Design and Analysis of a Self-Excited Brushless Wound-Field Synchronous Machine with Reduced Torque Ripple

Permanent magnet synchronous machines are commonly employed in variable-speed applications because they offer high efficiency, easy control, robust construction, and do not require external field excitation. However, rising costs of permanent magnets have significantly increased the overall price of these machines. Wound-field synchronous machines (WFSMs), which have no permanent magnets and are thus less affected by market price changes, can be a good alternative to permanent magnet synchronous machines. WFSMs use slip rings along with brush assemblies to energize the rotor field winding (RFW) with DC. However, the use of slip rings and brushes introduces sparking and increases maintenance requirements. Consequently, brushless wound-field synchronous machines (BLWFSMs) have recently been developed to overcome these limitations. This paper introduces a novel BLWFSM designed to mitigate the volatility of permanent magnet prices and the maintenance challenges of conventional WFSMs using slip rings and brushes. Unlike existing brushless topologies that necessitate dual inverters, the proposed design utilizes a single three-phase inverter. The proposed machine’s rotor incorporates an excitation winding and a field winding, connected via an embedded rotating bridge rectifier. The stator MMF is dominated by a 30-Hz subharmonic component and a 60-Hz fundamental component. Brushless excitation is achieved by utilizing a subharmonic stator MMF component (30 Hz) that induces an AC voltage in the excitation winding, which is rectified by a rotating rectifier and supplied as DC to the field winding. Two-dimensional finite element analysis validates the performance of an 8-pole, 18-slot configuration. The results demonstrate a rated torque of 8.975 Nm with an exceptionally low torque ripple of 0.501%. These characteristics, combined with a reduced volume and weight profile, position the proposed BLWFSM as a robust alternative for air blowing, aerospace power generation, and electric vehicle applications.