Design of External Rotor Ferrite-Assisted Synchronous Reluctance Motor for High Power Density

Round 1

Reviewer 1 Report

Design and experimental verification of the five-phase external rotor ferrite magnet assisted SynRM is presented in this paper. The design analysis and experimental verification is comprehensive; however, some points remain not clear:

1) It seems this motor was already considered in your previous works several times including the design and experiments. Can you please explain the novelty of the paper in more detail in the introduction?

2) In the introduction part, please, explain in which vehicle you plan to use the developed motor. Why did you chouse the rated speed of 1800 rpm? This rated speed is too high for in-wheel motors. For geared or chain driven applications, usually a motor with a higher rated speed will be used, as this allows for increased power density.

3) What is the total weight of the motor prototype in fig. 17b?

4) Please, add a table with masses of active materials (stator steel, rotor steel, copper, magnets) before and after optimization.

5) See line 221: Formula (6) [optimization function] has the wrong numbering. Please correct it.

6) In optimization function (6) the motor efficiency is included. However, the paper contains no data on the calculated and experimental efficiency. It is necessary to add information about the motor losses at various loading points important for the considered traction application.

7) In traction applications, motor efficiency in more than just one loading mode is important. Why does the optimization function (6) only include efficiency in one loading point?

8) What is the required inverter power rating? Why is a term that allows reducing the required inverter power (increasing the motor power factor) not included in formula (6)?

9) How many function calls did it take to find the optimal solution using the DE algorithm? How much time was spent on this search?

10) There are two "Figures 19" on page 16. Please check the numbering.

11) Please describe in more detail the method of measuring the cogging torque and torque ripple (figures in page 16). How is the braking torque set? Which motor control algorithm is used? How is the precise angular movement of the rotor ensured? It would be interesting to compare the calculated and experimental cogging torque and torque ripple in the same plot.

12) It would be interesting to compare the Fe-PMaSynRM with a traditional external rotor synchronous motor with rare-earth magnets on the rotor surface.

Author Response

Reviewer: 1
Comments: Design and experimental verification of the five-phase external rotor ferrite magnet assisted SynRM is presented in this paper. The design analysis and experimental verification is comprehensive; however, some points remain not clear:

1) It seems this motor was already considered in your previous works several times including the design and experiments. Can you please explain the novelty of the paper in more detail in the introduction?

Answers:

Thanks for the suggestions. The introduction has been updated to discuss the previous works in detail. The novelty and significance of this manuscript has also been discussed.

Following portion has been included in the revised paper as highlighted:

[Page 2 Section 1, Line 60]

However, articles [28-30]] have focused on some major and common concerns of external rotor PMaSynRM such as heat extraction, torque ripple reduction, and torque density increasing. The demagnetization aspects of the Ferrite PM, which is a major concern for using Ferrite-PM in external rotor PMaSynRM design, has not discussed in detail.

Comments:

2) In the introduction part, please, explain in which vehicle you plan to use the developed motor. Why did you chouse the rated speed of 1800 rpm? This rated speed is too high for in-wheel motors. For geared or chain driven applications, usually a motor with a higher rated speed will be used, as this allows for increased power density.

Answers:

Thanks for the question. The external rotor PMaSynRM discussed in the manuscript was originally designed to match a laboratory prototype of five-phase internal rotor permanent magnet assisted synchronous reluctance motor which is discussed in reference article [28]. That base motor was designed for the rated rpm of 1800 considering gear reduction of the speed in electric vehicle application.

Comments:

3) What is the total weight of the motor prototype in fig. 17b?

Answers:

Thanks for the question. The total weight of the prototype motor is 13.95kg.

Comments:

4) Please, add a table with masses of active materials (stator steel, rotor steel, copper, magnets) before and after optimization.

 

Answers:

I highly appreciate the reviewer’s comment to improve the contents of the paper. Following table has been added to manuscript to reflect the reviewer’s comment:

 

Page-15 Section-5

The weight of active materials of the initial model and optimal prototype Fe-PMaSynRM model have been shown in Table 5. 

 

Table 5. Comparison of active materials mass of initial model and optimal model of Fe-PMaSynRM

Item	Initial Model	Optimal Model
Stator steel mass [kg]	5.46	5.37
Rotor steel mass [kg]	2.78	2.85
Winding copper mass [kg]	3.32	3.20
Ferrite PM [kg]	1.03	0.93

 

Comments:

5) See line 221: Formula (6) [optimization function] has the wrong numbering. Please correct it.

