Comparison of Coaxial Magnetic Gears Using Rare Earth and Nonrare Earth Permanent Magnets

Mechanical gears are widely used because they allow traction motors with high efficiency at high speed and low torque to be driven with high performance at low speed and high torque. The friction of mechanical gears, however, causes noise and vibration and requires maintenance due to wear. Coaxial magnetic gears (CMGs) can overcome the problems of mechanical gears by transmitting torque without physical contact, but their heavy use of rare earth permanent magnets (PMs) has been an obstacle in the face of unstable supply and price of rare earth PMs. To reduce the dependence on rare earth PMs and increase the cost efficiency, four combinations of CMGs utilizing Neodymium (Nd) and ferrite are proposed. The comparative models were designed to satisfy a gear ratio of 7 with 4 inner PM poles and 28 outer PM poles. From the Lorentz force equation and flux density equation of CMG, it was confirmed that the product of magnetic flux densities of the inner PM and the outer PM is proportional to torque. Further, through the Fast Fourier Transform (FFT), it was verified that the harmonics of magnetic flux density product is divided into a component contributing to torque and a loss component converted to vibration. These relationships were validated by the performances of CMGs and the frozen permeability method (FPM) based on 2-dimensional (2D) finite element analysis (FEA). Considering the rare earth dependence, torque ripple, efficiency, and cost analysis of the optimal design results, the ferrite-Nd CMG showed a high potential performance among the proposed optimal models. Finally, the electromagnetic performance of ferrite-Nd CMG through experiments and FEA were compared.