Making electric cars lighter also involves reducing the weight of the motor. For this reason, researchers from the Fraunhofer Institute for Chemical Technology (ICT) and partners have developed a cooling concept for electric motors that enables the use of polymers as a housing material.
Sectional view of the electric motor: The core of the motor is a stator consisting of twelve individual teeth, which are wound upright using a flat wire.
Fraunhofer ICT
As part of the DEmiL project – a German abbreviation that stands for directly-cooled electric motor with integrated lightweight housing – researchers at Fraunhofer ICT are now working with the Institute of Electrical Engineering (ETI) and the Institute of Vehicle System Technology (FAST) at Karlsruhe Institute of Technology (KIT) to develop a cooling concept for electric motors that enables the use of polymers as a housing material. The researchers are convinced that this could enhance the power density and the efficiency of the drive.
The distinguishing thing about this concept is the direct cooling of the stator and rotor. "An electric motor consists of a rotating rotor and a static stator. The stator contains the copper windings that the electricity flows through – and this is where the majority of electrical losses occur. The novel aspects of our new concept lie in the stator," says Robert Maertens, a researcher at Fraunhofer ICT.
The rectangular flat wire has replaced the round wire
To prevent the motor from overheating, the heat in the stator is currently conducted through a metal housing to a cooling sleeve filled with cold water. The teams of researchers have replaced the round wire with rectangular flat wire that can be wound more tightly onto the stator. This creates more space for the cooling channel next to the flat wire winding phases, the researchers say. "In this optimised design, the heat losses can be dissipated through the cooling channel inside the stator, eliminating the need to transport the heat through the metal housing to an exterior cooling sleeve. In fact, you no longer need a cooling sleeve at all in this concept. It offers other benefits too, including lower thermal inertia and higher continuous output from the motor," says Maertens, explaining some of 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 produce than aluminium housings. They also lend themselves to complex geometries without requiring post-processing, so we made some real savings on overall weight and cost," Maertens says. The metal currently required as a heat conductor can be replaced by polymer materials, which have a low thermal conductivity.
The project partners chose to use fibre-reinforced, thermosetting plastics that offer high temperature resistance and high resistance to the aggressive coolants. Unlike thermoplastics, they do not swell when they come into contact with chemicals.
Electric motor is suitable for large series production
The polymer housing is produced in an automated injection moulding process. The cycle time for manufacturing the prototypes is currently four minutes. The stators are overmoulded with a thermally conductive epoxy resin moulding compound in a transfer moulding process. The team of researchers has chosen a design and manufacturing process for the electric motor that will allow it to be mass-produced.
According to the researchers, stator assembly is complete and the cooling concept has been experimentally validated. "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 heat losses. And we already have some promising approaches for dealing with the remaining heat losses of just under 20 percent, for example by optimising the flow of 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 Maertens, summing up the project's current status.