Abstract: Aiming at the rotor vibration characteristics of the traction motor in electric multiple units (EMU) with initial static eccentricity under gravity load, a unified analytical expression of unbalanced magnetic pull (UMP) of motor under no-load or load operations with static and dynamic air-gap eccentricity fault and arbitrary pole pairs is first given by using magnetic potential multiplying permeability method and Maxwell stress tensor method. Then, Jeffcott model is used to establish the dynamic equation of traction motor rotor system under action of UMP, mechanical unbalance force and gravity, and the Runge-Kutta algorithm is used to solve the dynamic response of the system. Finally, the influence of initial static eccentricity and gravity load on the orbits and displacement spectrum of the traction motor rotor is discussed. Results show that:① the effect of gravity load on the motor rotor system is equivalent to applying a constant static eccentricity in the direction of gravity, which superposes the initial static eccentricity vector to form a resultant static eccentricity in the air-gap; ② the magnitude and direction of the resultant static eccentricity affect the position and direction of the rotor orbit. The larger the eccentricity is, the larger the offset distance of the rotor orbit is and the offset direction is along the static eccentric direction; ③ The increase of static eccentricity makes the rotor orbits of this traction motor slightly increase, which can be considered as basically unchanged. At the same time, the increase of static eccentricity also makes the rotor displacement spectrum components more significant and diversified.

Key words: traction motor, initial static eccentricity, gravity load, unbalanced magnetic pull