 

Answers:

 

I appreciate your feedback to correct and improve the manuscript. Here is the correction are done as follows:

 

[Page-8 Section-3.3]

 

In this study, Equation (6) is the multi-objective optimization function that has been utilized to obtain the optimal external rotor PMaSynRM design with a high power density and low torque ripple.

 

Comments:

6) In optimization function (6) the motor efficiency is included. However, the paper contains no data on the calculated and experimental efficiency. It is necessary to add information about the motor losses at various loading points important for the considered traction application.

 

Answers:

I highly appreciate the reviewer’s comment. In the optimization function of the paper includes the efficiency of the motor at particular operating point. However, considering the major focus of the paper, which is emphasizing on the demagnetization aspects, the authors have not included the efficiency and loss plots of the motor. We are extremely sorry that we are unable to add the efficiency data of the motor at this moment.

 

 

Comments:

7) In traction applications, motor efficiency in more than just one loading mode is important. Why does the optimization function (6) only include efficiency in one loading point?

 

Answers:

Thanks for the question. The traction motors have to be optimized considering the efficiency at many operating points. Although, in this design process only one point has been considered in the objective function, we have considered the maximum torque at the maximum-speed, back-emf harmonics, cogging torque,

Comments:

8) What is the required inverter power rating? Why is a term that allows reducing the required inverter power (increasing the motor power factor) not included in formula (6)?

 

Answers:

The inverter that has been utilized for this experiment is rated for 5 hp. However, during the motor design, the minimum inverter power is chosen as 3.3 kW.

 

Comments:

9) How many function calls did it take to find the optimal solution using the DE algorithm? How much time was spent on this search?

 

Answers: I appreciate your feedback. It took 453 iteration to reach the optimal solution. The simulation time for the optimization was 2.5 hours.

Comments:

10) There are two "Figures 19" on page 16. Please check the numbering.

 

Answers:

 

I highly appreciate your feedback. The figure number has been corrected as follows:

 

Section-5 Page-17

 

Figure 20. Proposed motor results (a) developed electromagnetic torque (b) phase A currents.

 

Comments:

11) Please describe in more detail the method of measuring the cogging torque and torque ripple (figures in page 16). How is the braking torque set? Which motor control algorithm is used? How is the precise angular movement of the rotor ensured? It would be interesting to compare the calculated and experimental cogging torque and torque ripple in the same plot.

 

Answers:

 

The motor was connected to an DC machine which acted as a prime mover. The prime mover had a speed controller by which the motor shaft was spun at different speed. For this result the motor was spun at a speed of 2.5 Hz. The changes in the text are given below:

[Section-5 Page-16]

 

The cogging torque of the external rotor Fe-PMaSynRM is shown in Fig. 19.  To measure this quantity the Fe-PMaSynRM was connected to an induction machine which acted as the prime mover. The prime mover was spun at a speed of 2.5 Hz. A torque transducer was used on the shaft of the motor to capture the torque.  It is observed that the peak-to-peak cogging torque of the proposed five-phase external rotor Fe-PMaSynRM is only 0.024 Nm, which was predicted to be 0.19 Nm as per FEA results. Although, the difference is significant, both values are reasonably small. The difference might have occurred from a reasonably small error in measured torque data of the dyno setup.

Second, experimental test for full excitation is also carried out on the fabricated prototype to compare the generated torque under load. During this test, the prime mover was running at rated speed of 1800. A vector control using MTPA algorithm was implemented to generate the maximum torque with minimum current. The torque was captured using torque transducer. A position sensor was utilized to get the rotor information utilizing an incremental encoder. Before the test, the optimal angle offset was calculated to ensure MTPA method. Fig. 20(a) shows the output torque of the five-phase external rotor Fe-PMaSynRM. The test result shows the average torque developed under rated excitation by the five-phase external rotor Fe-PMaSynRM is 19.4 Nm. The predicted torque from the FEA data was 19.2 Nm, which is very close the test data.

 

Comments:

12) It would be interesting to compare the Fe-PMaSynRM with a traditional external rotor synchronous motor with rare-earth magnets on the rotor surface.

