New design allows smaller, lighter traction motors
High power density, small installation space within the electric vehicle and high efficiency play an important role in the success of sustainable mobility. In the DEmiL cooperation project, short for “Direct-cooled Electric Motor with Integral Lightweight housing”, researchers from Fraunhofer ICT are working together with the Electrotechnical Institute and the Institute for Vehicle Systems Technology at KIT to develop a new concept characterized by direct cooling of the stator and rotor. “An electric motor consists of a rotating rotor and a stationary stator. The stator contains wound copper wires through which current flows. This is where most of the electrical losses occur. The real innovation of our concept lies in the stator,” explains Robert Maertens, scientist at Fraunhofer ICT.
Electric motors already have a high degree of efficiency; typically, they reach more than 90 %. The remaining about 10 % of the electrical power is lost in the form of heat. To prevent the motor from overheating, the heat in the stator was previously dissipated through a metallic housing to a cooling jacket with water. The research teams are replacing the round wire with rectangular flat wire that can be wound tighter onto the stator. This creates more space for the adjacent cooling channel. This optimization allows the heat loss to be dissipated through the internal cooling channel and no longer has to be transported out through the metal housing to a cooling jacket. A cooling jacket is no longer required in this concept. Another consequence is that the motor achieves a higher continuous output and the thermal inertia is reduced. In addition, cooling of the rotor also allows its heat loss to be dissipated in the motor.
Since the heat is dissipated where it arises, the project partners can design the complete motor and the housing in plastic material and thus realize further advantages: They save weight, and plastic housings are also easier to produce than aluminum ones. Even complex geometries are possible without rework.
The project partners rely on fiber-reinforced, duromer plastics that are characterized by high temperature resistance and high resistance to aggressive coolants. Unlike thermoplastics, they do not swell when they come into contact with chemicals.
The plastic housing is manufactured using an automated injection moulding process. The prototypes are manufactured in a cycle time of four minutes. The stators themselves are encapsulated in a transfer molding process with a thermally conductive epoxy resin molding compound. The research team has designed the design and manufacturing processes of the electric motor so that it can be mass-produced.
The stator design has been completed and the cooling concept has been validated experimentally. The researchers have already been able to cool out more than 80 percent of the expected power loss. There are also approaches for the remaining almost 20 percent, for example by optimizing the cooling water flow. The rotors are currently being installed so that the engine can soon be tested on the test bench and validated in real operation.
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