Development of magnetic coupling with variable thrust structure for flywheel energy storage system in long lifetime UPS

Abstract

This paper introduces the flywheel energy storage system (FESS) in a long lifetime uninterruptible power supply. The first prototype FESS (3.0-MJ) uses low cost ball bearings and general purpose induction motor in terms of cost reduction. From the experimental results, it is confirmed that the charge and discharge efficiency of the FESS is 60.7% (charge 80.7%, discharge 75.1%). In order to improve the efficiency and lifetime, a permanent magnet synchronous motor, a spherical spiral groove bearing and a magnetic coupling with variable thrust structure are applied to the second prototype. The estimated charging efficiency is achieved to 86.9% and the estimated discharging efficiency is achieved to 82.8% at the second prototype. Then, the thrust of the proposed magnetic coupling is analyzed by the finite element method (FEM). As a result, the thrust can be changed by 11.1% with the electromagnet. In addition, the thrust analysis results are evaluated by using the prototype electromagnet.