 

Answers:

 

I appreciate your comment. We acknowledge that it will be interesting to compare external rotor Fe-PMaSynRM with a rare-earth magnet on the rotor surface. However, we have considered several disadvantages of rare-earth based surface PM motor such as poor field weakening capability and higher magnet cost. In addition, it will be difficult for us to make a fair comparison between a Fe-PMaSynRM with as rare-earth based surface PM motor. Therefore, have avoided the inclusion of surface PM motor in the manuscript.

 

 

Author Response File: Author Response.doc

Reviewer 2 Report

1- the objective in this design is to obtain a high average torque and robusteness. are you consider the torque ripple as an optimization parameter?

2-In section 3.1, the author said that a u shape for flux barrier is used, why this type as there is no reference added to defend this selection. 

3- the issue of number of flux barrier depends on staor slots is not considered?

4- The introduction part lacks the discussion abbout the previous published topics about this point.

5-please, add a full picture for the system with more discussuoion about the contriol program.

6- In experimental results you added some results for the proposed motor, this results shoul be ciompared with the obtained simulation results in details also the performance of the proposed motor shoulb be studied at fault case.

7- conclusions dosn't discuss the obtained results in a suitable manner.

 

Author Response

Comments:

1- the objective in this design is to obtain a high average torque and robusteness. are you consider the torque ripple as an optimization parameter?

 

Answers:

I highly appreciate your feedback to improve the paper and clear up different aspects. The multi-objectives optimization of the Fe-PMaSynRM include achieving higher torque density, lower torque density, robustness against irreversible demagnetization, and higher efficiency. The is simulation constraints also included a maximum allowable torque ripple and minimum allowable flux density in the permanent magnet.

 

 

Comments:

2-In section 3.1, the author said that a u shape for flux barrier is used, why this type as there is no reference added to defend this selection. 

 

Answers:

 

I highly appreciate your feedback. The U-shaped (arc-shaped) permanent magnets have been used in the proposed prototype motor as these types of magnets can offer higher torque density due to higher saliency ratio in a SynRM or PMaSynRM motor especially with external rotor setup.

 

A reference has been added which shows the comparison of saliency ratio and average torque of external rotor SynRM. This outcome of that study will be applicable for the proposed motor too.

 

[Page-5  Section-3.1]

 

The U-shaped flux barrier with an arc-shaped magnet will be a suitable selection for this type of pole piece [31].

 

[Page-19  Reference]

 

31. M. A. Raj and A. Kavitha, “Effect of Rotor Geometry on Peak and Average Torque of External-Rotor Synchronous Reluctance Motor in Comparison With Switched Reluctance Motor for Low-Speed Direct-Drive Domestic Application,” IEEE Transactions on Magnetics, vol. 53, no. 11, pp. 1-8, Nov. 2017, Art no. 8209108.

Comments:

3- the issue of number of flux barrier depends on staor slots is not considered?

 

Answers:

I highly appreciate your feedback. There is certain configuration of slot-pole combination which are better to achieve lower torque ripple. However, slot-pole combination can be independent of the number of flux barrier. To reduce torque ripple, we can employ several other techniques such notch incorporation, asymmetric flux barrier, etc. The authors have kept their major focus the demagnetization risks.

 

 

Comments:

4- The introduction part lacks the discussion about the previous published topics about this point.

 

Answers:

We highly appreciate your feedback. The introduction has been updated to discuss the previous works in detail. The novelty and significance of this manuscript has also been discussed.

 

Following portion has been included in the revised paper as highlighted:

 

[Page 2 Section 1]

 

However, all of these articles have focused on some major and common concerns of external rotor PMaSynRM such as heat extraction, torque ripple reduction, and torque density increasing. The demagnetization aspects of the Ferrite PM, which is a major concern for using Ferrite-PM in external rotor PMaSynRM design, has not discussed in detail.

 

 

Comments:

5-please, add a full picture for the system with more discussuoion about the contriol program.

 

Answers:

 

We highly appreciate your feedback. A picture of the test setup has been added in the updated manuscript.

