Brushless Excitation Solutions for Wound-Field Synchronous Motors in Electric Vehicles: a Survey

Electric vehicles can produce far less pollution than conventional fuel-based vehicles. Nevertheless, their manufacturing still involves issues about sustainability from both environmental and economical points of view, also concerning the use of rare earths for permanent magnets. Therefore, the use of popular Permanent Magnet Synchronous Machines (PMSMs) is increasingly questioned despite their high efficiency, compactness, and light weight. There is a growing interest in alternative solutions for electric motor technologies. Among them, wound-field synchronous motors (WFSMs) stand out as a promising option featuring high power density and operational flexibility thanks to full field control. However, the conventional use of sliprings and brushes to supply the field winding affects both reliability and maintenance, turning out to be a potentially significant drawback. This paper provides an overview and comparison of the main solutions proposed to eliminate such sliding contacts, highlighting their respective benefits and limitations.