motor because not only are the elements very tightly spaced, leaving little room for expansion, the ceramic insulating mate- rial is unconventional.
As Schmidt explained, “Thermal stress in the slot is key to setting the maximum operating temperature. It was critical to understand the different material properties, such as their coefficient of thermal expansion and heat transfer coefficient, along with their yield and ultimate strengths. So UTRC devel- oped high-fidelity modeling tools as part of this program to understand the design space and associated challenges and some clever ways of mitigating them.” Effective heat dissipation is also critical for maximum
efficiency, since electrical resistance increases with tempera- ture. The UTRC team currently plans to cool the motor with an external cooling jacket circulating a 50/50 mix of ethylene glycol and water (EGW). This will add weight to the system, but EGW cooling loops are already used in most hybrids and electric vehicles so this setup should be relatively easy to implement. The team expects proper cooling to support sig- nificantly higher current density than the baseline traditional induction motor, allowing them to achieve the target machine power density of at least 1.3 kW/kg.
Commercially available
AM technology does not perform well with the materials needed for the motor.
Additive Hasn’t Been a Total Success So far, so good. But actually building the motor has
proven to be even more difficult than expected. First, com- mercially available AM technology does not perform well with the materials needed for the motor. Second, combining these materials in one space makes the challenge tougher still. UTRC is part of United Technologies Corp., which also owns Pratt & Whitney (East Hartford, CT). Given Pratt’s met- alworking knowledge and UTRC’s independent studies of the market, the project team understands the state-of-the-art in the additive manufacturing of metal structures. According to Schmidt, each of the currently available methods is optimized for a fairly narrow range of specific materials and very few companies are making any effort to fabricate highly conduc- tive copper using AM. Furthermore, he characterized these efforts as being in the “early stage of development.” The cur- rent equipment simply doesn’t deliver the required resolution
or structural integrity. For example, the purity and density of the copper deposited would not be sufficient for their motor application.
The team considered changing the material to ease the manufacturing challenge, but adding anything to copper to improve the fabrication process can significantly reduce its conductivity, eliminating the benefits of the new design. Other alternatives are either far inferior as a conductor (e.g., aluminum) or too expensive. Silver is a better conductor, but not enough to justify the cost. Plus it would face similar AM challenges. There are similarly few attempts to target ceramic or other suitable insulating structures and no attempts to fabricate all three materials in one device, given their differing and complex postprocessing powder of choice through one or more nozzles to a location in which a laser beam is focused; sinter the material to create a small, well-defined surface; and move to another area, all while being able to switch between materials. Instead they are cooperating with several cutting- edge development efforts to solve the AM challenge for each material separately. Steel laminates suitable for motors are currently available commercially and have not been a focus of the AM effort.
Even then, the various AM development efforts are at dif-
ferent levels of maturity, so the team has struggled with how best to fabricate the copper and insulation separately and then make them mesh. One possible workaround is to start with commercially available honeycomb structures of dielec- tric material and then fill them with the copper conductor. The coils could then be integrated with the stator steel structure and the end-turns fabricated using AM.
Stay Tuned While it appears the project will not produce working
prototypes by the end of 2015 as originally contracted, the UTRC team has already solved a number of tough challenges and they remain optimistic. Success would not only eliminate the need for rare earth metals in small electric motors, the technology should be scalable to other sizes and designs. It might even lead to entirely new motor topologies given the freedom of AM. At the same time, the ability to mix this set of materials would offer exciting new possibilities for other electronic devices. No wonder the team keeps pushing.
Acknowledgement: Much of the information, data, or work presented herein was funded in part by the Advanced Re- search Projects Agency-Energy (ARPA-E), US Department of Energy, under Award Number DE-AR0000308.
51 — Energy Manufacturing 2015
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