 

 

 

Following dicussion has been added to the revised manuscript:

 

[Page-15 Section-5]

 

The fabricated Fe-PMaSynRM has been mounted on a 5 HP dynamo testbed to validate the design properties and support the high power density operation shown in Fig. 17. The dynamo is built with a DC machine as the prime mover and a digital electronic load. The test setup is shown in Fig. 17. An indirect field-oriented control method has been adopted. For this purpose, a five-phase inverter has been designed and developed in the laboratory. The control algorithm is developed and implemented in the digital signal processor (TI DSP F28335). The five-phase currents are sensed to transform it to 2D vector quantities namely d-q axes components. Conventional PI regulator was design to perform the closed loop controls. Field oriented control utilizing MTPA and FW strategies were chosen to continue the tests.

 

 

 

 

Comments:

6- In experimental results you added some results for the proposed motor, this results shoul be ciompared with the obtained simulation results in details also the performance of the proposed motor shoulb be studied at fault case.

 

Answers:

 

[Page-16 Section-5]

It is observed that the peak-to-peak cogging torque of the proposed five-phase external rotor Fe-PMaSynRM is only 0.024 Nm, which was predicted to be 0.19 Nm as per FEA results. Although, the difference is significant, both values are reasonably small. The difference might have occurred from a reasonably small error in measured torque data of the dyno setup.

 

[Page-17 Section-5]

The test result shows the average torque developed under rated excitation by the five-phase external rotor Fe-PMaSynRM is 19.4 Nm. The predicted torque from the FEA data was 19.2 Nm, which is very close the test data.

 

Comments:

7- conclusions dosn't discuss the obtained results in a suitable manner.

 

Answers:

 

We highly appreciate your comment. We have thoroughly updated the conclusion to better represent the topics discussed in the manuscript. The revised manuscript has been concluded as follows:

 

 

[Page-17 Section-6]

 

Economical rare-earth free Fe-PMaSynRM designs can be optimally designed to better the performance of traction motor designs. In this study, the optimal design of a rare-earth-free five-phase external rotor ferrite permanent magnet assisted synchronous reluctance motor (Fe-PMaSynRM) has been presented. The anti-demagnetization ability has been a major focus during the design process during selecting the slot-pole configuration, flux barrier design to achieve low torque ripple, higher efficiency, and high average torque. Proposed 3.7kW five-phase outer rotor Fe-PMaSynRM has shown competitive torque density to a rare-earth counterpart while withstanding 2.5 times of rated current without any signs of demagnetization. Optimal design has been fabricated and tested to validate the simulation results. Overall, the Fe-PMaSynRM proposed in this study can be considered as rare-earth free competitive design to the rare-earth based traction motor designs.

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

Comment 6) was not answered. Detailed data on efficiency and losses at different load points should be added to this paper.

Author Response

Comments:

Comment 6) was not answered. Detailed data on efficiency and losses at different load points should be added to this paper.

Answers:

I highly appreciate the reviewer’s feedback to improve the manuscript. The following FEA results of motor-losses have been added in the revised manuscript. Due to the pandemic, we had very little access to the test setup to carry on testing. Therefore, we could not perform the loss data validation for these operating points with the dyno testing. We have already added test data of the back-emf, cogging torque, torque curve under the rated current. Those results closely match the FEA results. Hence, the motor is expected to perform as desired in the operating points mentioned in the following table (Table 5). The following paragraph and the table is added in the revised manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

- you have answered my questions well. However my comment about full picture is not good responded.

- you have added a full picture for the setup as in figure 17. However this figure is screened from a previously published paper in transaction of industrial electronic "A. K. M. Arafat and S. Choi, "Optimal Phase Advance Under Fault-Tolerant Control of a Five-Phase Permanent Magnet Assisted Synchronous Reluctance Motor," in IEEE Transactions on Industrial Electronics, vol. 65, no. 4, pp. 2915-2924, April 2018, doi: 10.1109/TIE.2017.2750620."

- in the paper of AKM et all., the machine was five-phase PMaSynRM with inner rotor and 15slots/4poles. However in your paper the PMa SynRM is outer rotor with 25slots/12poles.

- Hence, it is mandatory to add the following:

1- the full setup picture using the proposed motor in your paper.

2- A full picture for your motor from different views with stator inside the rotor and the winding is included.

3- modify the resolution of experimental results in figures 19,20. these figures seem to have a bad resolution!

Author Response

Comments:

- you have answered my questions well. However my comment about full picture is not good responded.

