A New Multi-Axial Flux Pm Motor–Generator System for Flywheel Energy Storage

Comments 1:

The research gap of this article has not been well summarized, especially in the abstract section. Readers cannot recognize the necessity of the research conducted by the author.

Response 1: Thank you for pointing this out. I agree with this comment Therefore; the abstract was rewritten to highlight the research's contribution to the literature.

Page number 1; line 9-27.

Comments 2: This article contains many descriptions that are not very rigorous. For instance, in the last paragraph of the first part, the author states that the designed flywheel energy storage mechanism can obtain a considerable amount of energy in a relatively short time.

Response 2: Thank you for pointing this out. I agree with this comment; Therefore, the generalizations such as "relatively short time" were corrected in the study.

Page:1, line:38; page:2, line:81; 90, page:14, line:398; page:15, line:401,403,405,416,420

Comments 3: However, it is not clear how "considerable" and "relatively short" are quantified, whether it has been compared with other designs, and what its advantages are. All these aspects lack clear research and expression.

Response 3: Thank you for pointing this out. I agree with this comment; Therefore, In the study, words such as relatively short were supported by numerical data. The focus of the research was emphasized more clearly. However, points far from the focus were removed from the study.

Page:1, line:38; page:2, line:81; 90, page:14, line:398; page:15, line:401,403,405,416,420

Comments 4:The author presents the design of the prototype in Figure 3, but lacks the introduction of the experimental platform and the display of physical pictures, which is necessary in motor-related papers. It is suggested that the author provide picture displays.

Response 4: Thank you for pointing this out. I agree with this comment; Therefore, the experimental setup is given in Figure 6 in the study.

Page:8

Comments 5: The conclusion part of this article lacks comparison with other designs. It is suggested that some simple comparisons be made where possible to ensure that the design ideas and solutions provided by the author have advantages.

Response 5: In conclusion, a qualitative comparison was made with the previous studies to emphasize the focus of the study. In addition, there are some deficiencies due to the limited budget of the study. It is planned to develop the study with further research and new projects.

Comments 1:

This article provides a brief summary of existing related research and does not delve into the advantages and disadvantages of different methods, which affects the comprehensiveness of the research.

Response 1: Thank you for pointing this out. I agree with this comment; Therefore, The following paragraph has been added to highlight the advantages and disadvantages of the study compared to existing studies.

Page:2 line: 68-76

“When examining the literature, supercapacitors provide the high energy required to ensure the movement of the projectile in the coil launcher within less than 2 seconds [5-6]. However, the energy discharge time for a flywheel is 2 minutes, compared to just 1 minute for supercapacitors [22]. Therefore, when the CML is powered by a flywheel, the discharge time, traditionally 120 seconds, must be reduced to under 2 seconds. In this study, conducted to address the deficiency of the flywheel, a power output of 1499.89 watts was achieved in 0.5 seconds using the proposed new winding design. Additionally, a more compact structure was realized by placing the newly designed winding in the same slots as the motor-generator winding used in the traditional design.

Comments 2:

The conclusion lacks a comprehensive critical analysis of the proposed structure itself and experimental verification results.

Response 1: Thank you for pointing this out. I agree with this comment Therefore; A critical analysis of the proposed structure and experimental results is added to the conclusion section.

Page: 15-16

Comments 3:

The system adopts dual motors (2-pole AFPM high-speed motor+12 pole AFPM low-speed motor) and independent C-winding, resulting in a complex mechanical structure. The coaxial integration of multiple motors may introduce dynamic imbalance risks, especially during high-speed stages (1188 rpm), which may cause vibration or resonance issues. Please explain.

Response 3: Thank you for pointing this out. Although the experimental setup consists of a motor driving the system together with the flywheel, the final design is on a single shaft. Therefore, when the desired improvements are made in the system, it will be stable at higher speeds without including coupled connections. However, the most important element affecting the balance in the design is the disk structure of the axial flux machine. Due to the disk structure of the design, we encountered limitations at the production point. Therefore, we could not find the opportunity to try higher speeds. For this reason, we want to improve this test setup and make a new prototype in the following studies.

Comments 4:

The paper did not mention the mechanical connection details between the flywheel and the motor (such as shaft system design and alignment accuracy), nor did it provide vibration mode analysis or dynamic balance experimental data, which may hide potential mechanical fatigue hazards.

Response 4: Thank you for pointing this out. I agree with this comment Therefore; the experimental setup is added in Figure 7.

Page:9.

Comments 5:

The paper only conducted magnetic circuit analysis using ANSYS Maxwell, but did not quantify the effects of copper loss, iron loss (hysteresis and eddy current loss), and mechanical loss (bearing friction, wind resistance) on the overall efficiency of the system. The eddy current losses during the high-speed phase (2-pole AFPM) may significantly increase, but specific data has not been provided.

