In our high-power flywheel energy storage system, the heavy and large flywheel is rotated by the electromagnetic torque of a permanent-magnet (PM) synchronous motor (PMSM) and, in case of a breakdown of electric current, this motor used as a generator supplies electric energy to the various electric utilities using mechanical rotation energy of the flywheel. Thus, the dynamic performance of the flywheel is related to the electromagnetic torque for the motoring and generating power of the PMSM/G. Moreover, in the idling mode without generating loads, speed reduction of the flywheel is mainly caused by drag torque generated in the slotless iron-cored stator of this PM machine. In this paper, the operating torque, defined as the electromagnetic torque and drag toque, is estimated from the magnetic-field analysis by the PM rotor with diametrical magnetization. Also, the dynamic performance of the flywheel, according to the driving mode, is simulated from dynamic modeling of the PMSM/G with the operating torque and verified by the experiment.