Soft magnetic materials are key to the efficient operation of the next generation of power electronics and electrical machines (motors and generators). Many new materials have been introduced since Michael Faraday's discovery of magnetic induction, when iron was the only option. However, as wide bandgap semiconductor devices become more common in both power electronics and motor controllers, there is an urgent need to further improve soft magnetic materials. These improvements will be necessary to realize the full potential in efficiency, size, weight, and power of high-frequency power electronics and high-rotational speed electrical machines. Here we provide an introduction to the field of soft magnetic materials and their implementation in power electronics and electrical machines. Additionally, we review the most promising choices available today and describe emerging approaches to create even better soft magnetic materials.

http://science.sciencemag.org/content/sci/362/6413/eaao0195.full.pdf

Soft magnetic materials for a sustainable and electrified world

This paper gives an overview on the history and trends of magnetic materials used in electrical machines and motors. The presented materials include silicon–iron, nickel–iron, and cobalt–iron lamination steels, as well as amorphous and nanocrystalline magnetic materials and soft magnetic composites. Development trends and current usage of these selected materials are presented, giving an outlook on the new magnetic material research with regard to electrical machine applications.

Soft Magnetic Material Status and Trends in Electric Machines

Disruptive innovations in electrical machine design optimization are observed in this paper, motivated by emerging trends. Improvements in mathematics and computer science enable more detailed optimization scenarios that cover evermore aspects of physics. In the past, electrical machine design was equivalent to investigating the electromagnetic performance. Nowadays, thermal, rotor dynamics, power electronics, and control aspects are included. The material and engineering science have introduced new dimensions on the optimization process and impact of manufacturing, and unavoidable tolerances should be considered. Consequently, multifaceted scenarios are analyzed and improvements in numerous fields take effect. This paper is a reference for both academics and practicing engineers regarding recent developments and future trends. It comprises the definition of optimization scenarios regarding geometry specification and goal setting. Moreover, a materials-based perspective and techniques for solving optimization problems are included. Finally, a collection of examples from the literature is presented and two particular scenarios are illustrated in detail.

Modern Electrical Machine Design Optimization: Techniques, Trends, and Best Practices

This paper presents a comprehensive over-view of the latest studies and analyses of the cooling technologies and computation methods for the automotive traction motors. Various cooling methods, including the natural, forced air, forced liquid, and phase change types, are discussed with the pros and cons of each method being compared. The key factors for optimizing the heat transfer efficiency of each cooling system are highlighted here. Furthermore, the real-life examples of these methods, applied in the latest automotive traction motor prototypes and products, have been set out and evaluated. Finally, the analytical and numerical techniques describing the nature and performance of different cooling schemes have been explained and addressed. This paper provides guidelines for selecting the appropriate cooling methods and estimating the performance of them in the early stages of their design.

/pdf/cooling-of-automotive-traction-motors-schemes-examples-and-1j9lmz0zl4.pdf

Cooling of Automotive Traction Motors: Schemes, Examples, and Computation Methods

Six-step operation has many advantages in permanent-magnet synchronous machine (PMSM) drives such as maximum power utilization and widened flux-weakening region. However, due to the maximum utilization of inverter output, saturation of current regulator makes it difficult to maintain instantaneous current control capability. Accordingly, in most of conventional research studies, the six-step operation has been implemented by voltage angle control without current regulation, whose dynamic performance is quite unsatisfactory. This paper proposes a control scheme for the six-step operation of PMSM with enhanced dynamic performance of current control. By collaborative operation of dynamic overmodulation, flux weakening, and a technique for enhanced dynamic performance, the six-step operation is realized without losing instantaneous current control capability. Simulations and experiments are carried out to verify the effectiveness of the proposed control scheme. The experimental results show 17% extension of the constant torque region and 27% enhancement of torque capability at three times of the base speed.

