Cevelopaent of a :

A new uninterruptible power supply (UPS) having a flywheel for a energy storage unit is described. The developed single phase UPS of the rating of 5(kW), lOO(V) has following characteristics : (1) Battery less system which makes maintenance free and long life about three times of a battery UPS. (2) Simple power conversion circuit of a half bridge configuration which can realize good input and output waveforms, high efficiency, small size and light weight about 80(%) of the conventional UPS. Inside of the flywheel unit rotating 30000 (rpm) is vacuumized to eliminate windage losses by a small vacuum pump. And the flywheel rotor is made no mechanical touch in high speed states to get as long life as 30 years. A electrical main circuit is consisted of a half bridge converter, a symmetrical half bridge inverter, and a inverter / converter to drive the flywheel induction motor/generator. Charging or discharging control of the mechanical energy is achieved by controlling the slip frequency of the induction machine. The converter is controlled by a current follow-up method using a hysteresis comparator to get the sinusoidal input current and the unity input power factor. Whereas the inverter is controlled to get the sinusoidal voltage by using a voltage follow-up method. results of 5(kW) and 1 minute discharge rating system show the excellent characteristics such as the input power factor of 99 (%), the output voltage distortion of 0.4(%) and small losses of 51(W) for the flywheel and its drive systems at storage states. Experimental