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Peer-Review Record

Innovative and Highly Integrated Modular Electric Drivetrain

World Electr. Veh. J. 2019, 10(4), 89; https://doi.org/10.3390/wevj10040089
Reviewer 1: Piotr Piórkowski
Reviewer 2: Ming-Tsan Peng
Reviewer 3: Anonymous
World Electr. Veh. J. 2019, 10(4), 89; https://doi.org/10.3390/wevj10040089
Received: 11 October 2019 / Revised: 3 December 2019 / Accepted: 5 December 2019 / Published: 11 December 2019

Round 1

Reviewer 1 Report

Very good paper describing theoretical, simulation and laboratory analyzes at the same time, which does not happen often. The presented drive system is very complex and will definitely be used as an object for the next few papers. The use of a double planetary gear set (Ravigneaux gear set) is particularly interesting. Currently in electric and hybrid drives, the greatest emphasis is placed on the development of power electronics, so the presented drive system from a research point of view positively stands out from other solutions.

The only thing I suggest is to extend the number of items in references. Seven items in references for such a complex object is a bit too little.

Author Response

Dear reviewer, thank you for you work, the good critics and the improvement suggestions. We have revised the article according to your comments. As the main point was missing references, we added 13 additional references, which suport and justify statements we have made also before without references.

Thank you very much

Jonas Hemsen

Reviewer 2 Report

The article presents a promising modular propulsion module for electric vehicles. Through high-speed and high-frequency operation, the system clearly gains many advantages over other traditional counterparts. The article may be more clearly presented if the following information can be further supplemented:

The motor has operated at more than 15000 rpm, and it is better to include information about bearings as to support the high-speed motor's rotor.   The winding reconnection can cope with the fixed voltage supply in a clever way. A method has been described as to reduce cogging torque by shifting PWM 180 degree. However, it is not clear how this degree shifting can actually affect the voltage or current input to the windings.   The GaN device has many advantages, especially reducing passive components' size via high-frequency switching. However, it would be more complete if the article can include more reliability and thermal specifications of these devices since those are key factors for GaN to have successful high-power operation.

Author Response

Dear reviewer, thank you for you work and the improvement suggestions. We have revised the article according to your comments. Following is a list of changes and answers to your comments:

The motor has operated at more than 15000 rpm, and it is better to include information about bearings as to support the high-speed motor's rotor.

-> Information to the bearing concept has been added from line 354

 A method has been described as to reduce cogging torque by shifting PWM 180 degree. However, it is not clear how this degree shifting can actually affect the voltage or current input to the windings.

-> I discussed this topic with the complete consortium, unfortunately we could not find out what you specifically mean with this question. There might just be a misunderstanding, please get back to me if the misunderstanding cannot be solved. We anyways have put some more precision into the wording of some paragraphs.

The GaN device has many advantages, especially reducing passive components' size via high-frequency switching. However, it would be more complete if the article can include more reliability and thermal specifications of these devices since those are key factors for GaN to have successful high-power operation.

-> You are right, reliability is currently an issue of GaN devices, as there is too little experiences with high powers. The aim of the project is to prove its application readiness, long term reliability plays a minor role for now bur must surely be addressed later. We included a sentence in line 307 to make this more understandable to every reader.

Thank you very much

Jonas Hemsen

Reviewer 3 Report

This paper describes a new design of electric powertrain for passenger cars and its development.

The work is interesting but, for such a comprehensive topic, it is too concise and leaves many questions. Some of them:

1) The authors write: “The main goal for the EM development was to save more than 50 % magnet mass compared to existing PMSM with similar power, to reduce the active weight and to improve the efficiency”

What is the model of the benchmark PMSM motor? Please, add a more detailed comparison and a table containing main data of the benchmark PMSM motor and the proposed Permanent Magnet Hybrid Synchronous Machine to the text.

2) The authors write: “However, the key to save energy and battery size is mainly not peak efficiency, but a high drive cycle efficiency”

The statement is obvious and trivial. However, it is not suggested in the text how to reach the goal. For this reason, for a reader it would be interesting to know the objective function to optimize the HSM design

3) The authors write: ”Unfortunately, bonded magnets have less remanence flux density (Br) than sintered magnets but they contain no heavy rare-earth (which is more critical than Nd and whose price might raise more severely if a supply crisis were to occur)”.

Please, explain in more details what grade of the bonded magnets was chosen to use in the HSM design? What grade of the sintered magnets was chosen for the comparison? What specific heavy rare-earth elements do you mention?

4) The authors mention that one of the goal of the study is «low environmental impact».

However, rare-earth magnets are expensive, and mining rare-earth elements harms the environment [2,3].

Probably, there is another valuable option that provides even a “lower environmental impact”. It is to use non-rare earth magnets. For example, synchronous reluctance machines with ferrite magnets can be used [4,5] since mining of ferrites is less harmful for the environment comparing with rare earth.

