In this paper, stator axial pairing of tooth-tips is proposed to suppress the open-circuit DC winding induced voltage and torque pulsation in the wound field switched flux machine. The stator is divided into two segments axially, whilst these two segments have different tooth-tip arcs. The proposed technique can suppress the rated on-load torque ripple from 17.91% to 6.99%, reduce the peak-to-peak value of cogging torque from 5.14 Nm to 0.48 Nm, and reduce the open-circuit DC winding induced voltage at 1500 r/min from 39.79 V to 6.21 V. Although the efficiency of the machine is slightly reduced from 81.5% to 80.7%, the average electromagnetic torque can be maintained by >95%.

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In order to analyze the DC winding induced voltage in the wound-rotor synchronous machine, this paper uses the air-gap field modulation principle to investigate its operation mechanism and harmonic order. By establishing the analytical magneto-motive force (MMF)-permeance model, the DC winding induced voltage per electrical cycle under open-circuit condition, armature reaction condition and on-load condition are deduced. Analytical analysis shows that the MMF function, stator and rotor permeance function are critical factors that influence the harmonic order of the DC winding induced voltage. The analysis results are compared with those predicted by the finite element analysis (FEA). Both non-linear steel and linear steel conditions are accounted in the FEA analysis, and the results show that the analytical deduction result agrees well with the FEA analysis result.