energies Review A Review of Design Optimization Methods for Electrical Machines Gang Lei 1,*, Jianguo Zhu 1, Youguang Guo 1, Chengcheng Liu 2 and Bo Ma 1 1 School of Electrical and Data Engineering, University of Technology Sydney, Ultimo 2007, Australia; [email protected] (J.Z.); [email protected] (Y.G.); [email protected] (B.M.) 2 State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, Tianjin 300131, China; [email protected] * Correspondence: [email protected]; Tel.: +61-2-9514-1268 Received: 30 September 2017; Accepted: 22 November 2017; Published: 24 November 2017 Abstract: Electrical machines are the hearts of many appliances, industrial equipment and systems. In the context of global sustainability, they must fulfill various requirements, not only physically and technologically but also environmentally. Therefore, their design optimization process becomes more and more complex as more engineering disciplines/domains and constraints are involved, such as electromagnetics, structural mechanics and heat transfer. This paper aims to present a review of the design optimization methods for electrical machines, including design analysis methods and models, optimization models, algorithms and methods/strategies. Several efficient optimization methods/strategies are highlighted with comments, including surrogate-model based and multi-level optimization methods. In addition, two promising and challenging topics in both academic and industrial communities are discussed, and two novel optimization methods are introduced for advanced design optimization of electrical machines. First, a system-level design optimization method is introduced for the development of advanced electric drive systems. Second, a robust design optimization method based on the design for six-sigma technique is introduced for high-quality manufacturing of electrical machines in production. Meanwhile, a proposal is presented for the development of a robust design optimization service based on industrial big data and cloud computing services. Finally, five future directions are proposed, including smart design optimization method for future intelligent design and production of electrical machines. Keywords: electrical machines; multi-level optimization; multi-objective optimization; system-level optimization; manufacturing variations; manufacturing quality; robust optimization; industrial big data; cloud computing 1. Introduction 1.1. Energy and Environmental Challenges Electrical machines, as the main drive sources, have been widely employed from industry to agriculture, from defence to community facilities, from domestic appliance to electronic products, etc. They are the foundation of the power industry and the core components of industrial machinery. Electrical machines consume about 46% of total electricity generated worldwide, resulting in about 6040 Mega-tonnes of CO2 emission. This is the largest portion of electricity use up to now, as shown in Figure1. Therefore, motor energy efficiency is crucial for the energy conservation, environment protection, and global sustainable development. Consequentially, high-efficiency motors will dominate the market development of electrical machines worldwide [1–3]. Energies 2017, 10, 1962; doi:10.3390/en10121962 www.mdpi.com/journal/energies Energies 2017, 10, 1962 2 of 31 Energies 2017, 10, 1962 2 of 31 Figure 1. GlobalGlobal electricity demand by sector and end end-use.-use. Besides the energy efficiency,efficiency, therethere areare manymany otherother specificationsspecifications and/orand/or requirements for the design optimization optimization of of electrical electrical machines, machines, such such as astorque, torque, power power density, density, volume volume and andweight. weight. For specialFor special applications applications like geologic like geologic or petroleum or petroleum drilling drilling engineering engineering and aerospace and aerospace engineering, engineering, more requirementsmore requirements and extreme and extreme constraints constraints should should be beinves investigatedtigated [4– [46]–6. ].For For example, example, the the environment environment temperature for motors used in petroleum and geology maymay reachreach 300300 ◦°CC,, or even to 500 °C.◦C. The high surrounding temperature will will result in severe performance degradation due due to to the increase of the copper loss and the demagnetization of permanent magnets (PM) for PM motors. ThusThus,, all performanceperformance specificationsspecifications and and requirements requirements including including energy energy efficiency efficiency are are vital vital for thefor theapplication application of electrical of electrical machines. machines. Consequently, Consequently, improving improving motor performance motor performance is of great significanceis of great signito bothficance the environment to both the environment protection and protection the energy and sustainability. the energy sustainability. To achieve this To goal, achieve optimization this goal, is optimialwaysz necessaryation is always [2,7–11 necessary]. [2,7–11]. 1.21.2.. An An Overview of Design and Optimization of Electrical Machines Design optimization of electrical machines machines includes includes two two main main stages, stages, design design and and optimization optimization.. The main aim of the design stage is to findfind a feasible scheme (or several schemes) for a given application through the investigation of different materials and dimensions, motor typ typeses and topologies, multi multi-disciplinary-disciplinary analysis including electromagnetic analysis, and/or design design experience. experience. The analysis of this stage will provide information including motor parameter calculation and performance evaluation evaluation for for the the development development of ofoptimiz optimizationation model model that that will willbe used beused in the in next the stage next. Thestage. main The target main targetof optimization of optimization stage stageis to improve is to improve the performance the performance of the of motor the motor proposed proposed in the in designthe design stage stage through through some some optimization optimization algorithms algorithms and and methods. methods. As As the the outcome, outcome, an an optimal solution will be obt obtainedained for the the single single objective objective design design situation, situation, and and some some non non-dominated-dominated solutions (called Pareto optimal solutions solutions)) will be gained for the multi-objectivemulti-objective design situation after the completion of this stage. Figure Figure 22 illustratesillustrates aa briefbrief frameworkframework forfor thethe mainmain aspectsaspects coveredcovered inin thethe design and optimization of electrical machines.machines. There isis nono fixedfixed procedureprocedure for for the the design design optimization optimization of of electrical electrical machines. machines. However, However, there there are aresome some common common steps steps to be to followed.be followed. These These steps steps are are briefly briefly described described as follows.as follows. More More details details can can be beseen seen in thein the following following sections: sections: Step 1: Select/determine possible motor types and topologies, materials and dimensions according Step 1: Select/determine possible motor types and topologies, materials and dimensions to the requirements given by the applications and users. Requirements include steady-state and according to the requirements given by the applications and users. Requirements include steady- dynamic performances like efficiency, torque and torque ripple, material and manufacturing costs, state and dynamic performances like efficiency, torque and torque ripple, material and volume and others. The main aim of this step is to obtain a number of motor options which may be manufacturing costs, volume and others. The main aim of this step is to obtain a number of motor suitable for a specific applications. options which may be suitable for a specific applications. Step 2: Implement multi-physics design and analysis for each motor option. Due to the multi-physics Step 2: Implement multi-physics design and analysis for each motor option. Due to the multi- nature, many disciplines have to be investigated in this step, such as electromagnetics, structural physics nature, many disciplines have to be investigated in this step, such as electromagnetics, mechanics and heat transfer [2,12–19]. Moreover, power electronics and control should be included as structural mechanics and heat transfer [2,12–19]. Moreover, power electronics and control should be they are relevant to the dynamic responses of the machines, such as overshoot and settling time [20–23]. included as they are relevant to the dynamic responses of the machines, such as overshoot and This step targets to calculate some parameters for the evaluation of motor performance, including settling time [20–23]. This step targets to calculate some parameters for the evaluation of motor performance, including electromagnetic parameters like core loss, inductance and back electromotive force (EMF), and thermal parameters like temperature rise and distribution. Energies 2017, 10, 1962 3 of 31 electromagnetic parameters like core loss, inductance and back electromotive force (EMF), and thermal parametersEnergies 2017, 10 like, 1962 temperature rise and distribution. 3 of 31 Motor