In addition, you mention that the prices for rare earth elements are unstable and it can cause a crisis for production while the prices for ferrites are low and stable.

Therefore, in the case of use of synchronous reluctance motors with ferrite magnets it is possible to: 1) mitigate the environmental damage siсne the rare earth elements are not applied 2) reduce the total cost of production and avoid the crises due to the unstable prices. 

In the paper, it is needed to explain in more details why the HSM design with bonded magnets was chosen instead of the synchronous reluctance machine with ferrite magnets.

5) Also about the «low environmental impact» you mention.

If the bonded magnets with a comparatively low remanent flux density are used instead of the sintered magnets, it leads to increasing the magnet weight and, therefore, consumption of the rare earth by several times. It is known that mining rare-earth elements harms the environment. Maybe, then the HSM with bonded magnets requires a greater amount of the rare earth ore and therefore the environmental impact is even increased?

6) The rotor mechanical analysis provided for the Delta-type IPMSM in the paper doi:10.3390/en12040641. I think that it is necessary for high speed traction applications. Show the paper rotor von Mises stress distributions at the maximum speed.

7) What is the normalized PM flux linkage λPM/λrated of the Permanent Magnet Hybrid Synchronous Motor considered [6]? If λPM/λrated< 0.5 then, according to commonly accepted terminology, it would be more correct to define the machine as a “Permanent magnet assisted synchronous reluctance motor” (PMaSynRM). Otherwise, it is defined as “Internal permanent magnet synchronous motor” (IPM). The term "hybrid" is too uncertain. It can mean a few different things. For example, it can mean a synchronous machine with a rotor that consist of blocks of different structures [7]. However, as the reviewer was able to grasp based on text, it is rather a traditional IPM (λPM/λrated> 0.5) or a traditional PMaSynRM (λPM/λrated< 0.5).

Anyways, you should add several additional references to the recent works that describes motors/electrical machines similar to the one under consideration.

8) Please, add to the text the “Conclusion” section in which the main conclusions and the novelty of the work concisely summarize.

 

References:

T. Ault, S. Krahn, A. Croff, “Radiological Impacts and Regulation of Rare Earth Elements in Non-Nuclear Energy Production”, in Energies, 2015, 8, 2066-2081; doi.org/10.3390/en8032066 R. Schulze, “Reducing environmental impacts of the global rare earth production for use in Nd-Fe-B magnets, How magnetic technologies much can recycling contribute?“, PhD Thesis, Technical University of Darmstadt, December 2018. M. Obata, S. Morimoto, M. Sanada, Y. Inoue, "Performance of PMASynRM with Ferrite Magnets for EV /HEV Applications Considering Productivity", IEEE Transactions on Industry Applications, vol. 50, no. 4, pp. 2427-35, 2014. doi.org/10.1109/TIA.2013.2294999 H. Liu, K. Joo, Ye J. Oh, H. Lee, H. Seol, C. Jin, W. Kim, and J. Lee, “Optimal Design of an Ultra-Premium-Efficiency PMASynchronous Reluctance Motor With the Winding Method and Stator Parameters to Reduce Flux Leakage and Minimize Torque Pulsations”, in IEEE Transact ons on Magnetics, Vol. 54, No. 11, Nov. 2018. doi.org/10.1109/TMAG.2018.2846880 W.L. Soong ; N. Ertugrul, “Field-Weakening Performance of Interior Permanent-Magnet Motors”, in IEEE Transactions on Industry Applications, vol. 38, no. 5, pp. 1251-1258, 2002. doi.org/10.1109/TIA.2002.803013 Oguzhan Ocak, Metin Aydin, “A New Hybrid Permanent Magnet Synchronous Motor with Two Different Rotor Sections”, in IEEE Transactions on Magnetics, Volume: 53, Issue: 11, Nov. 2017, doi.org/10.1109/TMAG.2017.2709787

Author Response

Dear reviewer, thank you for you work and the improvement suggestions. They have really been helpful and improved the quality of the paper. We have revised the article according to your comments. Following is a list of changes and answers to your comments:

1) We added a table in line 88, comparing the reference motor (BMWi3) with the developed ones in terms of geometry, magnet mass and more

2) You are right, this statement is obvious. We reformulated the paragraph, stating more clearly that low load efficiency was an important but not the only criterion for the rotor design selection.

3)Grades of bonded and sintered magnets are included with their datasheet as well as the meantioned heavy rare earths (Dysprosium and Terbium) (Line 116, 132, 156 )

4) The reasons for preferring NdFeB magnets over ferrite based is now clearly stated in an extra paragraph (Line 79)

5) Your statement is correct (see table 1 in line 88), the lower environmental impact results from not needing heavy rare earths any more in comparison to sintered magnets (Dy and Tb). This explanation is added to the text for clearification.

