To be used as a robotic actuator, a surface-mounted permanent magnet synchronous motor (SPMSM) requires a low cogging torque characteristic. The aim of this study is to analyze and reduce the cogging torque's variation due to the assembly imperfection of the segmented stator that is widely used in the SPMSM. Considering the interconnection of the stator segments, an equivalent modeling method of the segmented stator with assembly imperfection is introduced. In addition, the variation in the cogging torque caused by the assembly imperfection is mathematically evaluated. Then, a robust design process that incorporates a sensitivity analysis and a robust design optimization is proposed. By using the proposed design, a robust optimum design model was built and its validity was verified through Monte Carlo simulation and tests.

This paper proposes an indirect method for predicting mechanical loss by considering the eddy current loss of permanent magnets (PMs) and conductors under the no-load condition. The mechanical loss has been conventionally predicted indirectly through experiments and numerical methods. The conventional method uses the no-load loss measured through experiments and the no-load iron loss calculated through a numerical method. With the increase in the demand for high power density motors, the PMs with high energy density and winding technology with high fill-factor are required. Thus, the proportion of eddy current losses of PMs and conductors is increasing among the electromagnetic losses. Therefore, we propose an indirect method for predicting the mechanical loss considering the eddy current losses. The accuracy of the proposed method is higher than that of the conventional method. Moreover, the proposed method is verified by comparing the estimated efficiency of the specimen obtained by using this method with the measured efficiency.

This paper proposes a method to suppress the torque ripple caused by misalignment of the gearbox. Misalignment is a common fault caused by the manufacturing tolerances, and generates torque ripple. Therefore, even if a constant electromagnetic torque is transmitted from the motor to the gearbox, a large torque ripple may be transmitted to the load because of misalignment. In the proposed method, a harmonic current injection method, in which harmonic current is injected into the armature current, is used to suppress the torque ripple produced by misalignment. The amplitude and phase of the harmonic current are calculated using the misalignment torque ripple, and a method for estimating the misalignment torque ripple, which does not require a torque sensor, has been presented. Using the estimation method and harmonic current injection method, the misalignment torque ripple can be suppressed effectively. The phenomenon of torque ripple production by misalignment is analyzed, and a current control algorithm based on harmonic current injection method is proposed. The effectiveness of the proposed method is verified through experiments at various load torque. Using this method, a high degree of torque control can be achieved with even a low quality gearbox.

In this article, the electrical and mechanical characteristics of multilayer interior permanent magnet synchronous motors (ML-IPMSMs) using ferrite permanent magnets (PMs) are compared when driven at high speeds with respect to the type of electrical steel. In high-speed operation, the mechanical and electrical characteristics of ML-IPMSMs vary depending on the number of PM layers, PM shape, number of bridges, and bridge thickness. Additionally, ferrite PMs have a lower coercive force than neodymium (Nd) PMs; therefore, irreversible demagnetization (ID) must be considered, and it is affected by the number of PM layers and PM shape of ML-IPMSMs. Thus, when designing a multilayer IPMSM using ferrite PMs for electric vehicle (EV) traction, all characteristics should be considered. Compared to conventional electrical steel (CES), high-strength electrical steel (HSES) offers advantages in mechanical properties, allowing for greater flexibility in ML shape. Consequently, it is possible to design a shape with improved electrical performance when compared to CES by adjusting the ML shape. In this article, all characteristics of each ML-IPMSM using HSES and CES are reviewed via finite element analysis (FEA), and the benefits of HSES applied motors for high-speed driving motors are presented. To verify the validity of the review content in this article, experiments are conducted on the manufactured motor, and FEA results are compared with the test results.

The traction motor in parallel 2 hybrid electric vehicles adopts a cooling method involving automatic transmission fluid (ATF). The scattered ATF directly cools the heat sources of the electric motor. A thermal model based on the lumped parameter network is developed to reflect the direct oil cooling effect. In the developed oil-cooled lumped parameter thermal network (LPTN), the cooling by the ATF acts as a current source. The oil scattered through the motor shaft has high cooling efficiency, but it is difficult to theoretically calculate the nonlinear fluid behavior. Because the flow rate and rotational speed affect the ATF cooling performance, the convective heat transfer coefficient (HTC) of the ATF is reflected in the direct oil cooling thermal model of the traction motor via the proposed correlation process. In this process, a kriging surrogate model-based optimization is performed to determine the convective HTC of the ATF under specific flow and load conditions. The objective function of the optimization is to minimize the difference between the temperature predicted by the thermal model and experimentally measured temperature. As the kriging surrogate model has high prediction accuracy for nonlinear performance, the direct oil-cooled LPTN developed during the optimization process provides a useful thermal model.

Owing to the high speed and frequency required in electric machine industry, the significance of AC resistance has been increasing. This study investigates the AC resistance of windings according to configuration of strands. The mechanism of generating the AC resistance in winding conductors was investigated with respect to the slot leakage flux and eddy current. Then, when applying the strands to the winding, the concept of unbalanced impedance was introduced to characterize the additional AC resistance due to circulating current. The analytic method to calculate AC resistance describing the above phenomena is derived and verified by FEA results. Afterwards, the AC resistances of varying numbers and arrangements of strands were separately studied via finite element analysis. In addition, a method to avoid increasing the AC resistance was suggested based on impedance equalization when applying strands. Finally, experiments were conducted to verify the effect of the strand configuration on AC resistance. An E- and I- shaped core, which can be the rendering of common electric machines, such as motors, generators, and transformers, was manufactured and tested with winding specimens. Consequently, the test results showed a similar trend as the analytical results.