Study and Optimal Design of a Direct-Driven Stator Coreless Axial Flux Permanent Magnet Synchronous Generator with Improved Dynamic Performance
Round 1
Reviewer 1 Report
1. The English should be improved. There are many grammatical errors in this paper.
2. Fig.1 should be improved to better understand the motor structure.
3. It is confused of equation (1)-(4). Please give more explanation.
4. In section 2.2.1, there are grammatical errors in sentence “With the assumption and condition of no stator core loss, no cogging torque, no saturation in magnetic circuit and ignoring some other complex magnetic field influence”. What are “other complex magnetic field influence”? Why can they be ignored?
5. Figure 3 is poor to read.
6. In section 2.2.3, the authors said that the dynamic response of AFPMSG to the direct driven variable speed condition can be obtained by adjusting the input condition and generator parameters. Please explain more about how to adjust them.
7. Figure 7 is hard to read.
8. GA is a common optimization method of the motor. Please claim your novelty in detail.
9. No experimental tests had been done to verify the simulation.
Author Response
Dear reviewers:
Thank you for your letter and for the comments concerning our manuscript entitled “Study and Optimal Design of Direct-driven Stator Coreless Axial Flux Permanent Magnet Synchronous Generator with Improved Dynamic Performance” (ID: energies-384356, the former name is Dynamic Mechanical Performance Study and Optimization Design of Stator Coreless Axial Flux Permanent Magnet Synchronous Generator for Direct Driven Variable Speed Application, which has been revised.). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the comments are list point by point as following:
Response to Reviewer 1 Comments
Point 1：The English should be improved. There are many grammatical errors in this paper.
Response 1: Thanks for your comments. We are sorry for the English and grammatical errors in this paper due to limited translation ability. Considering the English requirements, your kind suggestion and our English expression ability, we have adopted the MDPI English editing service to improve the English after we revise the paper. We can offer the English editing certificate if it is necessary. We hope the English of latest revised article will be more readable.
Thank you again for your comments which is important for me to know the shortcomings and do better in the future work!
Point 2：Fig.1 should be improved to better understand the motor structure.
Response 2：Thanks for the referee’s kind suggestion. We are very sorry for our unclear expression and we intend to introduce the motor structure with structural assembly drawing, axial section and simulation model. The structural assembly drawing showing the whole motor structure is added to the Fig.1 in the paper to enhance the readability. And the axial section illustrates the main structure parameters of each part. To improve the simulation efficiency, some simplifications are conducted which make the simulation model looks a little different with the original structure. Besides, we also do some adjustment to the simulation model to make it more readability. We hope the modification of Fig.1 will be helpful to better understand the motor structure. The revised Fig.1 is showed below.
Figure 1. APFMSG configuration without stator core and simulation model. (a) Assemble model; (b) Ironless stator structure; (c)simulation model
Point 3: it is confused of equation (1)-(4). Please give more explanation.
Response 3：We are sorry for not expressing the meaning of equation (1)-(4) clearly. Equation (1)-(4) are basic electromagnetic relationship of AFPMSG, which are refer to the book “Axial flux permanent magnet brushless machines”. The reference is shown below. The equations are given here for two purposes: the first one is to give a brief introduce about the theoretical relationship of AFPMSG, and the other is to show that the electrical performance and operation state are directly affected by the structure parameters. Equation (1), (2), (4) indicate the relationship between the EMF (electromotive force), electromagnetic torque, electromagnetic power and structural parameters. Equation (3) represents the influence of electrical parameters established by structural parameters on the operation state. As you suggested, the equation (1)-(4) are explained and the relate contents are modified in the paper, as show below. Thank you for the comments. We will pay more attention to specific contents to avoid confusion.
Gieras J F, Wang R J, Kamper M J. Axial flux permanent magnet brushless machines [M]: Springer Netherlands, 2005.
The reference has been inserted into the corresponding content in the paper.
“Electromagnetic relation is the basis of motor analysis and research. The basic electromagnetic relation of AFPMSG are the steady state equations and main sizing equations [3], as shown below. Equation (1), (2), (4) indicate the relation between the EMF (electromotive force), electromagnetic torque, electromagnetic power and structural parameters. Equation (3) represents the influence of electrical parameters established by structural parameters on the operation state. From the electromagnetic relation, the transformation relation inside the generator can be known. It can be seen that the steady state equations and main sizing equations respectively represent the operation characteristic under different conditions and relationship between the rated design power and structure dimensions. Also it is obvious that the structure and dimensions have great influence on output performance of generator.”
