Researchers at Fraunhofer IFAM have developed a casting technique that can be used to produce lightweight aluminum windings with a higher groove fill factor to replace copper windings in electric motors. In one study, they found that the aluminum coils increase the continuous output of the electrical machines compared to the copper windings, reduce the operating temperature and at the same time save weight and raw material costs.

After having already successfully made aluminum coils in precision casting in recent years, it was an obvious goal for the department Foundry Technology and Lightweight Construction at the Fraunhofer Institute for Manufacturing Technology and Applied Materials IFAM also to produce the coils for series production in die casting.

Cast coils are characterized by a flat conductor arrangement, which leads to a higher slot fill factor and thus to a better utilization of the available installation space.

Die-cast aluminum coil with seven turns and a conductor height of approx. 1.5 millimeters. © Fraunhofer IFAM

Although the cast aluminum coils have a higher electrical resistance relative to the wound copper coils, the larger cross-section results in less resistance with respect to the entire coil.

Due to the better connection to the laminated core and more favorable utilization of the installation space results in a much better thermal and electromagnetic behavior. For this reason, it is possible to replace wound copper coils with cast aluminum coils for improved performance and lower material costs.

To show this in a direct comparison, the Fraunhofer team used 250W commercial pedelec motors. Rebuilt engines with different laminated cores and coil combinations were tested on a bench.

Coil arrangement within a pedelec engine with copper coils (left) and an identical motor with cast aluminum coils (right). © Fraunhofer IFAM

Among the results with the aluminum windings:

• The groove fill factor could be increased from 32 to 60%.

• At the same time, there was a weight saving of 10%.

• Torque increased by 30%.

• Due to the better thermal behavior of the coils, the continuous power at operating temperature increased by almost 20%.

The aluminum coil can deliver the resulting heat better to the laminated core and thus to the environment. This results in an improved continuous performance, since the coils only reach the permissible continuous operating temperature at higher currents.

Even more advantageous were the measurement results for a laminated core optimized on the cast coils in another modified pedelec motor. At lower weight, the torque increased by almost 80% and the continuous power by 25% compared to the original engine. Design changes can further increase the performance of aluminum coil motors.