THE TECH
Alternatives to Permanent Magnet Motors in EV traction applications
By Jeffrey Jenkins
n my very first article for Charged over 10 years ago, I opined that the Switched Reluctance Motor, or SRM, would eventually come to dominate the EV traction market, if for no other reason than the fact that it is almost as cheap as dirt to manufacture. I won’t sprain my shoulder patting myself on the back for a prediction that took 10 years to (partially) come true, but it does seem that the days of the Permanent Magnet Synchronous Motor, or PMSM, and the AC Induction Motor, or ACIM, dominating EV traction applications are coming to a close, with both Toyota and even stalwart ACIM fan Tesla introducing reluctance motors into the mix (albeit not ones that purely employ magnetic reluctance, but more on that below). Despite the fact that the PMSM and ACIM types dominate the EV traction market, neither technology
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is particularly well-suited to the job, mainly because of their limited overload and starting torque capabilities. Torque is a function of the magnetic field strength of the motor’s field structure, and that is obviously fixed in a PMSM, because its field is being supplied by literal magnets, but it is also limited in the ACIM, because its field is induced into its rotor via transformer action, and like any electromagnetic structure employing a high-permeability core material, it is subject to saturation, or the point at which a further increase in current fails to increase the magnetic field strength thus developed. Since traction applications require maximum torque at 0 rpm (to overcome both the inertia and static friction, or stiction, of the load), having a hard and fast torque limit is definitely a downside. Furthermore, the PMs invariably employed in the PMSM are rare earth types—which, as the name implies, tend to be
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