Point 4: In section 2.2.1, there are grammatical errors in sentence “With the assumption and condition of no stator core loss, no cogging torque, no saturation in magnetic circuit and ignoring some other complex magnetic field influence”. What are “other complex magnetic field influence”? Why can they be ignored?
Response 4：Thanks for your comments, what you have mentioned is really something that needs to be clarified to make the article more rigorous.
First, the grammatical errors are corrected. The sentence are revised as follow. “Considering the conditions and assumptions such as no loss, torque pulsation, unsaturated magnetic circuit and ignoring the complex magnetic field effects”
Second, the complex magnetic field effects mainly include cross magnetization, mutual coupling between the d− and q−axis circuits, the interpole leakage or magnetic pole edge leakage. There are two main reasons for ignoring the complex magnetic field effects:
1. The magnetic field of motor is complicate, expecially when excitation system and prime mover are considered. There are main magnetic field, leakage magnetic field, alternating direct axis mutual inductance and so on. Therefore, in practical application, the mathematical model of synchronous motor is often simplified to different degrees so that it can be used in different applications. The AFPMSG studied in the paper is excited by the PM material and here is focused on the dynamic performance. Second order reduced model that ignoring some magnetic field effects is enough and adopted to the dynamic model of AFPMSG using synchronous d-q rotating reference frame to get the modeling equations, which has been verified and used in reference literature. The references are listed below and has been add to the paper.
(1) Bang D, Polinder H, Shrestha G, Ferreira JA. Review of generator system for direct- drive wind turbines. In: European Wind Energy Conference & Exhibition, Belgium; 2003. p. 1-11.
(2) Patil K, Mehta B. Modeling and control of variable speed wind turbine with permanent magnet synchronous generator[C]// International Conference on Advances in Green Energy. IEEE, 2015:258-264.
(3) Yin M, Li G, Zhou M, et al. Modeling of the Wind Turbine with a Permanent Magnet Synchronous Generator for Integration[C]// Power Engineering Society General Meeting. IEEE, 2007:1-6.
2. The stator winding of AFPMSG is made of non-ferromagnetic material, which makes the equivalent air gap length larger than other topologies. The complex magnetic fields ignored in this paper are not the main magnetic fields themselves, and their influence is further weakened due to the large air gap. Therefore, on the premise of meeting the research demand, the motor magnetic field is simplified due to the insignificant effects.
In order to make the article more rigorous, we add some precondition explanation of motor dynamic model equation derivation in the paper. Thank you for your comments again. Your comments will help us improve the comprehensibility and completeness in our future work.
“According to the practical need, Considering the conditions and assumptions such as no loss, torque pulsation, unsaturated magnetic circuit and ignoring the complex magnetic field effects , second order reduced model is adopted for dynamic study[] and the dynamic equations of output voltage are given by….”
Point 5: Figure 3 is poor to read.
Response 5：Thank you for your careful review and guidance. We are sorry for the unclarity in Figure 3 and it has been replaced.
Point 6: In section 2.2.3, the authors said that the dynamic response of AFPMSG to the direct driven variable speed condition can be obtained by adjusting the input condition and generator parameters. Please explain more about how to adjust them.
Response 6：Thanks for your suggestion. It does needs to be explained to be more understandable. The dynamic analytical model of AFPMSG is established by the generator parameters including structure and electrical parameters while the dynamic response is obtained by simulation based on the model. Keep the generator parameters unchanged and adjust the input torque or speed conditions, the response of AFPMSG itself to different operation condition can be studied. In the same way, keep the input condition constant but change one generator parameter, the sensitivity of the parameter to performance can be analyzed. Then we can optimize the parameter design of AFPMSG to improve the dynamic performance with unstable input conditions. As you suggested, more explanation are inserted in the paper.
“Three steps are taken to analyze the dynamic performance of AFPMSG. First, the dynamic response of generator under different input conditions is found by changing the input torque or speed condition according to the typical input disturbance. Second, the control variable method is applied to change one parameter while the others are kept unchanged to find out the sensitivity of main parameter of AFPMSG to the performance. The last step is to adjust the generator parameters on the basis of optimization to improve the dynamic performance.”
Point 7: Figure 7 is hard to read.
Response 7：We are sorry for not articulating the contents clearly in Figure 7. Left picture of Figure 7 shows the performance response of AFPMSG under constant start torque including the maximum output value and time to be stable. Similarly, right picture shows the performance response with step torque input. As you reminding, Figure 7 has been replaced and instructions are added.
Point 8: GA is a common optimization method of the motor. Please claim your novelty in detail.