- you have added a full picture for the setup as in figure 17. However this figure is screened from a previously published paper in transaction of industrial electronic "A. K. M. Arafat and S. Choi, "Optimal Phase Advance Under Fault-Tolerant Control of a Five-Phase Permanent Magnet Assisted Synchronous Reluctance Motor," in IEEE Transactions on Industrial Electronics, vol. 65, no. 4, pp. 2915-2924, April 2018, doi: 10.1109/TIE.2017.2750620."

- in the paper of AKM et all., the machine was five-phase PMaSynRM with inner rotor and 15slots/4poles. However in your paper the PMa SynRM is outer rotor with 25slots/12poles.

- Hence, it is mandatory to add the following:

1- the full setup picture using the proposed motor in your paper.

Answers:

We highly appreciate the comment. We are extremely sorry for the mistake of using the same photo for two different motors. We have a couple of internal rotor and external rotor PMa-SynRM motors in our laboratory. As all these motors are of similar power levels and sizes, we somehow missed adding the photo of the external-rotor Ferrite PMa-SynRM dyno setup. The original photo of the test setup has been added in the revised manuscript.

Following portion has been included in the revised paper as highlighted: 

[Page 16, Figure 17]

Comments:

2- A full picture for your motor from different views with stator inside the rotor and the winding is included.

Answers:

We appreciate your feedback to improve the quality of the manuscript. In response to your comment, we are presenting several photos of the motor assembly from several different angles as follows:

[Page 16, Figure 17]

Comments:

3- modify the resolution of experimental results in figures 19,20. these figures seem to have a bad resolution!

Answers:

We highly appreciate your feedback, and we are extremely sorry for the low-quality images in the manuscript. Although the original image was good in our manuscript-raw-file, the quality degraded during the file conversion. I hope the quality of the images in the revised manuscript will be acceptable.

 

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

The paper can be published in the journal

Author Response

Thank you for your kind review comments.

Reviewer 2 Report

1- please add experimental results at either open circuit fault or short circuit fault.

2- add some details and block diagram about control strategy.

3- figure resolution isnot good, please modify.

4- 

Author Response

Comment:

1. please add experimental results at either open circuit fault or short circuit fault.

Answers:

We highly appreciate your comment to add experimental results under open or short circuit faults. But we are extremely sorry to say that we do not have enough test results at this moment to add a comprehensive fault-tolerant control (FTC) analysis of the proposed external rotor motor. Considering a major benefit of these types of motors, we are considering the FTC analysis of the proposed motor as a future project. As part of that project, we will be conducting experimental testing of the proposed motor under one or two-phase-open circuited fault conditions. We have some previous studies that show the fault-tolerant operation of the five-phase internal rotor permanent magnet assisted synchronous reluctance motor (PMaSynRM). We will be using our past experiences to carry out the work you have suggested here.

In the end, I would like to mention that we considered the fault-tolerant analysis beyond the scope of this manuscript. I would request the reviewer to consider this study as a design optimization study only for the ferrite-based five-phase external rotor PMaSynRM. We have not claimed any fault-tolerant control outcome in the abstract or conclusion.

Comment:

2. add some details and block diagram about control strategy.

Answers:

I appreciate your feedback. A control block diagram is added and some details about the control strategy are also added in the revised manuscript as follows:

[Page 16 Section 5]

The test setup is shown in Fig. 17 (a) and the controller block diagram to drive the motor is shown in Fig 17 (b). An indirect field-oriented control method has been adopted. An in-house five-phase inverter has been developed. The control algorithm is developed and implemented in the digital signal processor (TI DSP F28335). As shown in Fig. 17 (b) the five-phase currents are sensed to transform it to 2D vector quantities namely d-q axes components. Conventional PI regulators were designed to perform the closed-loop control of d-q currents followed by inverse park-transformation and generation five-phase voltages at the output terminals of the voltage source inverter. Field-oriented control utilizing MTPA and FW strategies was chosen to conduct the tests.

Comment:

3. figure resolution is not good, please modify.

Answers:

I appreciate your comment. We have worked our best to ensure the highest possible quality of the figures. I understand that some figures are still not up to the mark of our original pictures. I would like the ensure that we have the original copy of the figures. If we get an acceptance, we will provide our original pictures separately so that the main articles have the original high-quality images. Also, I would appreciate it if the reviewer can mention specifically which one or two figures are not up to mark.

 

Author Response File: Author Response.pdf

Round 4

Reviewer 2 Report

Best luck for your future work. i mean experimental results figures.