Wheel-Hub-Motor Innovations in Flux

March 1, 2022

John Kendall

The packaging and design of electric motors has, not surprisingly, come under scrutiny in recent years as the pace of vehicle electrification has accelerated. Wheel hub motors, once dismissed as too costly and dynamically problematic for most automotive applications, are increasingly back in focus for EV use. Vehicle developers recognize that hub motors offer advantages – particularly in all-wheel-drive applications. At the same time, hub motors tend to favor the packaging advantages offered by axial-flux motor designs.

The compact dimensions of these motors would be far easier to package in a hub-mounted application. In addition, the more powerful axial-flux designs are larger in diameter than comparable radial-flux motors, offering a packaging advantage in line with wheel dimensions: Small-wheeled vehicles such as scooters need smaller, less-powerful motors, while the larger wheel diameters of trucks and buses parallels the larger diameter and more powerful motors needed for these vehicles.

U.K.-based axial-flux motor designer and producer Saietta recently bought the Dutch supplier of electric drivetrains and high-voltage power electronics, e-Traction, from the Chinese Evergrande New Energy Vehicle Group. “I was in Slovenia and suddenly a lot of Dutch friends started texting, “Electric motor company about to go bankrupt in Holland,” noted Saietta CEO Wicher (Vic) Kist. This was shortly after Saietta had been listed on the London Stock Exchange, following earlier investment from the U.K. Enterprise Investment Scheme (EIS).

“I thought, ‘This is an opportunity,’” Kist told SAE Media. He immediately returned to the U.K. and watched e-Traction’s asking price tumble over the coming days. Further investigation showed that e-Traction provided some complementary technologies that Saietta needed. “I flew the entire M&A [mergers and acquisitions] team over that delivered the Saietta IPO and four weeks later, we signed”, Kist said. “What I found is it’s almost like the brother we didn’t realize we had. Fifty-five people, very similar to the U.K. team and power electronics, electromagnetics and especially on the power electronics side, an inverter.”

Saietta had developed an axial-flux technology (AFT) motor in response to British rival axial-flux motor designer YASA, which was acquired by Mercedes-Benz in July 2021, two weeks after Saietta was given its London Stock Exchange listing. Since e-Traction produced an 800-V inverter, it neatly fulfilled Saietta’s need for one. The acquisition also added e-Traction’s e-axle technology to the Saietta portfolio, which will help the company to realize its desire to produce electric drive systems for trucks and buses.

The e-axle designed by e-Traction has the inverter packaged within the drive motors. Kist described it as “a fairly standard outrunner motor” but with direct drive, similar to Saietta’s AFT philosophy. He noted that the company will have multiple solutions for powering buses and trucks up to 55-tonne (60.5-ton) trucks “especially with the hydrogen charging infrastructure coming,” he added.

Inside the motor

The first prototypes of the e-Traction-designed e-axle have been built and tested. Kist noted the technology can be implemented in a scalable fashion. “You can even take the motors back out and replace them with our AFT axial-flux technology and bring it back into the hub,” he suggested.

The principal components of the Saietta AFT motor include a pair of cold-formed, copper hairpin-shaped coils. “That pair is a discrete coil, which you can hold in an assembly line so you can make the entire stator fully automated,” Kist explained. The stator ring is extruded and the lamination packs are of grain-oriented steel. The rotor is formed from mild steel and the motor is surrounded by a small cooling jacket. “Torque is r times force and basically because our effective diameter is bigger (compared with a radial-flux motor), we can achieve a higher torque for the same current,” he said.

Acquiring the e-axle technology has accelerated Saietta’s plans for powering heavy vehicles. He had been planning to work on similar technology with the aim of introducing the company’s own power electronics in three years. Acquiring e-Traction has provided him with the systems and components that could shorten that development time. The e-axle also could be used to re-power existing diesel-powered buses, using the space vacated by the diesel engine to house batteries.

Outboard marine drive

Saietta launched the latest application for its AFT at the Marine Equipment Trade Show (METS) in Amsterdam in November 2021. The company launched a new division devoted to marine propulsion with two new designs under the brand name Propel. The first is an outboard motor and the second a replacement for light marine diesel engines used in leisure craft and work boats, both powered by Saietta AFT motors.

The D1 inboard engine range uses low-voltage motors and does not need a gearbox. Included in the package are a sunlight-readable 5-inch (12.7-cm) display and a side-mounted “throttle” lever, as well as a DC inverter to power onboard 12-V devices. For the Propel S1 outboard motor, Saietta has drawn on existing designs for the shaft leg and also designed and produced a closed-loop cooling system filled with glycol, so the motor could be left in place through the winter. It also removes the need for a cooling-water intake below the waterline.

The motor and control electronics all are housed above the waterline. Saietta claims a 10-kW continuous power output from a 48-V battery. Using Saietta’s own four-bladed propeller, this should produce 3.75 kW of propulsive power. “You can optimize your propeller because you don’t need an exhaust in the middle of that prop, so that helps a lot,” Kist said. “Then, it’s just a shaft that goes up with an internal spline in our motor, with integrated inverter and it just goes on top.”