SynRMs Could Change the Electric Vehicle Game
Removing the Need for Rare Earth Magnets Improves EV Sustainability
Widespread adoption of electric vehicles (EVs) could make carbon-laden tailpipe emissions a thing of the past. But producing these vehicles has meant trading one environmental problem for another. While EVs don’t produce greenhouse gasses themselves, mining and refining the raw materials needed to make their batteries and electric motors produces a significant amount. What’s more, the demand for these raw materials across industries continues to drive up the cost of manufacturing EVs, resulting in sticker prices that remain out of reach for many consumers.
In the aptly named VEHICLE project, researchers at INSA-Strasbourg and ICube Laboratory in France are trying to address these issues with a technology commonly used outside the auto industry: an electric motor called the synchronous reluctance machine, or SynRM. Traditionally used in applications ranging from robotics to wood grinders, a SynRM optimized for the electric vehicle drivetrain could mean more sustainable and affordable EVs in the future.
Currently, the EV industry’s motor of choice is the permanent magnet synchronous machine (PMSM), accounting for about 84% of the electric vehicle market. But the magnets underlying this technology rely on rare materials mined from the Earth, addressing one sustainability problem by creating another. Those materials are also in high demand for electronic products and come with a price tag that’s only increasing. There are other types of motors used in EV drivetrains, such as induction motors and rotor synchronous motors, but their efficiency and overall performance fall short of PMSMs.
Simplified electric vehicle drivetrain architecture propelled by SynRM. (Image credit: Dr. Yakoub Saadi)
Primarily funded by the European Union, the VEHICLE project seeks to bring down the cost of EV ownership by improving battery performance and energy storage on top of developing a SynRM suitable for an electric vehicle. The VEHICLE project is co-funded by the INTERREG V Upper Rhine program and by the Franco-German regional partners of the Science Offensive initiative—the Grand Est region, Baden-Württemberg, and Rhineland-Palatinate—which finance cross-border research projects of excellence.
Old Machine, New Solution
Tedjani Mesbahi, an associate professor of electrical engineering at INSA-Strasbourg, has worked on this problem for years. Mesbahi began his research on electric mobility as part of a research and innovation team at the automotive supplier Valeo and L2EP Laboratory of Lille University. There, he worked to develop a low-consumption gasoline-electric hybrid car. Later, he moved from industry projects to academia at INSA-Strasbourg and ICube Laboratory, where he joined work to optimize the EV drivetrain.
With no permanent magnets, a lower cost, and decent performance, the SynRM strikes a compromise between PMSMs and rotor synchronous or induction motors.
“When I came to INSA-Strasbourg, I found my colleagues working on a different type of electric vehicle motor, the synchronous reluctance machine,” said Mesbahi.
Though SynRMs are widely used in other industries, their drawbacks have made them a poor fit for traditional EV drivetrains. Yakoub Saadi, a former research engineer on the VEHICLE project at INSA-Strasbourg, now with SATT Ouest Valorisation, recalled some of the issues. “The shortcomings included low power density, low power factor, and relatively high torque ripple. SynRMs can also generate significant heat, demanding strategies to decrease the temperature,” said Saadi. “But they also offer advantages. They’re free of magnets, very robust, and efficient.”
A SynRM takes advantage of reluctance, the property of magnetic materials to move from areas of low to high magnetic permeability, analogous to resistance in electric circuits. In a SynRM, the stationary outer portion of the motor, the stator, houses wire coils that produce electromagnetic fields. The inner component, the rotor, contains no magnets but does contain iron, and due to the material’s low reluctance, the rotor’s air gaps align with the rotating magnetic field, spinning the rotor and generating torque.
Cross section of the synchronous reluctance machine. (Image credit: Dr. Yakoub Saadi)
The SynRM’s high torque ripple, or fluctuations in torque, results in loud noise and poses a particularly difficult problem to solve. Controlling torque ripple requires control strategies beyond the classical control algorithms.
Without the high torque ripple and resulting noise, and with a higher power density, SynRMs could be a solid substitute. With no permanent magnets, a lower cost, and decent performance, the SynRM strikes a compromise between PMSMs and rotor synchronous or induction motors. SynRMs are also simpler to produce and more efficient than other electric motors. Their weaknesses, however, have hindered their entry into the EV market.
A Rough Start
In partnership with Hochschule Karlsruhe and Hochschule Trier universities and led by Mesbahi at INSA-Strasbourg and ICube Laboratory, the VEHICLE project began in 2019. “We had to integrate the synchronous reluctance motor in the EV powertrain. This was the first challenge,” said Saadi, then a research engineer at INSA-Strasbourg and ICube Laboratory. “The second challenge was figuring out how to overcome the torque ripple, and the third challenge was implementing advanced control strategies in this electric motor.”
