Analysis of electromagnetic torque in sinusoidal excited switched reluctance machines having DC bias in excitation

This paper investigates the average torque in the novel switched reluctance machine (SRM) excited by sinusoidal bipolar excitation with DC bias by employing the Fourier series and FE analysis. The optimal leading angle of phase current ahead of the inductance for producing maximum torque and the optimal ratio of AC to DC bias for maximum torque per copper loss are derived analytically from the first and second harmonic components of inductance (L₁ and L₂). Thus, with the optimal leading angle and optimal ratio of AC to DC bias an analytical approach to determine the optimal rotor pole arc can also be developed. It shows that when L₁ and L₂ are anti-phase, the optimal leading angle decreases as the ratio of AC to DC bias increases, but increases when L₁ and L₂ are in phase. The optimal leading angle and optimal ratio of AC to DC increase slightly with the increase of L₂ to L₁ ratio. The ratio of L₂ to L₁ is observed to be negative for small rotor pole arc, but positive for large rotor pole arc. The optimal ratio of AC to DC bias is determined by the amplitude of L₂ to L₁ ratio. The magnetic saturation makes the optimal leading angle less sensitive to the ratio of AC to DC bias. The similar torque capability can be achieved for the unipolar excitation and sinusoidal bipolar excitation with DC bias.