6) The rotor von Mises stress was of course considered in the development but not mentioned yet in the article. We provided a stress plot for speeds over rated max speed (Line 100). Unfortunately EM developper did not answer me yet for a picture with higher resolution. I will update it once I received it.

7) we added additional references and a statement, that the motor can also be called PMsSynRM. The exact distribution into reluctance and electromagnetic torque is confidential according to the EM developer.

8) We added a conculsion chapter.

 

Hopefully we could fulfil your request.

Thanks and best regards

Jonas Hemsen

Round 2

Reviewer 3 Report

Dear authors,

Thank you for your answers. However, there are some issues to clarify.

1) You consider two motors with sintered magnets and bonded magnets. However, Figure 1 shows only the rotor von-Mises stress for the motor with sintered magnets. Please, show the rotor von-Mises stress for the motor with bonded magnets.

2) The authors write: ”Unfortunately, bonded magnets have less remanence flux density (Br) than sintered magnets but they contain no heavy rare-earth (Dy and Tb which are more critical than Nd and whose price might raise more severely if a supply crisis were to occur)”.

If the heavy rare-earth elements are a big problem, why do you choose for your motor with sintered magnets the magnets grade of N45UH with the heavy rare-earth elements? There are Dy free и no heavy rare-earth sintered magnets [1-4]. The characteristics of these magnets are much better than that of 5SP-12ME.

 

References:

Neodymium Magnets, NEOREC Series, TDK Corporation, Neodymium Magnets, NEOREC Series, TDK Corporation, 2017. Available online: https://product.tdk.com/info/en/catalog/datasheets/magnet_neo_summary_en.pdf X. Xu, T. Sasaki, J. Lia, Z. Dong, H. Sepehri-Amin, T. Kim, T. Ohkubo, T. Schrefl, K. Hono, “Microstructure of a Dy-free Nd-Fe-B sintered magnet with 2 T coercivity”, in Acta Materialia, Volume 156, 1 September 2018, Pages 146-157. doi.org/10.1016/j.actamat.2018.06.037 E. Castle, R. Sheridan, W. Zhou, S. Grasso, A. Walton, M. Reece, “High coercivity, anisotropic, heavy rare earth-free Nd-Fe-B by Flash Spark Plasma Sintering”, in Scientific Reports volume 7, Article number: 11134 (2017). doi.org/10.1038/s41598-017-11660-9 K. Hono, “Towards Dy-free high coercivity Nd-Fe-B permanent magnets”, Presentation. Available online: http://komag.org/2013winter/Hono.pdf

Author Response

Dear Reviewer,

thank you for your comments and improvement suggestions.

1) You consider two motors with sintered magnets and bonded magnets. However, Figure 1 shows only the rotor von-Mises stress for the motor with sintered magnets. Please, show the rotor von-Mises stress for the motor with bonded magnets.

-> Unfortunately, the injected (bonded) magnet rotor cannot be shown. It is a very special design with "organic" magnet forms and special retainers to enhance mechanical stability. A patent is pending for this way of design. The EM developer does not want to show the design until the patent is accepted/rejected. I added a remark about this in the text above the image.

2) The authors write: ”Unfortunately, bonded magnets have less remanence flux density (Br) than sintered magnets but they contain no heavy rare-earth (Dy and Tb which are more critical than Nd and whose price might raise more severely if a supply crisis were to occur)”.

If the heavy rare-earth elements are a big problem, why do you choose for your motor with sintered magnets the magnets grade of N45UH with the heavy rare-earth elements? There are Dy free и no heavy rare-earth sintered magnets [1-4]. The characteristics of these magnets are much better than that of 5SP-12ME.

-> You are right, there is DY free magnets with slightly higher Energy density (390 vs 374 kJ/m^3). However Dy-free magnets do not fulfil the temperature criteria. The DY-free versions in the sources you attached are only capable to withstand 110 Deg C, which is too low for a high performance motor. Also Demagnetization plays an important role and is difficult tp prevent with Dy-free magnets. So for the sake of high performance we chose the stated grade 45UH

For the comparison with 5SP-12ME you mention: Since the bonded magnets are needed in a much larger portion in order to provide the neccessary flux, they are also more robust to demagnetization even with no Dy

We hope this clarifies all questions, please get back to me if something is open. Best regards

Jonas Hemsen

Round 3

Reviewer 3 Report

The main novelty of this paper is the motor with injected (bonded) magnet, however its design is not shown in the paper which seems strange, for the application for the invention is already filed by authors. The authors write about this fact «A patent is pending for this way of design».

However, despite that not all details are disclosed in the paper, the paper can be interesting for the readers of the WEVJ journal.

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