A 40,000,000 rpm Magnetically Levitated Spinning Ball Motor

Abstract

Electrical power is largely converted to or from mechanical energy using electric motors. Several applications have driven the miniaturization of these machines, which requires high rotational speeds to achieve a desired power level at a decreased size. Rotational speeds of several hundred thousand revolutions per minute (rpm) have been used industrially, and drive systems in research environments have reached rotational speeds of up to one million rpm. The highest measured rotational speed achieved with an electrically driven rotor was 37.98 million rpm and dates back to 1947. Since then, this speed has not been reproduced or exceeded, despite subsequent efforts. In conjunction with the application-driven trend toward high rotational speeds, this has fostered interest in the underlying physical limitations regarding the achievable rotational speed, independent of a specific application. To explore these boundaries and to provide technological solutions for the related challenges, the design and implementation of an electric motor capable of reaching 40 million rpm is reported in this work. Submillimeter scale steel spheres are used as rotors, which were selected based on their structural and dynamic stability. To limit bearing and gas friction losses at such ultra-high rotational speeds, the rotor is magnetically suspended inside a vacuum without mechanical contact. Acceleration is carried out by the principle of a solid rotor induction machine, for which a detailed analytical model is provided. A suitable stator design using power ferrite cores for magnetic field frequencies in the megahertz range is developed. The design of an optical sensor system, which measures the rotor position without mechanical contact in all degrees of freedom, is outlined. This system provides the input to the high bandwidth digital control scheme required for active magnetic stabilization of the rotor. A power electronic converter system capable of generating the high frequency drive currents as well as the bearing currents necessary for magnetic suspension is presented. All subsystems were assembled to provide an experimental prototype of the ultra-high-speed motor. During a series of acceleration experiments with various rotor sizes and materials, a rotational speed slightly above 40 million rpm was reached with a rotor of 0.5 mm in diameter, which is, to the knowledge of the author, the highest rotational speed achieved by an electrically driven rotor to date. Circumferential speeds exceeding 1000 m/s and centrifugal accelerations of more than 4*10^8 times the gravitational acceleration were reached. The results open up new research possibilities, such as the testing of materials under extreme centrifugal load, and provide insights for the development of future ultra-high-speed electric drive systems.