Citation: Barmpatza, A.C. The Neutral Voltage Difference Signal as a Means of Investigating Eccentricity and Demagnetization Faults in an AFPM Synchronous Generator. Machines 2023, 11, 647. https:// doi.org/10.3390/machines11060647 Academic Editor: Ahmed Abu-Siada Received: 30 April 2023 Revised: 30 May 2023 Accepted: 5 June 2023 Published: 14 June 2023 Copyright: © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). machines Article The Neutral Voltage Difference Signal as a Means of Investigating Eccentricity and Demagnetization Faults in an AFPM Synchronous Generator Alexandra C. Barmpatza Department of Mechanical Engineering, Hellenic Mediterranean University, 71004 Heraklion, Greece; abarmpatza@hmu.gr; Tel.: +30-694-2479943 Abstract: This article investigates the neutral voltage difference signal, V NO signal, for fault diagnosis. The aforementioned signal is the signal of the voltage between the common star point of the stator and the common star point of the load. The under-study faults are demagnetization and static eccentricity faults, while the machine in which the faults are investigated is an axial flux permanent magnet (AFPM) synchronous generator, suitable for wind power applications. This study was conducted using a 3D finite element method (3D-FEM), and the machine’s FEM model was validated through experiments. This method is one of the most accurate methods for electrical machine computation, allowing for a detailed study of electromagnetic behavior. The components that constitute the V NO signal were determined using a 3D-FEM software program (Opera 18R2). Subsequently, further analysis was performed using MATLAB R2022b software, and a fast Fourier transform (FFT) was applied to this signal. In all the investigated faulty cases, new harmonics appeared, and the healthy amplitudes of most of the already existing harmonics increased. These findings can be used for fault identification. The analysis revealed that the harmonic frequency of 1.5f s was the most dominant in the case of demagnetization, while in the case of static eccentricity, the most dominant harmonic was a frequency equal to the machine’s operating frequency, f s . The novelty of this study is that this signal has not previously been used for fault identification, especially in AFPM synchronous machines. This signal depends on EMF voltage and stator phase currents but is less sinusoidal. Consequently, it can detect faults in cases where the aforementioned signals cannot be used for detection. Keywords: demagnetization; eccentricity; fault diagnosis; permanent magnet; synchronous generator 1. Introduction Fault diagnostics is a very important subject in the field of electrical machines, as faults reduce the efficiency and increase the cost of every electromechanical system. The majority of existing studies investigate faults in asynchronous machines, which are the most popular machine type in the industry [1–5]. Regarding synchronous machines, there are also many studies that are dedicated to faults in this machine type. However, these investigations are focused on radial flux synchronous machines (RFSMs), which are a more widespread topology than axial flux synchronous machines (AFSMs). Recently, AFSMs, especially those with topologies containing permanent magnets, have increasingly appeared in the industry, with many applications such as in wind power energy systems, electrical vehicles, ship propulsion, elevators, floppy disk drives, low-torque servomotors, etc. [6–8]. This type of machine presents many advantages compared with conventional RFSMs. AFSMs used in wind energy conversion (WEC) systems provide high output power, high torque, operational ability in a significant speed range, significant reliability, and the opportunity to eliminate the gearbox by adding more magnetic poles in the generator. Indeed, the price of permanent magnets (PMs) is still high; however, a mechanical part such as the gearbox also increases the need for maintenance and the overall cost of the system. In addition, Machines 2023, 11, 647. https://doi.org/10.3390/machines11060647 https://www.mdpi.com/journal/machines