Response 5: Thank you for pointing this out. The issues raised regarding bearing friction are significant and should be prioritized in the design of electric machines. While supercapacitors are primarily utilized for energy needs in coil launchers, some research has explored the capability of discharging a flywheel within two minutes, making it suitable for short-term power outages or as a backup for the power grid. This study proposes a new winding structure specifically for the flywheel in CML, demonstrating its advantages and disadvantages with experimental data. Consequently, including detailed analyses of the axial flux machine structure design could detract from the main focus of the study.

Comments 6:

The paper does not specify the collaborative control logic of dual motors (such as high and low speed switching thresholds, dynamic response strategies during load transients), which may lead to a decrease in energy conversion efficiency or fluctuations in flywheel speed. The selection of power electronic devices (such as the switching frequency and losses of IGBT modules) is not mentioned, which affects the dynamic performance of the system.

Response 6: Thank you for pointing this out. The study on flywheel speed control and efficiency is considered valuable. However, the publication does not focus on controller development, switching thresholds, or losses. It is possible that the prototype system presented could be the basis for a comprehensive study on these topics in future research, provided that its deficiencies—particularly mechanical balance problems—are addressed.

Comments 7:

The description of the structure of the 12 pole AFPM motor generator in the article mentions the winding design serving the functions of the motor and generator, as well as the determination of some design parameters. However, there is a lack of detailed explanation of the overall structural layout, connection methods of various components, and assembly relationships. Suggest adding how the spatial layout of the motor and generator components achieves functional synergy in a compact structure; The specific positions of each winding on the stator and rotor, as well as their relative positions with permanent magnets, iron cores, and other components; The connection methods between different components and their impact on system stability and performance.

Response 7: Thank you for pointing this out. I agree with this comment; Therefore, the winding layout is given with the structure added to the motor-generator structure given in Figure 5. In addition, the magnet placement angle was selected as 30 degrees in the design. An explanatory paragraph is also included.

Page:7, line:211-215

Comments 8:

The paper mainly focuses on the energy output performance of the system and the design of the motor generator structure, with less analysis on the reliability of the entire flywheel energy storage system structure. Considering that the system is applied to coil transmitters and will face high energy output, fast charging and discharging conditions, structural reliability is crucial. Suggest increasing research on the reliability of system structure, studying whether structural components will experience fatigue damage, loosening, and other issues after long-term operation, as well as the impact of these issues on system performance and safety; At the same time, corresponding structural optimization measures or reliability design criteria can be proposed to improve the reliability and stability of the system in practical applications.

Response 8: Thank you for pointing this out. The problems such as structural reliability, fatigue damage, and loosening of the system suggested by the referee are important for reliability and stability. These issues suggested by the referee will be worked on especially in the prototype development phase in future studies. For this, new publications including long-term work tests can be considered. I can say thank you or I am grateful because the referee's contribution is a guide for future studies.

Comments 1:

For a non-specialist reader, the description of the system considered in the article is not exhaustive and may not be understandable. The article mainly considers the flywheel energy storage system (FESS). An important element of this system is the coil magnetic launcher (CML). But the construction and operation of CML is not sufficiently discussed. In particular, it would be useful to present the energy flows between CML and FESS for different possible operation states. In addition, the article completely ignores the discussion and description of the power systems used for   two AFPM motors and motor control systems.  It is not clear, in what way the reference speeds for the flywheel system are determined.

Response 1: Thank you for pointing this out. I agree with this comment; Therefore,

CML’nin yapısı ve çalışmasını içeren bir bölüm eklenmelidir. Burada farklı çalışma durumları için CML ve FESS arasındaki enerji akışlarını sunmak gerekiyor.

Control sistemleri için kullanılan güç sistemlerinin tartışılması ve açıklanmasını da istiyor.

Comments 2: In the studies presented in the article the Author used modern and advanced numerical methods. Computational research was supported by experimental research, which ensures great usefulness of the research performed.

Response 2: Thank you for pointing this out.

Comments 3: The article is written using mostly short, grammatically simple sentences, which makes it easier to understand the content of the article.

Response 3: Thank you for pointing this out.

Comments 4: The commentary to Fig. 6 and 7 is excessively short. It is not stated under what conditions this characteristics were determined.    The horizontal axes of Figures 6 and 7 are marked as Revolution per minute (rpm), but in the titles of these  Figures there are the terms: V-F characteristic.

