In an interior permanent magnet synchronous motor, an adhesive such as bond is generally injected into the magnet tolerance to prevent vibration of the permanent magnet within the insertion space. In this case, a disadvantage is that the magnet tolerance does not contribute to the performance. In this paper, ferrofluid is inserted to improve the torque density, utilizing the magnet tolerance. When inserting ferrofluid into the magnet tolerance, it is important to fix the magnet because conventional adhesives are not used, and it is important that the ferrofluid does not act as a leakage path within the insertion space. In this study, a new rotor configuration using a plastic barrier that satisfies these considerations was introduced. The analysis was conducted through finite element analysis (FEA), and this technique was verified by comparing the simulation results and the experimental results through a dynamo test. It was confirmed that the no-load back electromotive force in the final model increased through ferrofluid injection.

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It has advantages such as high power density and miniaturisation when the motor operates at high speed. However, as a side effect, the loss density is much higher than that of a constant speed motor. In addition, a retaining sleeve is inserted in the high‐speed motors to prevent the rotor from scattering. This insertion deteriorates the performance due to the increase of airgap, and eddy current loss occurs in the sleeve. In order to improve these shortcomings, the sleeve is redesigned in this paper. A groove was dug into the retaining sleeve and ferrofluid was injected. Ferrofluid refers to water or oil mixed with ultra‐fine powder. Since the permeability of ferrofluid is greater than that of airgap, it is possible to have high output and high efficiency. Since the material of the sleeve is non‐permeable, the performance is reduced with the same effect as increasing the airgap. However, if a ferrofluid is inserted by digging a groove in the retaining sleeve, the magnetic flux from the magnet flows into the stator through the ferrofluid, which has the same effect as reducing the airgap. Therefore, the torque density increases. Also, the eddy current loss is reduced through the retained sleeve groove. Moreover, a sleeve skew structure was used to further reduce the eddy current loss. In this paper, the mechanical rigidity at the rated speed was also taken into consideration. As a result, eddy current loss was reduced and torque density was improved. This is verified through the final finite element analysis.

Conventional surface mounted permanent magnet synchronous motor (SPMSM) at high speed has large iron loss in the stator and eddy current loss in the rotor. The higher the eddy current loss, the higher the rotor temperature. Therefore, a plan to reduce the loss must be devised. Also, weight at high speed is related to mechanical stiffness and electromagnetic vibration noise. Therefore, a plan to reduce the weight must be devised. In this paper, we propose to change the shape of the conventional 15 kW 110 000 rpm class into a slotless shape. A process of protocol change was proposed, and the sizing design of the rotor and stator was devised. The advantages of the proposed shape were verified through performance comparison through finite element analysis (FEA) and mechanical stiffness analysis.

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