  Citation: Prajzendanc, P.; Paplicki,P. Performance Evaluation of an Axial Flux Machine with a Hybrid Excitation Design. Energies 2022, 15, 2733. https://doi.org/10.3390/ en15082733 Academic Editors: Quntao An, Bing Tian and Xinghe Fu Received: 25 February 2022 Accepted: 4 April 2022 Published: 8 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. 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/). energies Article Performance Evaluation of an Axial Flux Machine with a Hybrid Excitation Design Pawel Prajzendanc * and Piotr Paplicki Faculty of Electrical Engineering, West Pomeranian University of Technology, 70-313 Szczecin, Poland; piotr.paplicki@zut.edu.pl * Correspondence: pawel.prajzendanc@zut.edu.pl Abstract: Variable speed, permanent magnet synchronous machines with hybrid excitation have attracted much attention due to their flux-control potential. In this paper, a design of permanent magnet axial flux machines with iron poles in the rotor and an additional electrically controlled source of excitation fixed on the stator is presented. This paper shows results pertaining to air-gap flux control, electromagnetic losses, electromagnetic torque, back emf and efficiency maps obtained through field-strengthening and weakening operations and investigated by 3D finite element analysis. Moreover, the temperature distribution of the machine was analyzed according to the fluid–thermal coupling method. The presented machine was prototyped and experimentally tested to validate the effectiveness of numerical models and achieved results. Keywords: permanent magnet machines; axial flux machine; hybrid excitation; variable speed machines 1. Introduction Nowadays, electrical machines with high efficiency and good reliability are required. Although permanent magnet machines are suitable and have excellent efficiency, they have some flux-control limitations, especially in the field-weakening region at high rotor speed [1]. Permanent magnet (PM) machines with flux-weakening features are desirable in drives with a wide range of rotational speeds (e.g., in electrical vehicles or in wind turbine generators operated under different weather conditions). Alternative design solutions for permanent magnet machines with hybrid excitation (HE) have been proposed, in which there is an additional source of field excitation, usually in the form of an additional coil electrically controlled by DC current and fixed on the machine stator [2,3]. The air-gap magnetic flux density is created by PMs and an additional magnetic field excited by the DC coil. In contrast to a parallel HE system, a serial one has an additional magnetic flux that directly affects the PM and changes the operating point of the magnet. Hence, the parallel system is more suitable for magnets from a demagnetization point of view. Significant progress in hybrid excitation machines can also be observed in reluctance or synRM machines [4–7] and permanent magnet synchronous machines [8–12]. An additional excitation coil can be fixed on the stator [13,14] or in the rotor [15,16], which depends on the concept. In the literature, axial flux machines with hybrid excitation concepts [15–23] can be found where an additional coil is most often fixed on the stator. In this paper, a concept of axial flux machines with permanent magnets, iron poles and an additional DC coil for magnetic flux control fixed in the middle on the stator is presented. The advantages of the proposed machine design solution are: a good range of magnetic flux regulation, brushless supplying stator windings and an additional coil, no demagnetization risk for the magnets, low cost and volume of the magnets compared to conventional machines and a high efficiency of up to 95%. The main drawbacks are: extra Energies 2022, 15, 2733. https://doi.org/10.3390/en15082733 https://www.mdpi.com/journal/energies