additive manufacturing
technology into new materials and combining those materi- als side by side. The team? The United Technolo- gies Research Center (UTRC) in East Hartford, CT, working in partnership with Penn State’s Applied Research Lab (Univer- sity Park, PA), Ricardo Inc. (Van Buren Township, MI), and the Connecticut Center for Advanced Technologies (also East Hartford, CT).
Permanent Magnets Versus Bundles of Wires The newest permanent magnet motors are up to 95%
effi cient and achieve a power density of 1.5 kW/kg, mak- ing them suitable for vehicles. But their permanent magnets typically contain rare earth metals amounting to 30 g/kW. So a 30-kW motor would contain roughly a kilogram in rare earth metals, a huge amount compared to other applications such as cell phones and TVs. Conversely, induction motors require no rare earth metals, but offer a power density of only 1 kW/ kg (a third less) and about 85–90% effi ciency. The difference is in how the motors are constructed. In an induction motor, copper wires are wound around
a ferrous core in the motor’s stator to create a magnetic fi eld when electrical current is applied. What’s more, these windings must skip poles and cross over each other at the ends of the stator to generate the required magneto motive force in the air gap between the stator and rotor. That’s why the ends of a typical induction motor’s stator look like the Gordian knot. Yet all the wire going from pole to pole (called “end turns” or “end windings”) is not contributing to the mo- tor’s electromagnetic torque. Its only purpose is to maintain the continuity of the circuit. Only the wire you don’t see, the straight wire overlapping the ferrous core, is contributing to the motor’s power. The amount of wire “wasted” in end turns is determined by the bend radius of the wire, and it’s signifi - cant both in space and weight. That’s where AM comes in.
Additive Manufacturing to the Rescue? Since AM can now fabricate metallic components with
An early attempt to build copper strands onto a steel substrate using laser-based AM. The micro-structural cross section revealed some residual porosity in the build and perhaps some reaction with the underlying substrate.
almost limitless geometric fl ex- ibility, it should be possible to build circuits that overlap at the ends of the stator without arching outward and wasting space and mate- rial (i.e., without large end-turns). That alone would make a big improvement
in the power density of induction motors. Besides that, AM could also enable a higher concentration of copper within the stator coils, further boosting power density and effi ciency. The team at UTRC has designed just such a motor. Led by Wayde Schmidt, the team designed a motor that runs dozens of strands of copper through a ceramic insulator to build one “stator slot.” Each strand of copper is several inches long but only 1 mm in diameter. Since the insulator walls could be even thinner (possibly down to 0.1 mm), the ratio between bare copper and the surrounding material in a slot could theoretically approach 90%, versus the 35–40% “fi ll factor” of conventional wound coils or the 50–55% found in concentrated windings.
By modeling the interplay between key attributes, the team determined that a design with four poles and four slots per pole per phase with a 75% fi ll factor leads to the highest power density while satisfying other key performance indices. Their models also showed that increasing the fi ll factor from 75 to 90% made only a marginal improvement in the power density. Plus they needed to allocate insulation for ground insulation and phase-phase insulation. So UTRC is focusing on a design with 48 slots and a 75% fi ll factor for continued testing and development. But as the headline above hinted, even the wonders of AM are not making it easy.
Lots of Copper Isn’t Cool The project’s lead designer, Jagadeesh Tangudu, said that “while a higher copper fi ll factor improves the machine power density, it comes at the cost of higher heat dis- sipation density. This poses challenges for the machine’s thermal management and its reliability due to the operational thermomechanical stresses.” That’s especially true for this
50 — Energy Manufacturing 2015
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