transfer between the winding and the cooling medium, and increase power density and efficiency of the motor.

The cooling spaces created within the housing eliminate the need for the transfer of heat through the metal housing to the cooling sleeve. Consequently, it eliminates the need for cooling jackets. Other benefits include reduced thermal inertia and higher output. The new concept also consists of a rotor cooling system which allows heat from the rotor to dissipate within the housing. Since the heat is dissipated within the motor, the housing can be made from lightweight materials such as polymers. Housings created from polymers are easier to develop and can be molded to the desired shape without the need for post-processing.

The researchers adopted the automated injection molding process to develop housing from fiber-reinforced, thermosetting plastic technology.  Thermally conductive epoxy resin was used to mold the stators via a transfer molding process. The motor assembly was experimentally validated by the research team, making it ready for mass production.