Novel cooling enables a plastic electric motor

Lighter, cheaper and more efficient – this is how researchers at the Fraunhofer ICT describe an electric motor to which they have missed a new cooling concept. The modified heat conduction makes conductive metal superfluous.

This sectional view of the electric motor shows the principle: The core of the motor is a stator consisting of twelve individual teeth, which are wound upright with a flat wire. This creates space for a cooling system.Photo: Fraunhofer ICT

This sectional view of the electric motor shows the principle: The core of the motor is a stator consisting of twelve individual teeth, which are wound upright with a flat wire. This creates space for a cooling system.

Photo: Fraunhofer ICT

The acceptance of electric cars depends to a large extent on their range – and this can not only be increased by more powerful batteries: are the E-vehicles lighter, of course, less energy is needed to move them. Researchers at the Fraunhofer Institute for Chemical Technology ICT and the Karlsruhe Institute of Technology KIT have found a way to do just that. You save weight on the engine by making it from fiber-reinforced plastics. According to the team, the new technology increases the power density and efficiency of the drive at the same time.

A flat wire ensures a higher continuous output of the electric motor

Electric motors are characterized by a very high degree of efficiency of over 90% out. Most of the electrical energy is thus converted into mechanical power. The reason for the usual metal housings is the remaining 10% of electrical power. They accumulate as a loss in the form of heat and are passed over the metal to a cooling jacket with cold water. This avoids overheating of the engine. The cooperation project “Direct-cooled electric motor with integral lightweight housing (DemiL)” is based on this principle.

Robert Maertens, scientist at the Fraunhofer ICT, explains: “An electric motor consists of a rotating rotor and a fixed 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. “The scientists replaced the round wire with rectangular flat wire, which due to its shape can be wound more tightly onto the stator. In this way they gain space for a cooling channel, which they have set up next to the flat wires. So it lies inside the housing.

That means the heat loss no longer has to be conducted through the housing. The outer cooling jacket is completely dispensable in this system. “As a further consequence, the thermal inertia is lower, and in addition the motor achieves a higher continuous power,” says the researcher. In addition, by cooling the rotor, its heat loss can also be dissipated in the engine.

The electric car with plastic engine becomes lighter and cheaper

The bottom line is that the heat is dissipated where it originates and the housing no longer needs to be made of conductive metal. Instead, the scientists have designed the complete motor and the housing in plastic construction. This makes the electric car much easier. However, according to the scientists, there are other advantages associated with this method. Because the plastic makes it possible to produce complex geometries without reworking, resulting in significantly lower costs in production.

The project partners chose fiber-reinforced thermosetting plastics that are characterized by high temperature resistance and are highly resistant to wear are aggressive coolants. Unlike thermoplastics, they do not swell when they come into contact with chemicals.

Mass production of the new engines would be easily possible.

The plastic housing is manufactured in an automated injection molding process. A prototype is already finished after 4 minutes. The stators use the so-called transfer molding process. The stators are encapsulated with a thermally conductive epoxy molding compound. The scientists have designed their new system so that it could easily be mass produced. However, some questions have to be clarified beforehand.

Because the cooling concept is not working perfectly yet. So far, the scientists are only able to cool about 80% of the expected power loss. For the remaining 20%, they have to find new approaches. A possible solution could, according to the researchers, be to improve the flow of cooling water. However, they do not want to work on these last subtleties exclusively through simulations, but rather in real operation on the test bench of the Electrotechnical Institute. The rotors are being set up for this.