Response 8：Thank you for your professional question. GA has many outstanding advantages, such as generality, parallelism, robustness, simplicity and strong global optimization ability, and is suitable for solving complex and difficult global optimization problems. Mahmoudi, Rostami N et al. have applied GA to optimize the design of AFPM motor including minimizing the active material cost, maximum power density, low cogging torque[1][2]. However, the aim of this paper is to analyze the relation between generator parameter and dynamic performance and optimize the design to improve the dynamic performance. Due to the multiplicity of parameters involved in the design and the nonlinear equations complicating the relationship among the parameters, the problem at hand will not yield an optimized outcome through manual design, requiring soft computing methods. And there are little application of GA for dynamic performance optimization. GA was proposed in the paper to optimize the design of AFPMSG to improve the dynamic performance. Simple and utility algorithm is preference. To avoid the problems of slow convergence and "precocity", adjustable crossover probability and mutation probability are used in the GA process. The GA in this paper mainly is used as a tool to obtain the optimization design results for dynamic performance. The Flowchart of the genetic algorithm optimization design procedure is revised and the sentences are inserted for further describe the genetic algorithm used in the paper.
1. Rostami N, Feyzi M R, Pyrhonen J, et al. Genetic algorithm approach for improved design of a variable speed Axial-Flux Permanent-Magnet synchronous generator [J]. IEEE Transactions on Magnetics, 2012, 48(12): 4860-4865.
2. Mahmoudi A, Kahourzade S, Rahim N A, et al. Design, analysis, and prototyping of an Axial-Flux permanent magnet motor based on genetic algorithm and Finite-Element analysis [J]. IEEE Transactions on Magnetics, 2013, 49(4): 1479-1492.
The Flowchart of the genetic algorithm optimization design procedure
“GA was adopted in the paper to perform the calculation as a part of design procedure for the best optimization result. Thus simple and utility algorithm is preference. To avoid the problems of slow convergence and "precocity", adjustable crossover probability and mutation probability are used in the GA process.”
Point 9: No experimental tests had been done to verify the simulation.
Response 9：Thank you for your comments. The comparison you mentioned makes a lot of sense. In fact, the work done in this paper is part of a series of design and optimization. The AFPMSG studied in the paper is used in the direct drive wave power generation system, which is supported by national support project of PRC and key project of PLA. The actual application environment of generator is bad and the unstable input conditions is one of them. This paper mainly focuses on the relationship and optimization of generator parameters and dynamic performance on terms of AFPMSG itself, which is only part of the AFPMSG direct drive wave power generation system application. Other researches on the application in the direct drive wave power generation system are undergoing. The need for generator optimization in this paper comes from the real wave power converter device and the original design data refers to real AFPMSG. The results come from optimization calculation and original data, which are credible. In order to get closer to the actual application of the motor, all the simulation condition including the accurate 3D model and settings are fully considered. In the future, it will be compared with the laboratory experiment and the actual sea experiment results as the generator device of direct drive wave power generation system, which will be fully considered in the subsequent research process, so as to improve the research results. Thank you for your advice!
We tried our best to improve the manuscript and made some changes in the manuscript according to the reviewer’s comments. What’s more, after careful examination and modification, we also made some small changes on grammatical errors and so on. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in red in revised paper. We appreciate for editors/reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Looking forward to hearing from you.
Thank you and best regards.
Yours sincerely
Weijun Wang
Author Response File: 
Author Response.docx
Reviewer 2 Report
-Title is too long, it is recommended to shorten it.
-the abstract should be reformulated to reveal the main contribution of the paper.
-Several English typos are observed e.g. “With the
increasing application of” should be “With the increased application of”. The paper
should be fully revised to correct English and grammar mistakes.
-Authors are to clarify whether design data in Table 1 are for real machine or
not?
-legibility of parameters labels in Fig. 3 needs to be improved.
-It is not clear how the dynamics of the machine is taken into account in the simplified model shown in Fig. 4. Would this model provides accurate dynamic performance similar to the actual machine?
-how the nonlinearity of the machine dynamics been taken into account?
-The disturbance scenario investigated in Fig. 7 assumes, offline disturbance. What this fault could represent in practical applications?
-The performance of the optimized machine should be compared with the dynamic performance with the un-optimised one.
-Legend in Fig. 8 is not clear.
Author Response
Dear reviewer:
Thank you for your letter and for the comments concerning our manuscript entitled “Study and Optimal Design of Direct-driven Stator Coreless Axial Flux Permanent Magnet Synchronous Generator with Improved Dynamic Performance” (ID: energies-384356, the former name is Dynamic Mechanical Performance Study and Optimization Design of Stator Coreless Axial Flux Permanent Magnet Synchronous Generator for Direct Driven Variable Speed Application, which has been revised.). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the comments are list point by point as following:
Response to Reviewer 2 Comments
Point 1：Title is too long, it is recommended to shorten it.