Six-Step Operation of PMSM With Instantaneous Current Control

One important factor in the design process and optimization of electrical machines and drives are iron losses in the core. By using new composite materials and low-loss electrical SiFe steels, the losses in the magnetic flux conducting parts of the machine can be reduced significantly. However, it is necessary to have accurate but also easy implementable iron loss models to take the loss effects into account, preferable already during the first design steps and simulations of new electrical machines.The goal of this paper is to give an overview of available iron loss models and to summarize and compare certain models for analytical and numerical machine design methods.

/pdf/overview-and-comparison-of-iron-loss-models-for-electrical-3k5f554zqz.pdf

Overview and Comparison of Iron Loss Models for Electrical Machines

This paper presents a practical approach to model thermal effects in directly cooled electric machines. The main focus is put on modeling the heat transfer in the stator winding and to the cooling system, which are the two critical parts of the studied machines from a thermal point of view. A multisegment structure is proposed that divides the stator, winding, and cooling system into a number of angular segments. Thereby, the circumferential temperature variation due to the nonuniform distribution of the coolant in the cooling channels can be predicted. Additionally, partial computational fluid dynamics (CFD) simulations are carried out to model the coolant flow in the cooling channels and also on the outer surface of the end winding bodies. The CFD simulation results are used as input to the analytical models describing the convective heat transfer to the coolant. The modeling approach is attractive due to its simplicity since CFD simulations of the complete machine are avoided. The proposed thermal model is evaluated experimentally on two directly cooled induction machines where the stator winding is impregnated using varnish and epoxy, respectively. A good correspondence between the predicted and measured temperatures under different cooling conditions and loss levels is obtained.

Thermal Modeling of Directly Cooled Electric Machines Using Lumped Parameter and Limited CFD Analysis

This article presents an up-to-date magnetic material investigation and overview on soft magnetic materials used in rotating electrical machines. The focus is on small-to-medium-sized high-performance and high-efficiency permanent-magnet and induction motors for different application scenarios. The investigated materials include fully processed silicon-iron (SiFe), nickel-iron (NiFe), and cobalt-iron (CoFe) lamination steels as well as soft magnetic composites (SMCs) and amorphous magnetic materials. This article focuses on the magnetic properties and iron losses as well as the manufacturing influence and required thermal treatments during the manufacturing process. A new loss-to-flux-density factor is introduced to compare the magnetization curve and the iron losses of different materials within the same diagram. This article provides a review and comparison of magnetic substances for use in high-performance machines.

/pdf/magnetic-materials-used-in-electrical-machines-a-comparison-1350yaz8tk.pdf

Magnetic Materials Used in Electrical Machines: A Comparison and Selection Guide for Early Machine Design

In this paper, the thermal impact of using different impregnation materials on high-performance liquid-cooled electric machines is studied. In this regard, varnish, Epoxylite, and a silicone-based thermally conductive material are considered. To study thermal effects of using different impregnation materials in theory, an advanced lumped-parameter thermal model of the studied electric machines is developed. In addition to the simulation studies, three identical induction machines using the aforementioned materials are manufactured and evaluated. Experimental tests are carried out at a wide range of current magnitudes and cooling conditions. A good agreement between the temperature measurements and corresponding simulation results is observed. It is demonstrated that using innovative thermally conductive materials in the stator slots and the end winding bodies of liquid-cooled electric machines results in a significant reduction in the winding hot spot temperature. Additionally, the influence of the critical parameters on the impregnation material performance, e.g., impregnation goodness and slot fill factor, is studied.

Andreas Krings

Papers

Overview and Comparison of Iron Loss Models for Electrical Machines

Soft Magnetic Material Status and Trends in Electric Machines

Thermal Modeling of Directly Cooled Electric Machines Using Lumped Parameter and Limited CFD Analysis

Magnetic Materials Used in Electrical Machines: A Comparison and Selection Guide for Early Machine Design

Evaluation of Impregnation Materials for Thermal Management of Liquid-Cooled Electric Machines