The development and testing of an AMB supported, 125 kW energy storage flywheel is discussed. The flywheel is being developed for a number of industrial applications to provide: 1) ride-through power, 2) voltage support in rail applications, 3) power quality improvement, and 4) UPS service in-lieu of standby batteries. The flywheel, which operates in a vacuum, is supported by AMBs to minimize bearing losses, and has a high power motor/generator coupled to an efficient power conversion module. The magnetic bearing system is designed to minimize losses for both energy storage efficiency and to reduce heat generated on the rotating assembly. The magnetic bearing controller uses gain scheduling to stabilize the gyroscopic rotor and uses synchronous cancellation to minimize dynamic loads. Dynamic data from high speed testing is presented. Rotor temperature measurements from thermal equilibrium testing are presented and discussed.