Field-Modulation Electrical Machines: Origin, Unification, and Development

The airgap field harmonics were considered to undermine the performance of electrical machines for more than a century. Since the proposal of general airgap field modulation theory (GAFMT) in 2017, the magnetic field modulation effect (MFME) has received unprecedented attention. Harmonics are proven to be no longer a hindrance, but a benefit by artificial design. Accompanied field-modulation electrical machines (FMEMs) that work relying on harmonics are becoming increasingly popular and expected to satisfy the hunger for high torque and high efficiency simultaneously in the modern industry. In this review, the development histories of FMEMs, with particular reference to brushless doubly-fed machines, coaxial magnetic gears and magnetically geared machines, permanent magnet Vernier machines, and stator-PM machines, are presented. Then, the necessity of proposing the GAFMT for harmonic usage is highlighted. The latest developments of FMEMs, including theoretical, technological, and application aspects, are systemically analyzed. Finally, challenges to be addressed are discussed.