The electric motor and the battery are the key elements of the electric drive train. High power density, small installation space inside the electric vehicle and high efficiency are key features to ensure sustainable mobility. With electric motors, about 10 percent of the electrical power is lost in the form of heat. To prevent the motor from overheating, in the past the heat in the stator was dissipated through a metallic housing to a cooling sleeve with chilled water.
In collaborative project DEmiL – a German abbreviation that stands for »directly-cooled electric motor with integrated lightweight housing« –, researchers from the Fraunhofer Institute for Chemical Technology ICT, together with colleagues from the Electrotechnical Institute and the Institute of Vehicle Systems Technology of the Karlsruhe Institute of Technology (KIT), have developed a new concept in which stator and rotor are directly cooled. To do so, they have replaced the usually used circular wire with a rectangular flat wire. Such a wire can be wound tighter onto the stator. This creates more space for the adjacent cooling channel next to the flat wires.
»The thermal losses can therefore be dissipated through the internal cooling channel and do not have to dissipate through the metal housing to an external cooling sleeve. Our concept therefore does not require such a cooling sleeve. As a further consequence, the thermal inertia of the motor decreases, and in addition it achieves a higher continuous output from the motor«, explains Robert Märtens, researcher at the Fraunhofer ICT, the advantages of the new system. In addition, the new design incorporates a rotor cooling solution that also allows the rotor’s heat loss to be dissipated directly within the motor.
By dissipating the heat close to where it is generated, the project partners were able to construct the entire motor and housing from polymer materials, leading to further advantages. »Polymer housings are lightweight and easier to manufacture as aluminum housings. Even complex geometries are possible without post-processing, so that we can save some overall weight and cost«, Märtens says. The metal previously used as a heat conductor can now be replaced by polymer materials – a poor heat conductor.
The project partners chose to use fiber-reinforced, thermosetting plastics that offer high temperature resistance and high resistance to aggressive coolants. Unlike thermoplastics, thermosets do not swell when they come into contact with chemicals.
Suitable for Mass Production
The polymer housing is produced in an automated injection molding process. The cycle time for manufacturing the prototypes is currently four minutes. The stators themselves are overmolded with a thermally conductive epoxy resin molding compound in a transfer molding process. The team of researchers chose a design and manufacturing process for the electric motor that will allow it to be mass-produced.
The team has already completed the stator assembly and experimentally validated the cooling concept. »We used an electrical current to introduce the amount of heat in the copper windings that would be generated in real operation according to the simulation. We found that we can already dissipate over 80 percent of the expected thermal losses. And we already have some promising approaches for dealing with the remaining losses of just below 20 percent, for example by optimizing the flow of the coolant. We are now at the stage of assembling the rotors and will soon be able to operate the motor on the test bench at the Institute of Electrical Engineering and validate it in real operation«, says Märtens, summing up the project’s current status.