“MATLAB and Simulink allowed us to model this electrical machine quickly.”
Soon after it started, the VEHICLE project ran into a roadblock: COVID-19. “We had to change our methods when working remotely,” said Mesbahi. Originally, they planned to approach the project by developing their control strategies and trialing them on the testbench in tandem. But once the lockdown forced them to work from home, the team relied entirely on simulations for testing iterations of their control algorithm. MATLAB® and Simulink® were instrumental in ensuring they could still make progress.
“We chose to propose a new controller based on sliding mode control theory, or H-infinity theory. It’s a new algorithm in cascade control to reduce the torque ripple,” said Saadi. “Typically, the EV industry uses a classic control strategy, the proportional-integral (PI) controller. But classic controllers can’t significantly reduce the torque ripple.”
The control strategy to reduce motor torque ripples. (Image credit: Dr. Yakoub Saadi)
Using Simulink and its prebuilt SynRM and EV drivetrain models, the INSA-Strasbourg group got to work developing and testing advanced control strategies. Using Simscape Electrical™, they modeled the battery and converter of an electric vehicle. “MATLAB and Simulink allowed us to model this electrical machine quickly,” said Saadi.
The team validated their advanced control strategies from home, using simulation to identify one that maximally reduced torque ripple. The team also tested their proposed advanced algorithm against typical methods, such as the (PI) controller.
Back to the Lab Again
Once COVID-19 restrictions eased, Mesbahi, Saadi, and their team returned to the lab to test their controller in the prototype SynRM. They used a Speedgoat® machine, which allowed them to run their tests in real time without any cumbersome conversions to different software or languages. But it wasn’t only convenience that made them choose Speedgoat as their target machine: They also needed a powerful control port.
“Speedgoat offered a powerful port. It is compatible with MATLAB and Simulink, allowing us to test our Simulink model in real time.”
“Speedgoat offered a powerful port,” said Saadi. “It is compatible with MATLAB and Simulink, allowing us to test our Simulink model in real time.”
With just a click, Saadi could deploy the MATLAB and Simulink algorithms to the Speedgoat target machine and test them in the SynRM prototype. Initially, the results weren’t perfect, as expected for early tests. While they had tested their control strategy in simulations, the theoretical model couldn’t perfectly mimic real-world conditions. The team noticed perturbations in their results and had to adjust on a trial-and-error basis.
Finally, Saadi and his colleagues found a strategy that worked. After testing it, Saadi began to grin as he examined the measurements and calculations of the torque ripple rate. The algorithm worked, reducing the torque ripple to a manageable level. “There were a lot of tests that led up to this moment. It was the best feeling,” said Saadi, recalling that first successful test. “It encompassed all our work over the course of two years in one moment.”
Synchronous reluctance motor testbench. (Image credit: Dr. Yakoub Saadi)
The Future of SynRMs
While other research groups are also working to improve SynRMs with advanced control strategies, Saadi said what sets the VEHICLE project apart is modeling all parts of the electric vehicle drivetrain—the motor, battery components, and converter—to optimize the motor controller, a feat MATLAB, Simulink, and Speedgoat made possible. Research groups in electric vehicles typically zero in on one aspect of the vehicle rather than each piece of the drivetrain and how they work together. “The electrical motor is just one part of the VEHICLE project, which also focuses on batteries and energy management,” said Saadi.
“A machine without magnets will have an advantage in this new market.”
But the VEHICLE project’s SynRM isn’t ready for the auto industry yet. While high torque ripple was a major obstacle to SynRM use in EVs, researchers still need to increase the power density of the machines and improve heat management. “Our researchers are working to address this challenge and make synchronous reluctance machines more viable for EV applications,” said Mesbahi. The INSA-Strasbourg team is developing AI algorithms in MATLAB to create a heat management solution.
In addition to further refinement of control algorithms for SynRMs, this motor may become more appealing to the auto industry as the European Commission’s Digital Product Passports program goes into effect over the next few years. The program aims to promote sustainability and a circular economy by requiring transparency about the materials and supply chains involved in making products so that customers can get a better idea of a product’s environmental impact. “A machine without magnets will have an advantage in this new market,” said Mesbahi.
The main goal of the project, however, is to reduce the total cost of EV ownership. “We still have much more development to do,” said Mesbahi. But in creating algorithms that power a smoother and quieter ride, they’re a step closer. “In line with this, our research activities continue toward optimizing the EV total cost of ownership with a large-scale European research project boasting a budget of 5 million euros, involving over 14 European partners across seven countries.” The ENERGETIC project aims to optimize energy storage performance using smart battery technologies and connectivity, integrated with a new generation of Battery Management Systems (BMS).
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