Traditional shaft-sealed centrifugal pumps exhibit problems, such as liquid leakage and reduced efficiency, due to mechanical seal failures. To address the issue, the bearingless direct-drive centrifugal pump system (BDDCPS) has been designed with bearingless motor technology. This may cause a new issue of rotor eccentricity displacement, which can lead to a reduction in pump performance. Compared to a stable state, the changes in characteristics, such as flow rate, head, and pressure, during the transient start-up process are more severe, increasing the likelihood of system failure and requiring greater attention. In order to ensure safety and reliable operation during the transient start-up process, this paper investigates the working principle of the BDDCPS and analyzes transient start-up characteristics with three different start-up methods. The external characteristics, pressure field distribution, velocity field distribution, and hydraulic losses analysis of the pump are determined for comparative analysis. Finally, it is found that the linear acceleration start-up method is the most suitable and efficient for the BDDCPS.

Rotor suspension characteristics of magnetically levitated centrifugal pump under passive control is investigated by numerical simulation. Numerical models of the studied system are built, including a bearingless permanent magnetic slice motor (BPMSM) and a centrifugal pump. The authenticity of the numerical model is validated by comparing both simulated radially restoring electromagnetic force and fluid-induced axial force with theoretical values. Then, the performance of passive control of the simulated BPMSM is revealed, including axial restoring electromagnetic force and restoring torque exerted on the deviated rotor. It is found that the axial force induced by water suction in the pump impeller is crucial to affect the passive levitation stability of the rotor. Different modification schemes are studied to reduce fluid-induced axial force on the pump impeller so as to alleviate the requirement on the axial restoring force of BPMSM. It is found that balance hole and back blades can significantly diminish the axial force while twisted impeller blades can slightly increase the axial force. Finally, an improved impeller with balance hole and back blades is proposed for the centrifugal pump. Then, the net axial force and titling torque exerted by fluid and gravity on the assembled impeller and rotor are predicted and the two values are loaded on the rotor of the numerical model of BPMSM. The simulation results demonstrate that the current design of BPMSM can provide with enough restoring axial force and torque when initial position of the rotor is located at both centre and extreme positions.