Response 1: Thank you for your kind suggestion. As you suggested, the title are shorten as below.
“Study and Optimal Design of Direct-driven Stator Coreless Axial Flux Permanent Magnet Synchronous Generator with Improved Dynamic Performance”
Point 2: the abstract should be reformulated to reveal the main contribution of the paper.
Response 2: Thanks for your evaluation and kind suggestion. We are very sorry for our unclear expression and we intend to reformulate the abstract in the following paragraph.
“In this paper, the study and optimization design of stator coreless axial flux permanent magnet synchronous generators is presented for direct-driven variable speed renewable energy generation system application considering the requirement of reliability and dynamic performance with unstable input conditions. The dynamic analytical model is developed based on the investigation of AFPMSG structure and basic electromagnetic equations to find out the relationship between generator parameters and dynamic performance. Simulation via Matlab/Simulink platform is carried out to obtain the sensitivity of generator parameters to dynamic performance. An integrated optimization model taking the key parameters as variables is proposed aiming to improve the mechanical dynamic performance of AFPMSG. For accurate design, the design procedure is modified by combining the nonlinear iterative genetic algorithm (GA) to perform the calculation. A 3kW AFPMSG is optimal designed to minimize the output voltage overshooting—the index of dynamic performance for direct driven variable speed generation application. Finally, 3D finite element model of the generator is established by Maxwell ANSOFT and simulation results confirm the validity of the dynamic performance analysis and optimal design procedure.”
Thank you again for your kind suggestion. We will improve our wring in the future work!
Point 3: Several English typos are observed e.g. “With the increasing application of” should be “With the increased application of”. The paper should be fully revised to correct English and grammar mistakes.
Response 3: Thank you for your comments. We are sorry to have made mistakes while writing and translating. Considering our English level and translation ability, we have adopted the MDPI English editing service to correct the English and grammar mistakes. We can offer the English editing certificate if it is necessary. We hope the English of latest revised article will be more readable.
Thank you again for your comments which is important for me to know the shortcomings and do better in the future work!
Point 4: Authors are to clarify whether design data in Table 1 are for real machine or not?
Response 4: Thank you for your question. We are sorry for not clarity the design data in Table 1. And it is confirmed that design data in Table 1 are for real machine. The AFPMSG studied in the paper is used in the direct drive wave power generation system, which is supported by national support project of PRC and key project of PLA. The need for generator optimization in this paper comes from the real wave power converter device and the original design data refers to real AFPMSG. Instructions about the design data are added:
“The configuration and simulation model schematic as well as the specifications referring to the real AFPMSG are given in Fig. 1 and Table 1.”
Point 5: legibility of parameters labels in Fig. 3 needs to be improved.
Response 5: Thank you for your comments. We are sorry for the illegibility of parameters labels in Fig. 3 and the Fig.3 has been replaced by a new one. We will pay more attentions on the clearly expression of research material.
Point 6: It is not clear how the dynamics of the machine is taken into account in the simplified model shown in Fig. 4. Would this model provides accurate dynamic performance similar to the actual machine?
Response 6: Thank you for your comments. The question you mentioned is the basis of this paper. Fig.4 shows the dynamic model of AFPMSG, which including the input and output parameters, electrical parameters and structural parameters. The simplified model in Fig.4 is established based on the voltage equations, electromagnetic power and torque equations as given (5-13) in the paper. Those equations also called basic equations illustrate the relationship between generator and structural parameters. The basic equation is derived from the second order practical model of synchronous motor， which has been widely used dynamic study in PMSG[1-4]. Therefore the model in Fig.4 is enough to simulate the AFPMSG including the dynamic performance. The sentence “the model in Fig.4……generator parameters” has been added.
“The model in Fig.4 contains all the main parameters needed for dynamic performance. From the model, it is obvious that the dynamic performance can be simulated with different input conditions and can be improved by choosing the proper generator parameters.”
Equations of synchronous motor are very complicated if considering the electromagnetic and mechanical transient process. Thus, in the actual engineering process, the mathematical model of synchronous motor is simplified to different degrees according to the needs of research and application. The model in Fig.4 simplified from the synchronous motor model cannot being 100 percent similarity to the actual machine, but it provides enough accurate and easier for dynamic performance analysis.
1. Ahmed D, Karim F, Ahmad A. Design and modeling of low-speed axial flux permanent magnet generator for wind based micro-generation systems[C]// International Conference on Robotics and Emerging Allied Technologies in Engineering. IEEE, 2014:51-57.
