Rare-earth magnets have long been treated as the price of admission for a compact, efficient traction motor, but that assumption is exactly what a system-level design approach challenges. The following sections trace how magnetless synchronous motors work, where their electrically excited rotors create new engineering burdens, and how those burdens are resolved not by any single fix but by treating excitation, cooling, control and manufacturing as one coordinated design.
Understanding Magnetless Synchronous Motors
Magnetless synchronous motors establish rotor flux without rare-earth magnets through two principal architectures. Synchronous-reluctance machines develop torque from rotor saliency, while electrically excited synchronous motors (EESMs) use a rotor field winding whose current sets the magnetic field directly. The latter adds an excitation-control variable, letting field strength be adjusted alongside stator current as speed and torque demand change. EESMs are already being developed for both industrial and automotive use, where rotor and stator geometry must be optimised carefully to protect efficiency once that extra control freedom is introduced [1].
That controllability has practical value outside traction applications too: an EES upgrade for industrial gas production was designed specifically to add control functionality and improve motor performance [2]. The underlying engineering question carries over to any application, industrial or automotive, that adopts this architecture: can the added excitation system deliver these control benefits without imposing unacceptable losses, complexity or thermal burden of its own?
The UESM’s Innovative Design Framework
The UESM builds on this same principle, replacing fixed magnetic flux with electrically controlled rotor excitation. Rather than accepting the field strength baked into a permanent-magnet rotor, its controller sets excitation to suit torque and speed demand, turning rotor flux into another controllable operating variable rather than a fixed property of the hardware. This follows the conventional EESM layout, pairing a three-phase stator with a DC rotor field winding [3].
That flexibility does not remove the engineering burden it creates: generating rotor field consumes power, and rotor losses are inherently harder to reject than stator losses because the rotor sits further from any stationary cooling path [3]. The design question is therefore not simply whether magnets can be removed, but whether excitation, inverter control and the motor’s physical architecture can be coordinated so that controllability doesn’t become an efficiency penalty in its own right.
Challenges Faced by Current Magnetless Designs
Magnetless operation shifts the constraint from magnet material to heat and production discipline. Reluctance and wound-field machines need higher current to produce comparable torque, which increases I²R losses in the windings, so sustaining output depends on extracting that heat without adding excessive mass, volume or parasitic power draw. EV traction requirements, namely compact packaging, high efficiency and performance across a wide speed range, make this balance especially severe [4].
Manufacturability compounds the problem. Rotor excitation introduces winding, insulation and connection requirements that must survive speed, vibration and repeated thermal cycling, while any added cooling hardware brings its own integration complexity. Axial-flux layouts can intensify the thermal challenge further, since coolant has less physical access to active parts than in radial-flux machines. Current rare-earth-free axial-flux research openly acknowledges these trade-offs between torque density and installation space, underscoring that removing magnets alone does not settle the thermal question [5].
Advancements in Magnetless Motor Solutions
Progress on this front means treating thermal capacity and control as performance variables to be engineered, not remedial additions bolted on afterward. Since magnetless machines often carry higher current to generate a given torque, I²R losses rise and continuous output becomes dependent on how effectively that heat is removed. Improved cooling can widen the operating window, but it also adds circuitry and integration complexity that must earn its keep in mass, cost and reliability.
Control has to evolve alongside the hardware for this trade-off to pay off. Reluctance torque depends on rotor position and flux paths, while wound-field designs add excitation as a further controllable variable; one recent EV study went as far as developing a dedicated brushless wound-field synchronous-motor drive system to manage exactly this interaction [6]. The task is coordinating torque, field and temperature limits in real time, rather than falling back on a conservative fixed compromise between them.
Cooled Motors’ System-Level Solution
The UESM treats rotor excitation, winding architecture, control and thermal management as one design problem, rather than asking cooling or electronics to compensate for a motor that was compromised at the outset. Electrically generated rotor field gives control over excitation, but that control consumes power in doing so; winding and control choices therefore have to limit I²R losses while still delivering the torque the drivetrain needs.
That integration also targets manufacturability directly. The UESM is designed to run on a standard three-phase inverter, avoiding specialist rotor power electronics and the platform changes they typically require. Its architecture combines a compact exciter, rotor winding and control strategy so the motor can replace a permanent-magnet unit without separate power-electronics rework downstream. [Cooled Motors Internal]
Concluding Insights on System-Level Thinking in Motor Design
A rare-earth-free motor is not defined by the absence of magnets alone, but by whether its excitation, control, windings, cooling and production method work together as one system. Treating these elements independently merely relocates the constraint: electrically generated rotor field can raise current and heat, and a cooling solution can add its own integration complexity if it isn’t designed into the machine from the outset rather than added afterward.
That is why progress has to be judged at the platform level, weighing continuous performance, manufacturability, cost and supply-chain resilience together, rather than by a single material substitution. YASA’s rare-earth-free axial-flux development illustrates that multiple rotor technology paths are being explored for differing vehicle performance, cost and manufacturing requirements, with no single approach yet settled as the industry answer [7]. The durable answer is an architecture engineered from the start to remove magnets without quietly accepting a new compromise elsewhere.
Sources
- IEEE Xplore — ieeexplore.ieee.org
- Engineer Live — engineerlive.com
- Rare-earth-free propulsion motors for electric vehicles — upcommons.upc.edu
- Electric Motor Engineering — electricmotorengineering.com
- electrive.com
- World Electric Vehicle Journal — mdpi.com
- The Engineer — theengineer.co.uk