Haruo Koharagi

High electrical resistance hot-pressed NdFeB magnet for low loss motors

Fluoride-coated hot-pressed NdFeB magnets with electrical resistivity of 1.4mΩcm, which is ten times larger than that of a noncoated sintered NdFeB magnet, were prepared using fluoride coating powders. The high resistive NdFeB magnets consist of coated powders and are sintered with a neodymium fluoride layer, which was grown using a fluoride solution. No degradation of coercivity was observed in the fluoride coated NdFeB powders. The increase in magnet temperature caused by an alternating magnetic field (Eddy currents) was eightfold with the use of a fluoride-treated magnet. Furthermore, the increase in temperature at the magnet’s rotor was reduced by 50% when the high resistive hot-pressed magnet was substituted for the conventional commercial magnet.

Evaluation of experimental permanent-magnet brushless motor utilizing new magnetic material for stator core teeth

We prepared motors with experimental cores using a soft magnetic composite and an amorphous metal and evaluated their performance. We measured the magnetic and motor characteristics, such as iron loss, when the new materials were used in the stator core teeth. The amorphous metal was found to have very high permeability and low iron loss, whereas the soft magnetic composite had low permeability and had a comparable iron loss as low-grade magnetic steel. The soft magnetic composite had the added benefits of being able to form a three-dimensional shape and having relatively low core loss in high frequency conditions. The magnetic properties we measured were able to predict the motor characteristics with high accuracy based on numerical and finite-element-method analysis.

Motor performance and magnetic properties of rotors with directly injected bonded magnets in laminated Fe-Si sheets

Directly injected bonded magnets in rotors have attracted attention because the injection process has various advantages compared to the application of sintered magnets. Anisotropic Nd2Fe14B, Sm2Fe17N3, and Sm2Co17 compounds were directly injected in rotors with laminated Fe-Si sheets. All bonded magnets had easy magnetization direction in the radial direction. The value of the coercive force for injected Nd2Fe14B was 17.9kOe at 298K and was smaller than that for Sm2Co17 at 398K. Demagnetization was found for all the bonded magnets in a magnetic field of −1kOe at 398K. The induced voltage wave forms for the three rotors showed sine waves. The value of induced voltage for Nd2Fe14B bonded magnet rotor reduced with increasing temperature by −0.16%∕K, which is in agreement with the temperature coefficient of Br.

Magnetic field analysis of a squirrel cage induction motor considering rotor skew and higher harmonics in the secondary current

This paper describes a method to analyze harmonic magnetic fields of squirrel cage induction motors considering rotor skew and higher harmonics in the secondary current. The proposed method is based on a two-dimensional finite element method. The rotor skew structure is expressed by multiple 2D finite element mesh models, produced in suitable axial positions, and the magnetic field in each mesh model is calculated by the revised secondary current taking into account the skew effect. The secondary currents, magnetic flux densities, and electromotive forces are calculated by the proposed method. Then the differences between a skewed rotor and a nonskewed rotor are discussed. From the comparison between the calculated and the experimental results, the proposed method is shown to be appropriate and useful for quantitative estimation of harmonic components of induction motors.