2. Patil K, Mehta B. Modeling and control of variable speed wind turbine with permanent magnet synchronous generator[C]// International Conference on Advances in Green Energy. IEEE, 2015:258-264.
3. Yin M, Li G, Zhou M, et al. Modeling of the Wind Turbine with a Permanent Magnet Synchronous Generator for Integration[C]// Power Engineering Society General Meeting. IEEE, 2007:1-6.
4.Tobías-González A, Peña-Gallardo R, Morales-Saldaña J, et al. Modeling of a wind turbine with a permanent magnet synchronous generator for real time simulations[C]// IEEE International Autumn Meeting on Power, Electronics and Computing. IEEE, 2016:1-6.
Point 7: how the nonlinearity of the machine dynamics been taken into account?
Response 7: Thank you for professional question. In my opinion, there are two aspect about the nonlinearity of machine dynamics. The first aspect is about the input conditions. If the unstable input condition is random and nonlinear, the output dynamic performance can be obtain by linearization and superposition. The input instability condition is linearized, and the multi-segment is computed and then superimposed. The other aspect is that the effect of motor parameters on its dynamic performance is non-linear and interaction. In the paper, the sensibility of main motor parameters to dynamic performance are analyzed and an integrate optimization model considering all the main motor parameters is proposed to optimize the design of AFPMSG. Besides, GA has been used to perform the calculation to get the best optimization result.
Point 8: The disturbance scenario investigated in Fig. 7 assumes, offline disturbance. What this fault could represent in practical applications?
Response 8: Thank you for your comments. The proper choice of simulation scenario is important for the whole research. The disturbance scenario investigated in Fig.7 is mainly to analyze the dynamic response of AFPMSG to the disturbance. This fault presents step torque input disturbance caused by a short-term prime mover or load change in practical applications. Although the disturbance scenario assumes offline step torque disturbance, it can be added to any stable online system in practical applications. A similar disturbance can occur with decelerating wind turbines or fueling the machine. In addition, step disturbance is a common disturbance. Some complex disturbances can be decomposed into overlapping multi-step disturbances. Thus, the step torque disturbance scenario as offline disturbance is study in the paper.
Point 9: The performance of the optimized machine should be compared with the dynamic performance with the un-optimised one.
Response 9: Thank you for your suggestion, the comparison you mentioned makes a lot of sense. In fact, the comparison of electrical performance and dynamic performance before and after optimization is considered and completed but we are sorry for not clearly state. Fig.16 (b) compares the electrical performance including output voltage and power before and after optimization. And Fig.17 (b) compares the output voltage response to random disturbance, from which we can see that the optimized generator has lower voltage leap than the original one. In other word, the dynamic performance of optimized generator behaves more stable to the disturbance. Fig.17 (c) compares the maximum voltage overshoot with different disturbance input. It is obvious that the inhibition of voltage leap for the optimized generator is more apparent when the input is heavy disturbance. In addition, comparison data on the electrical performance, cost of PM and dynamic performance are listed in Table.3. According to your suggestion, some sentences are inserted in the paper to make the comparison more understandable.
“To compare the dynamic performance, output voltage response to the same continuous disturbance torque input is simulated and analyzed before and after the optimization of the AFPMSG. The output voltages before and after the optimization are shown in Fig. 17(a) while Fig. 17(b) illustrates the envelop curve of maximum output voltage. The output voltage response of optimized generator is slower and smaller than the preliminary one. It is observed that the dynamic response of output voltage after optimization became more stable, which exerts better inhibition of voltage leap in case of heavy disturbance. The comparison of maximum output voltage overshoot with different input torque is indicated in Fig. 17(c). The markings in Fig.17 (b) and (c) indicates that the output voltage becomes more stable and the maximum output voltage overshoot is reduced making the system being more safety and reliability after the optimization. Moreover, the inhibition of voltage leap for the optimized generator is more apparent when the input is heavy disturbance. Conclusions are obtained that the dynamic performance of optimized AFPMSG is better and the optimized AFPMSG can operate more stable and reliable with unstable input conditions.”
Point 10: Legend in Fig. 8 is not clear.
Response 10: Thank you for your reminding. We are sorry for not expressing the legend clearly. Fig.8 has been replaced with a clear legend. We will learn the lesson and improve ourselves in the future!
We tried our best to improve the manuscript and made some changes in the manuscript according to the reviewer’s comments. What’s more, after careful examination and modification, we also made some small changes on grammatical errors and so on. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in red in revised paper. We appreciate for editors/reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Looking forward to hearing from you.
Thank you and best regards.
Yours sincerely
Weijun Wang
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I am happy with the revision. This paper can be published.
Reviewer 2 Report
Authors have addressed all my concerns.
