Making electric cars lighter also involves reducing the weight of the motor. One way to do that is by constructing it from fiber-reinforced polymer materials. A new cooling concept was developed that will enable polymers to be used as motor housing materials, increasing the power density and efficiency of the motor compared to the state-of-the-art.

The two key components of an electric drivetrain are the electric motor and the battery. Three issues play important roles when it comes to using an electric motor for eco-friendly mobility: high power density, a compact configuration that fits snugly within the electric vehicle, and high levels of efficiency.

Sectional view of the electric motor. The core of the motor is a stator consisting of 12 individual teeth, which are wound upright using a flat wire. (© Fraunhofer ICT)
Coolant circuit in the stator. (© Fraunhofer ICT]

The new approach incorporates direct cooling of the stator and rotor.

Electric motors have efficiency of over 90 percent, which means a high proportion of the electrical energy is converted into mechanical energy. The remaining 10 percent of the electrical energy is lost in the form of heat. 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. In the new approach, the round wire was replaced with rectangular flat wire that can be wound more tightly in the stator. This creates more space for the cooling channel next to the flat wire winding phases. In this optimized 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. It offers other benefits including lower thermal inertia and higher continuous output from the motor. 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 researchers were able to construct the entire motor and housing from polymer materials. Polymer housings are lightweight and easier to produce than aluminum housings and lend themselves to complex geometries without requiring post-processing. The metal currently required as a heat conductor can be replaced by polymer materials, which have a low thermal conductivity compared to metals. Fiber-reinforced thermosetting plastics were chosen that offer high temperature resistance and high resistance to aggressive coolants. Unlike thermoplastics, ther-mosets do not swell when they come into contact with chemicals.

The polymer housing is produced in an automated injection molding process using the phenolic molding compound Vyncolit X7700. The cycle time for manufacturing the prototypes is currently four minutes. The stators are overmolded with a thermally conductive epoxy resin molding compound (Sumikon EME-A730E) in a transfer molding process.

An electrical current introduced the amount of heat in the copper windings that would be generated in real operation according to the simulation. More than 80 percent of the expected heat loss was dissipated.

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