The characteristics of an active magnetic bearing (AMB) supported energy storage flywheel are discussed. The flywheel was developed for a number of industrial applications to provide: 1) ride-through power in turbine or diesel generator sets, 2) voltage support in rail applications, 3) energy recovery in crane applications, 4) power quality improvement, and 5) load support in uninterruptible power supplies (UPS). The flywheel system operates in a vacuum to minimize windage losses, uses AMB to minimize bearing losses and eliminate bearing maintenance, 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 synchronous cancellation to minimize dynamic loads (and losses). This is demonstrated by dynamic data from high speed testing. Rotor temperature measurements from thermal equilibrium testing are also presented.