Flywheel energy storage systems (FESSs) store kinetic energy in the form of J omega(2)/2, where J is the moment of inertia and omega is the angular frequency. Although conventional FESSs vary w to charge and discharge the stored energy, in this study a fixed-speed FESS, in which J is changed actively while maintaining w, was demonstrated. A fixed-speed FESS has the advantage of being capable of direct grid-connection without requiring a power electronic interface. A prototype with an output power of several hundred watts and a charge/discharge period of several seconds has been developed and discharge/charge operations have been conducted while mechanically measuring the output energy E-m from the flywheel and mechanical work E-c, to vary J. Theoretical analysis suggests a ratio of 2.2 for E-m/E-c, and experimental values of E-m/E-c show high reproducibility, approximately 1.8 and 2.7 in discharge and charge operations, respectively. The reason for the errors can be explained by the loss incurred in varying J. (C) 2018 Elsevier Ltd. All rights reserved.