Compared with traditional electrochemical batteries, flywheel energy storage systems are attractive in certain aerospace applications due to their high power density and dual-use ability to achieve attitude control. A small flywheel energy storage unit with high energy and power density must operate at extremely high rotating speeds; i.e., of the order of hundreds of thousands of revolutions per minute. In this paper, initial test data is provided on a prototype permanent magnet flywheel motor/generator with design goals of achieving 100W of power conversion over a speed range from 150,000 rpm and 300,000 rpm. A field-oriented position-sensorless control scheme is implemented on the machine. The initial test results indicate the system has the potential to meet the desired performance.