Electromagnetic Force Characteristics of Doubly-Fed Induction Machines Considering Harmonics from Rotor-Side Converters

Doubly-Fed Induction Machines (DFIMs) are widely used in wind turbines and variable-speed hydropower plants due to their ability of flexible adjustment, however, the electromagnetic vibrations caused by the high-frequency harmonics from the converter at the rotor side still challenge their applications in industry. This study investigates the electromagnetic force (EMF) characteristics of DFIM considering the harmonics from the rotor-side converters. With the slotting and saturation effects considered, an analytical model is proposed to analyze the characteristics of the harmonic fields and the related EMFs, such as the spatial order, frequency and magnitude. The influence of these EMFs on the vibration of DFIM is also predicted. The harmonic fields and the vibration responses are analyzed by Finite Element Method (FEM) for a 30-kW, 6-pole DFIM. The conventional EMFs without the rotor-side pulse width modulation (PWM) supply and the additional EMFs caused by this supply are compared under different operating conditions to validate the proposed analytical model. The analysis indicates that the radial EMF of the 6th order in 3900 Hz and 8100 Hz significantly increases and the corresponding vibration increases by about 10%, which is attributed to the high-frequency harmonics from the rotor-side converter. A prototype of the 30-kW, 6-pole DFIM is manufactured and tested for the vibration spectrums, and the results predicted by the proposed analytical model agree well with the measured results.