A Hybrid-Excitation Synchronous Motor with a Change in Polarity

Round 1

Reviewer 1 Report

This paper presents an electric motor concept by mixing magnets with excitation coils in the rotor as flux sources. Linked to two inverters feeding the stator winding to allow a polarity switch. The concept is interesting to reduce rare earth material and I always welcome this kind of concept for publications when analyzed properly in FEM (not always demanding a prototype as some have as a standard). But for a journal article, the paper value in my opinion would be the design considerations and their reasons for application in the motor. Showing these criteria in a technical way would make the reading easier. The paper lacks elaboration on this aspect. Some comments are put below but please extend the paper as you see appropriate.

 

 

1. Introduction: in general is ok but by making a quick search I found a review paper by Wardach that mention similar concepts. Include the difference between these and the proposed concept in the introduction.
2. Stator winding:
1. The winding is designed for the configuration Q36p4. After checking it, the winding design is correct. The winding is divided in half for each inverter. So having 12 coils per phase, 6 coils go for each inverter. I can see the pattern that every 3 coils are given for each inverter, i.e., 3 coils for inverter A' and the next 3 for inverter A'' and so on. What would be the design criteria for a proper division of the winding? Is there a pattern that could always be followed to have correct winding distribution? Please elaborate on it and reflect it in the paper.
2. Matrix KA' after line 89 is missing a slot, only 34 slots are put.
3. It is said that in order to change the polarity to four poles, the winding needs to be fed by a negative sequence besides shifting 180° in one inverter. So a question arises if this is the only way. Please reflect in the paper the technical reason for changing the sequence to obtain the four poles configuration that shows how doing differently would not lead to the wanted configuration.
4. When computing the maximum winding factor for a Q36p4 I get 0.945 with a coil span of 4 slots. How did you get a winding factor of 0.96?
   Besides, please put in the paper the computation of the 0.62 winding factor.
5. There are 100 conductors per slot. Do you use parallel paths for them? How many?
3. Stack length:
1. The stator stack length is 40 mm while the magnet length is 25 mm. So piling 2 magnets would lead to 50 mm or if only one is used we would have a lower rotor. What is the case? Do we have a lower rotor because of a reason, for instance, because of the excitation winding? What would be the design criteria that would need to be considered for such a concept? Because of the end-winding length of the rotor coils. Please elaborate on it.
4. Flux density map:
1. Figures show flux lines that disappeared to the shaft area may be due to boundary conditions. So it could be that the saturation might be higher?
5. Excitation winding:
1. Units would be Ampere-turns not Amperes only.
2. In this case, 450 A-turns are used in the excitation coils for the N35UH magnet. The criteria to set it is ok.  But It would be good to have the design criteria of the excitation slots and coils at least for this design. Since going to higher magnet grades is common, the ampere-turns would increase so this is important in my opinion to check the feasibility of the design.
3. As far as I understand there are no ribs for the coil slots in the rotor. Any particular reason for it? Would this be the best way to go considering mechanical aspects? I am not requiring a mechanical analysis but simply your opinion on what and why would be the way to go if somebody wants to design a similar concept.
6. FEM:
1. Please put the software and version used within the paper for FEM computations.
2. What process did you follow to compute the step skewing in the 2D simulation? Or what post-process did you use?
7. IPM comparison:
1. The comparison between the IPM2 motor is irrelevant or at least I couldn't get the point of doing so.
2. So focusing on the comparison with the IPM1 motor, it would be appropriate to compare at least one operating point, e.g., the base point (i.e., same torque and speed) for 8 and 4 poles. No need to compare the 4-pole configuration with a 4-pole IPM only the 8-pole would be ok but the performance parameters when operating with 4 poles should be included. The following parameters should be shown (besides current, voltage, power factor, etc.):
1. Iron losses (and their way of computation). It seems the hybrid concept could have higher rotor iron losses because of the excitation slots. Not sure but the paper doesn't show anything about it.
2. Copper losses. Besides the stator Joule losses, it would be good to see how much losses the excitation winding put into the system compared to a typical IPM.
3. Efficiency.

 

Clearly, the magnets are reduced to 4 instead of 8 but in terms of performance, what would be the trade-off on doing so? That is why the comparison would give a better perspective of the application of the concept. 

Author Response

Dear Reviewer,

The Authors would like to thank you for your valuable comments. We have carefully evaluated them and put in our best to answer all the questions in order to improve the quality of our work.

For the Reviewers convenience, the main parts which have been updated in the
paper are written in red.

You can find the detailed answers to your comments in the pdf file attached (in the Section Reviewer 1).
We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

The article considers theoretically a synchronous motor with an excitation winding and permanent magnets on the rotor to increase the maximum operating speed. The reviewer believes that, given the methods of extending the speed control range already adopted by the industry, the proposed solution looks impractical, and its application is not clear. Here are just a few thoughts on it:

1)  As shown in Figure 2, the proposed solution requires two inverters. Please, provide an estimation of the cost difference of the invertors for the proposed motor and the inverter for the PM motor of similar power and size.

2)  HE-IPM requires rotor skew. Taking into account the magnets in the rotor, it is difficult and expensive.

3)  What kind of the exciter in the rotor is there in HE-IPM? If it is a wireless exciter, it is a separate machine increasing the volume and the cost significantly.

4)  Torque vs speed characteristic are shown in Fig.16. It is rather moderate characteristics for conventional drives in the industry. It is well known that the characteristics of modern traction drives are better. For example, BMW i3 uses a 12-pole 125 kW permanent magnet assisted synchronous reluctance machine (PMaSynRM) with maximum speed of 11400 rpm, rated speed of 4500 and the constant power speed range (CPSR) of 2,53:1. 

