Long Term Backup Bearing Testing Results

An energy storage flywheel was developed in 2005 to provide ride-through power for industrial UPS applications, and its application was expanded to provide regeneration power for mobile cranes. The flywheel, which operates in a vacuum, is supported by Active Magnetic Bearings (AMB) to minimize bearing losses, and has a high power motor/generator coupled to an efficient power conversion module. The flywheel can maintain energy for emergency use by holding at full speed or can be cycled up and down in speed to absorb and discharge energy rapidly. Over 400 Flywheel systems have been delivered and are operating in the field since product release. In addition to the AMB, the Flywheel system uses a backup bearing system to support the rotor; 1- when the system is not in operation, including shipping; 2- during low speed safe spin-downs when external power is disconnected, 3- during intermittent overload events (only expected in crane systems as a result of crane motion) and 4- during high speed spin-downs when system events/failures occur. Low speed spin-downs typically start at 4,000 rpm in crane systems and can happen multiple times a day, while UPS systems typically start at 6,000-8,000 rpm and may only happen once a year. Overloadevents in crane systems can happen at any speed in the operation range of 10,000 rpm to18,000 rpm and last for up to 5 seconds. The backup bearing system of every flywheel is tested with a series of backup bearing drop tests culminating with a drop and spin down from full speed. Additionally, several engineering flywheels have been used extensively for testing and development of the backup bearing system. The original system was capable of three 2.75 hour spin downs from 36,000 rpm and developed with the UPS system requirements in mind. Methodology and results for this test program were described in [1] and [2]. Recent testing has validated the system for use in crane system with added low speed spin-downs and momentary overload conditions at high speeds. In addition, incremental changes to the backup bearing system have resulted in extending the capability to eight 2.75 hour spin downs from 36,000 rpm for UPS applications. These changes are the result of continued analysis of backup bearing operation during spin-down. This ongoing test program is reported here – including a discussion of some of the test results.