Wound-Rotor Synchronous Motor (WRSM/EESM)
A wound-rotor synchronous motor (WRSM), also called an electrically excited synchronous motor (EESM) or wound-field synchronous motor (WFSM), is an AC motor whose rotor magnetic field is created by current in rotor windings rather than by permanent magnets.
What it means in an EV
The traction inverter energizes the stator windings, while a separate excitation circuit supplies direct current to the rotor winding. The resulting rotor poles follow the stator's rotating magnetic field at synchronous speed. Because the excitation current is controllable, the drive can adjust rotor flux as operating conditions change.
Unlike a Permanent-Magnet Synchronous Motor (PMSM), a WRSM does not rely on permanent rotor magnets. It also differs from an Induction Motor (Asynchronous Motor), whose rotor field is induced and normally operates with slip relative to the stator field. WRSM, WFSM and EESM describe the same broad motor family; the label alone does not specify the excitation hardware, cooling, losses or performance of a particular drive unit.
For the detailed operating principles and engineering trade-offs, see Electrically Excited Synchronous Motors.