Analysis of Two-Terminal Hybrid Connection Method for Reducing Armature MMF Space Harmonics of Six-Phase Electric Machine

Fractional-slot windings, widely applied in multiphase electric machines, can further reduce cogging torque. However, concerns arise from the adverse effects of space harmonics introduced by the rich armature winding magnetomotive force (MMF). To reduce the space harmonics of armature winding MMF, a Two-terminal Hybrid Connection Method (TTHCM) of fractional slot electrical machine is proposed. Firstly, the general principle of harmonic elimination is derived, outlining the specific armature MMF harmonic orders that TTHCM can eliminate under different pole-slot combinations. Taking the example of a machine with 24-slot/10-pole configuration, the analysis process is presented. And the harmonic orders eliminated include the 1st, 7th, 11th, 13th, 17th, 23rd, and 25th space harmonics of the armature MMF. Next, models for a conventional three-phase machine, star-delta, dual three-phase, and TTHCM are established and compared. It is demonstrated that, compared to the dual three-phase connection, the TTHCM can enhance the torque, reduce torque ripple, decrease permanent magnet eddy current loss, and improve vibration performance without increasing the complexity of the control system. Finally, a prototype of TTHCM machine is manufactured and tested. The reliability of the theoretical and simulation analyses is confirmed.