The reliability assessment of electric machines plays a very critical role in today’s engineering world. The reliability assessment requires a good understanding of electric motors and their root causes. Electric machines mostly fail due to mechanical problems and bearing damage is the main source of this. The bearings can be damaged by mechanical, electrical, and thermal stresses. Among all stresses, the researcher should give special attention to the electrical one, which is bearing current and shaft voltage. This review paper introduces a comprehensive study of bearing current and shaft voltage for inverter-fed electric machines. This study aims to discuss several motor failure processes, as well as the sources and definitions of bearing current and shaft voltage. The different kinds of bearing currents are addressed and the parasitic capacitances, which are the key component to describe bearing current, are determined. Several measurement approaches of bearing current will be discussed. Furthermore, modeling of bearing current will be covered together with the machine’s parasitic capacitances. Moreover, the different bearing current mitigation techniques, as described in many papers, will be thoroughly addressed. The use of rewound multiphase machines for mitigation of bearing current will be proposed and compared to a three-phase machine. Finally, various pulse width modulation techniques of multiphase systems that reduce bearing current and shaft voltage will be investigated, and the findings described in the literature will be summarized for all techniques.

Due to the significant advantages of cost savings, low temperature rises and high efficiency, synchronous reluctance machines (SynRMs) have attracted growing interest from academia to industrial applications. Numerous articles have been published to obtain a SynRM with improved performance (i.e., a high torque density and efficiency and a lower torque ripple) using different focusing points at healthy and faulty conditions such as rotor design, magnetic steel grade, stator winding, and fault tolerance control (FTC) strategy. Hence, this article reviews the scientific researches about SynRMs aiming to improve their performance. These scientific researches can be categorized into four sections: 1) optimal rotor design; 2) magnetic steel grade; 3) winding configurations; and 4) FTC strategies.

Recently, controlling a wind energy conversion system (WECS) under fluctuating wind speed and enhancing the quality of power delivered to the grid has been a demanding challenge for many researchers. This paper provides a comprehensive review of synchronous generator-based WECSs. This paper will investigate the growth of wind energy in Egypt and throughout the world, as well as the technological and financial significance of wind energy. The block diagram of a typical grid-connected WECS, power control techniques, characteristic power curve-based maximum power point tracking (MPPT), and MPPT techniques are also presented in this study. Moreover, this study compares different power converter topologies for grid-connected and independent WECSs that use a permanent magnet synchronous generator (PMSG).

A five-phase fault-tolerant combined star-pentagon synchronous reluctance machine (SynRM) can provide a high torque density, minimal torque ripple, and superior fault tolerance. To enhance the performance of SynRM in the case of an open-circuit fault, this article proposes an optimal current angle to maximize the torque and minimize the torque ripple under the fault condition. The drive system consists of a five-phase combined star-pentagon SynRM fed from a three-to-five-phase matrix converter (MC). When determining the operating point of maximum torque per ampere (MTPA), the influence of fault and saturation on the $d$ - and $q$ -axis inductances is considered. The healthy phases’ currents are reconstructed to guarantee a null value of zero-sequence current. Space vector modulation is applied to control the MC under the fault condition. Under the fault condition, the $z$ -subspace is properly controlled to reduce the overall harmonic distortions of current. Moreover, the performance of the drive system is analyzed using the finite element method (FEM) simulation and MATLAB program. Finally, the experimental tests validate the effectiveness of the suggested approach on a SynRM prototype drive system with a three-to-five-phase MC.

Multiphase machines with a prime number of phases (five and seven) have shown better torque density than machines with other phase numbers. Unfortunately, these machines require a special stator design. Hence, this article proposes general rewinding techniques to obtain a five-phase winding from the existing standard three-phase stator frames. The proposed rewinding techniques are applied to construct a five-phase star-connected synchronous reluctance machine (SynRM 1) and a novel combined star-pentagon winding (SynRM 2). Simple mathematical formulations are introduced to compute the equivalent winding factor. The performance of five-phase SynRMs is analyzed using two-dimensional Ansys Maxwell transient simulations under the same copper volume for both healthy and faulty cases. SynRM 1 and 2 provide 6.56% and 13.35% higher torque compared to the three-phase SynRM at rated current and at optimal current angle. The torque ripple is decreased by 17% and 30%, respectively. Moreover, SynRM 1 and 2 offer a better performance at the faulty case; e.g., at one phase opened, the average torque of SynRM 1 and 2 is 82% and 108% higher compared to the three-phase SynRM. In addition, the torque ripple is reduced by about 63% and 81%. Finally, experimental results are done to validate the proposed winding techniques.

This article analyses the performance of synchronous reluctance motors (SynRMs) when an existing three-phase stator is rewound (REW) to a higher number of phases. The introduced rewinding technique offers an optimized distribution for air-gap flux, higher winding factor, and higher value for the fundamental component of the magnetomotive force with lower harmonics. The number of turns of the proposed multiphase winding is calculated, keeping fixed copper volume. Finite element simulations verified that the torque density of the REW five- and seven-phase SynRMs is enhanced by 6.56% and 3.37%, respectively, over the existing machine under rated conditions. The torque ripple is also reduced by 17.13% and 15.87%, respectively. Furthermore, the efficiency is enhanced with the REW machines by 0.3% at rated condition. Moreover, the torque gain is greatly increased to 23.48% at higher speed (9000 rpm) and the efficiency is also increased by 3%. The main advantage of the REW machine is clearly observed in case of an open-circuit fault. With one-phase opened, the REW machine can work at 78% of the healthy rated torque, while the three-phase machine works at only 43% with a very huge torque ripple about 228%. Experimental measurements are obtained to validate the theoretical work.