Frequency response function measurement of a rotor system utilizing electromagnetic excitation by a built-in motor

In this paper, we propose a practical method for evaluating the robustness and stability of rotating machines. This entails using electromagnetic excitation and measuring the frequency response functions (FRFs) of the rotor system during rotation. The difficulty of applying excitation can be solved by utilizing a built-in motor with static eccentricity and adding a sinusoidal sweep d-axis current to the motor. A test rig with a bearingless motor (BELM) was used to verify the validity of the proposed method. A consequent-pole-type BELM is a combination of a consequent-pole-type motor and a radial magnetic bearing, in which both the motor and the suspension windings are arranged in one stator core of the BELM. The center of the motor is defined by the zero-power controller and the static eccentricity is determined by the suspension control system. The FRFs measured utilizing the electromagnetic excitation generated by the motor windings was compared with reference FRFs measured utilizing the suspension windings. The natural frequencies and damping ratios were determined from the two different FRFs. The difference between the identified natural frequencies and the difference between the damping ratios for the two different FRFs were calculated. The effectiveness of measuring the dynamic characteristics of the rotor system under various rotational conditions, such as the rotational speed and eccentricity, was clarified and the differences between the measurements obtained from the two different FRFs were generally less than 7.7%.