Evaluation of Adhesion Strength of Coils of a High-Speed Motor Applying Welding-Less Coils

Motors for electric drive systems applied to electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) need to be compact and to have high power density to reduce vehicle weight, to expand the living space, and to lower costs. Several means of reducing the size of motors can be utilized. In this study, we focused on shortening the axial length of the stator coil. The axial length of the entire motor can be shortened by reducing the axial length of the stator coil end, and the length of the frame that houses the motor can be reduced. In our previous report, we proposed a permanent magnet synchronous motor that has a stator consisting of a stator coil with rectangular wires joined at the axial center of the stator core to shorten the axial length of the entire motor. However, one of the issues with the proposed coil, the possibility of the coil falling off due to thermal stresses generated in the coil, had not been considered. In addition, a large difference was found between the measured and analyzed coil temperatures, along with a problem in the evaluation of high-frequency iron loss. As a result, thermal stress could not be evaluated accurately. Therefore, we conducted a preliminary study to evaluate the thermal stress of the coil. The study, reported herein, included an investigation on the effect of harmonic iron loss on the temperature of various parts of the motor to improve the accuracy of loss analysis. We directly measured the iron loss of electromagnetic steel sheets at various frequencies to investigate up to what frequency the iron loss data used in the building factor calculation for the iron loss analysis would be required. Then, temperature analysis was performed at the operating point under the most severe temperature conditions, and shear stress was evaluated using the calculated temperatures of each part to determine whether or not the welding-less coils would pull out. As a result of our review, the maximum frequency of iron loss data for electromagnetic steel sheets used to calculate the building factor for iron loss calculations was found to be at least 20 kHz. In addition, the coil temperature was within ±10 K of the value obtained when the motor was designed under the condition of no mating area, and the motor with the welding-less coil exhibited the same characteristics as those of the designed value. In the next step, the coil’s adhesion strength exceeding the thermal stress, we found that the insulation configuration did not break under thermal stress and that coil dislocation did not occur. Therefore, we found that the welding-less coils are useful as coils constituting a motor.