5)  The electric drives used in electric vehicles have a continuous constant power speed range. The proposed solution has the maximum power at a speed of about at 1000 rpm in 8-pole mode and of about 2800 rpm in 4-pole mode. Based on Figure 16b, there is a power dip between these values. Explain, how can this machine meet the typical requirement for drives of an electric vehicle?

6)  What is the purpose of changing poles of an electric machine fed by invertor? A wider CPSR up to 15:1 can be obtained in a simpler way, for example by using an inverter with a higher current rating [doi.org/10.1109/ICELMACH.2018.8507222].

7) In 4-pole mode, there are big gaps in the stator surface with zero MMF produced by given phase. The stator surface is used by half. It deteriorates the machine characteristics.

8)  BMW developed PMaSynRMs with reduced quantity of magnets due to the reluctance effect. Also, BMW and Renault developed synchronous machines with an excitation winding in the rotor for electric vehicles. Provide the advantages of the proposed machine for its practical application. Or is it a theoretical paper?

Considering the above, novelty and practical applicability of the proposed solution, it is not clear. The reviewer cannot recommend this article for publication in the Journal.

Author Response

Dear Reviewer,

The Authors would like to thank you for your valuable comments. We have carefully evaluated them and put in our best to answer all the questions in order to improve the quality of our work.

For the Reviewers convenience, the main parts which have been updated in the
paper are written in red.

You can find the detailed answers to your comments in the pdf file attached (in the Section Reviewer 2).
We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for addressing the comments. Comment 7b has not been addressed properly and still stands.
It is true that the torque-speed curve for a fixed temperature for the whole operating region is ok (although the paper does not mention the temperature), and it should stay in the paper. However, I disagree in that the single operating point isn’t important. To better understand the hybrid concept, many readers might find it interesting and important to see the performance comparison of the motors at least at a single point (e.g. at base speed).  I said for the same torque-speed point but for the same current-speed point is ok. Please also add in the paper the conditions for temperature results for both stator and rotor (windings and magnets), i.e., cooling system specs and thermal model.

Author Response

Dear Reviewer,

The Authors would like to thank you for your valuable comments. We have carefully evaluated them and put in our best to answer all the questions in order to improve the quality of our work.

For the Reviewers convenience, the main parts which have been updated in the
paper are written in red.

You can find the detailed answers to your comments in the pdf file attached (in the Section Reviewer 1).
We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper still has significant drawbacks. For example, the magnets are arrange so as to contribute the flux in 8-poles mode. They won't contribute the flux in 4-poles mode but significantly worsen the flux spectrum in the gap.

Author Response

Dear Reviewer,

The Authors would like to thank you for your valuable comments. We have carefully evaluated them and put in our best to answer all the questions in order to improve the quality of our work.

For the Reviewers convenience, the main parts which have been updated in the
paper are written in red.

You can find the detailed answers to your comments in the pdf file attached (in the Section Reviewer 2).
We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

The motor topology is interesting and could be an alternative to current ones. Unfortunately, the proposed theoretical paper lacks some results. Performance comparison among motors should include the thermal behavior. Presumptions of temperatures are not valid in this case since it seems that the rotor temperature in the proposed topology would be higher or might require a special cooling system (which is the case of large synchronous machines with excitation systems). In any case, the study conditions should be included in the paper considering the thermal models (or prototype results) for a correct comparison. I am not expecting that the proposed motor would perform better than the IPM, if it does great, but if it doesn’t, I think it will also be a contribution to see its performance in a paper describing the design criteria for manufacturers and designers' consideration for its implementation. The paper is close but still needs work.

Author Response

Dear Reviewer,

The Authors would like to thank the Reviewer for his/her observations. We have carefully evaluated them and put in our best to improve the quality of our work, collecting all comments received.

For the Reviewers convenience, the main parts which have been updated in the
paper are written in red.

You can find the answers to your comments in the pdf attached, in the Section Reviewer 1.
We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

The concept of the paper is clear now.  The proposed motor has good characteristics at low speeds and at high speeds depending on the number of the poles.  However, there is a gap in characteristics between the ranges of low and of high speeds. Modern solutions provide the motors with a large constant power speed range without the gap in the motor characteristics. So, such motor is inferior to these solutions. However, electrical engineering needs fresh ideas which, perhaps, will be developed in demanded solutions. The paper can be published.

Author Response

Dear Reviewer,

The Authors would like to thank the Reviewer for his/her observations. We have carefully evaluated them and put in our best to improve the quality of our work, collecting all comments received.

For the Reviewers convenience, the main parts which have been updated in the
paper are written in red.

You can find the answers to your comments in the pdf attached, in the Section Reviewer 2.
We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf

Round 4

Reviewer 1 Report

The whole point of asking for the thermal model is for motor performance comparison, and it is not done. So the study it's ok, but the point is that for a given exact same cooling system for both motors (Hybrid and conventional IPM) (e.g. the one you proposed) and a given continuous operating point (torque - speed):

1. In the hybrid motor: you get X1 stator winding temperature, X2 rotor winding temperature, and X3 magnet temperature. (steady state temperatures)

2. In the IPM motor: you will get different steady state temperatures (cooler, I guess).

So, what would be the performance once you have the temperatures for a given operating point for both motors? Put a table with the performance comparison, including the temperatures. And these are the results expected to be presented in Table 5 (arranging the section order after the proposed cooling system).

Author Response

Dear Reviewer,

You can find the Authors answer in the pdf file attached.

The main parts which have been updated in the paper are written in red.

We hope our replies can be found satisfactory and we look forward to receiving your feedback.

Sincerely,

The Authors

Author Response File: Author Response.pdf