Using 3D printing and some unique semiconductors, researchers have devised a power inverter which could make electric vehicles more attractive by making them lighter, more powerful and more efficient.

A team at the Oak Ridge National Laboratory used a wide bandgap material, made of silicon carbide, which demonstrates qualities "superior to standard semiconductor materials."

A power inverter converts direct current into the alternating current used to drive electric vehicles, and this latest inverter boasts a higher power density while also achieving a significant reduction in weight and volume.

"Wide bandgap technology enables devices to perform more efficiently at a greater range of temperatures than conventional semiconductor materials. This is especially useful in a power inverter, which is the heart of an electric vehicle," says Madhu Chinthavali, the leader of the Power Electronics and Electric Machinery Group at ORNL.

Madhu Chinthavali

It's the ability of additive manufacturing techniques to make complex geometries which led the researchers to techniques for increasing power densities in the 30-kW inverter prototype.

"With additive manufacturing, complexity is basically free, so any shape or grouping of shapes can be imagined and modeled for performance," Chinthavali said.

Specifically, the researchers optimized the critical heat sink used in the inverter to improve heat transfer by placing more efficient cool components close to the high-temperature elements of the device. The arrangement reduces electrical losses and the total volume and mass of the complete package.

The design uses a number of small capacitors connected in parallel to ensure better cooling and lower cost, and the first prototype, a liquid-cooled, all-silicon carbide traction drive inverter, is made from 50 percent 3D printed parts.

Yet another team of researchers are working on an induction motor which was specifically built to take advantage of additive manufacturing technologies as well.

The motor design task leader, Jagadeesh Tangudu, says the new methodologies made possible by AM and 3D printing free his team of constraints imposed by traditional manufacturing methods.

"A major portion of the project is devoted to finding a motor design that maximizes power efficiency under the assumption that the structures can be made with additive manufacturing. We haven't yet constrained the motor design. It could end up being quite non-traditional," Tangudu said. "But there are a lot of challenges. Traction applications, for example, need constant power over a wide range of speed. The usual approach is to use 'flux weakening' as a way of hitting high-speed operation, but induction motor characteristics don't lend themselves to this sort of field weakening."

Tangudu is working with researchers at United Technologies Corp Research Center via a program called the Advanced Research Project Agency-Energy to create the induction motor. The idea is to build a motor which doesn't require rare-earth magnets.

Working in conjunction with partners from the Connecticut Center for Additive Manufacturing, Penn State University, and an engineering consulting firm, Wayde Schmidt, the UTRC project leader on the induction motor effort, says the process itself changes the researcher's vision of what the end product will become.

"Our original proposal was to devise a single machine that made copper conductors, dielectric components, and steel laminations that were are co-located and adjacent to each other, with all having state-of-the-art properties," Schmidt said. "But there is a large technical gap to be bridged before we can fabricate parts of all three materials simultaneously on a single machine. So, we are dividing the problem into smaller chunks that can be addressed with a focused effort."

As part of the total effort, a team from Lawrence Livermore National Laboratory and researchers from Brown University are attempting to design a magnet which would both be super-strong, and reduce or eliminate the amount of rare-earth materials in favor of an "exchange-spring" set of magnets.

Making an exchange-spring magnet means using a layout in a checkerboard pattern of hard and soft magnetic material spaced at a tiny 5 to 10 nm. The necessary hard-soft magnetic materials are made by coating a hard magnet with a shell of soft magnet material. The core material must be smoothed as nanoparticles are prone to jagged edges when ground down from bulk materials. The researchers say the development of a method to coat the hard magnet cores relies on precisely controlling the thickness of the soft layer.

It's the extreme material-delivery precision of 3D printing which might allow the researchers to combine the two types of magnetic materials needed to realize the design.

"The trick is that you need to have the hard material spaced between soft materials in such a way that the separation distance between them is only on the order of a magnetic domain wall. In many materials, that turns out to be a handful of nanometers," said Dr. Scott K. McCall, a project expert. "It's a simple idea, but trying to control materials at that level is something people have been working on for 20 years. We think with some of our advanced manufacturing methods we will be able to build up exchange-spring magnets brick-by-brick."