Experimental Verifications of Uninterruptible Power Supply Using a Flywheel Motor-generator and a Planetary Gear

Flywheels have several advantages such as long life, high reliability, and high environmental resistance. This paper describes the application of flywheel to uninterruptible power supplies that rotate a motor-generator at commercial frequencies of 3000min-1. Conventional systems that use flywheel generators require high-capacity semiconductor power converters for energy conversion. Moreover, a specially designed generator is required because the generator operates at variable speed. To overcome these weaknesses, this paper proposes a flywheel generator using a planetary gear, which is a mechanical shaft coupling mechanism. The use of a planetary gear enables the flywheel and generator to rotate at different speed. As a result, even if the flywheel speed drops during a power outage, the generator speed can be kept constant. This paper reports on the design guidelines and experimental verifications for a 1-kW system.