Response 4: Thank you for pointing this out. I agree with this comment; Therefore, The explanation of Figure 6-7 has been expanded with the paragraphs added below. In addition, the incorrectly written V-F has been corrected and equation 9 has been added for the relationship between speed and frequency. In addition, the frequency values ​​corresponding to the speed according to equation 9 have been added next to it.

“The values shown in Figure 6 represent the phase-neutral voltage (Vpn) obtained during the generator mode (idle operation) of the axial flux machine. An oscilloscope was utilized in the experimental study to measure these Vpn values. Additionally, the motor rpm values indicate the operating frequency. The frequency values corresponding to the Vpn-rpm pairs of 586.2-17.9, 885.6-27.6, 1182-35.9, and 1482-45.3 (Vpn-rpm), as shown in the graph, are 9.77, 14.76, 19.7, and 24.7 (Hz), respectively, according to Equation 9.”

“The Vpn values obtained from oscilloscope are shown in Figure 7.a-b. Figure 7.a presents the frequency values for winding A as follows: 12.53 Hz at 125.3-24.9 Vpn-rpm, 19.7 Hz at 197-39.1 Vpn-rpm, 29.67 Hz at 296.7-59.9 Vpn-rpm, 35.92 Hz at 359.2-72.3 Vpn-rpm, 42.33 Hz at 423.3-84.2 Vpn-rpm, and 45.92 Hz at 459.2-91 Vpn-rpm, calculated using Equation 9.”

Page:9, line:240-246; 253-257.

Comments 5: The Author does not refer to formulas by giving formula numbers in round brackets, which is commonly used in the literature.

Response 5: Thank you for pointing this out. I agree with this comment; Therefore, Reference to formulas has been added.

Comments 6: In the title of the manuscript, the plural form: Systems is used, but in the article only one system is discussed.

Response 6: Thank you for pointing this out. I agree with this comment; Therefore, the typo has been corrected to "system”.

Comments 7: On P.1, L.15 there is the term: watts of energy. But watts are connected with the power and energy with Joules.

Response 7: Thank you for pointing this out. I agree with this comment; Therefore, "watts of energy" has been corrected to "watts of power".

Comments 8: The article contains a relatively large number of editorial errors that should be corrected:

Response 9: Thank you for pointing this out. I agree with this comment; Therefore, I tried to make all the corrections.

Comments 10: On P.3, L.95-96 : there is a lack of symbol σ for denotation of tensile strength.

Response 10: Thank you for pointing this out. I agree with this comment; Therefore, necessary corrections have been made.

Comments 11: On P.3, L.100 there is grammatical mistake.

Response 11: Thank you for pointing this out. I agree with this comment; Therefore,

“For different materials, other constants, and energy amounts per unit volume, unit weight is given in Table 1.”

Table 1 provides the unit weight for various materials, along with their respective constant and energy amounts per unit volume.

Comments 12: On P.3, Table 1 in the Table the symbol P should be changed to ρ (ro).

Response 12: Thank you for pointing this out. I agree with this comment; Therefore, The errors regarding the icons occurred during formatting. Thank you for drawing attention to this point. necessary corrections have been made.

Comments 13: On P.4, Table 2 there is the mistake in the title of Table.

Response 13: Thank you for pointing this out. I agree with this comment; Therefore, Corrected “Comparison” to Comparison

Comments 14: In the Equations the inertia is denoted with small letter j, but not with capital letter J.

Response 14: Thank you for pointing this out. I agree with this comment; Therefore, "J" has been corrected to "j".

Comments 15: L.118-119 there is the term: the voltage and frequency value of the energy. What do you mean as    the voltage and frequency value of the energy.

Response 15: Thank you for pointing this out. I agree with this comment; Therefore, “Since the speed goes from the reference speed value to zero, the voltage and frequency value of the energy approaches zero.”

“As the speed decreases from the reference value to zero, both the voltage and frequency approach zero.” correction was made.

Comments 16: 5, L.157 the used symbol EASM is not explained.

Response 16: Thank you for pointing this out. I agree with this comment; Therefore, “EASM” has been corrected to “AFPM”.

Comments 17: In what sense the energy described in the article as Em is called the maximum energy.

In the list of Abbreviations the ns and X have the same definitions.

Response 17: Thank you for pointing this out. I agree with this comment; Therefore, “X” has been corrected to “x”.

Comments 18: In the list of Abbreviations the symbol with capital letter X is introduced. But in Eq. (5)-(8) the small letter x is used.

Response 18: Thank you for pointing this out. I agree with this comment; Therefore, “X” has been corrected to “x”.

Comments 19: In the list of Abbreviations the symbol with capital letter Yx is introduced. But in Eq. (6)-(8) the small letter yx is used.

Response 19: Thank you for pointing this out. I agree with this comment; Therefore, “Yx” has been corrected to “y_x”.