Analysis of Vibration Characteristics of Homopolar Inductor Machine with DC-Biased Air-Gap Magnetic Field

Vibration analysis is very essential for the electromagnetic design and stable operation of homopolar inductor machine (HIM) applied in flywheel energy storage systems. In the research of traditional integer slot machines, it is generally believed that there is only zeroth-mode vibration, and the armature reaction only affects the amplitude of radial forces. However, the stator excitation structure is adopted in the HIM, which results in the presence of a direct current (DC) component in the air-gap magnetic field and the magnetic circuit of alternating current (AC) components of the armature magnetic field is different from that of the wound-excited magnetic field (WMF). The effect of the DC-bias air-gap magnetic field on vibration is analyzed by calculating the concentrated force on the stator tooth surface. The results indicate that there are both zeroth- and pth-mode vibrations in the HIM. The zeroth-mode vibration is mainly generated by the DC component of the air-gap flux density, while the pth-mode vibration is generated by both the DC and fundamental components. In addition, the armature reaction will cause the frequency and order of radial forces to shift, and the d-axis armature reaction can reduce the zeroth-mode vibration. Finite-element analysis and experiments have been done to verify the analysis results, and the error of both was less than 23.5%.