An Investigation into Magnetic Gears and Magnetic Springs for Marine Hydrokinetic Power Generators

Abstract

Marine hydrokinetic generators often use mechanical gears to convert the low-speed motion into the high-speed rotary motion that is required for conventional electrical generators. While mechanical gearboxes have a high torque density and can reach high efficiencies, they suffer from serious reliability issues. Recently, magnetic gearing technology has been proposed as a means of making hydrokinetic generators more reliable. As magnetic gears use magnetic field modulation to create speed change, they offer a high reliability, nonfriction, power transmission mechanism that limits the wear to only the bearings and makes the magnetic gears almost maintenance-free. During an over-torque condition, a magnetic gear will pole-slip thereby providing inherent overload protection, preventing the magnetic gear from catastrophically failing. This dissertation investigates the torque capabilities of a 63.3:1 gear ratio 5 kW dual-stage magnetic gear. The dual-stage magnetic gear is composed of two coaxial magnetic gears connected in series. This arrangement makes it possible to have two separately designed magnetic gear with high torque density. Both 2-D and 3-D finite element analysis was used to maximize the magnetic gear's volumetric torque density. The number of magnet rotors' pole-pairs and modulating rotor ferromagnetic segments was selected based on minimizing the torque ripple and unbalanced radial force. An electromagnetic harmonic loss analysis and magnetomechanical deflection analysis were performed to ensure that the magnetic gears were energy efficient and mechanically robust. The magnetic gear rotors use a Halbach array and a flux-focusing magnet arrangement. To mitigate the permanent magnet tolerance inaccuracies, and help to retain the magnets in place, a unique isosceles trapezoid magnet arrangement was used. The measured torque of the sub-scale stage 1 and stage 2 magnetic gears were 1220 N·m and 188.4 N·m, respectively, which makes their torque densities 268.7 N·m/L and 236.5 N·m/L, respectively. The total active region torque density of the 63.3:1 gear ratio dual-stage magnetic gear is 231.1 N·m/L and 42.6 N·m/kg. This dissertation also presents the design, fabrication, and testing of a 50 kW high mass torque density magnetic gear with a 7.67:1 gear ratio for use in a rotary-base marine hydrokinetic generator application. The lessons learned in fabrication and testing of the 5 kW 63.3:1 gear ratio dual-stage magnetic gear led to several innovative design changes for the 50 kW magnetic gear design such as the use of a dual axial stack, the use of non-conductive circular modulating rotor rod supports, utilization of laminated magnets and the use of