AC Electric Powertrain without Power Electronics for Future Hybrid Electric Aircrafts: Architecture, Design and Stability Analysis

Round 1

Reviewer 1 Report

1. Highlight (mention) the issues generated due to the absence of power electronics converters in powertrain in the abstract as well as its solution in the conclusion in a more explanatory manner.

2. Section 2, Power electronic-less powertrain for aircraft propulsion needs some more explanation, as this is a very sensitive application of power electronics converters, and in the absence of power electronics converters lot of changes is to be made in the design. So please highlight all the necessary changes in this section.

3. Stability is major concern in aircrafts. In manuscript, it is explained in a good manner using software. But in figure 8, 9, 10, bandwidth values are missing in the caption, like it is available in figure 11, 12, 13. Please write it so that comparison can be understandable. Also mention the values of the necessary parameters in the caption of figures, where it is not available.

4. For the reference [3-9], the text has very less matter. Please increase and mention the references appropriately.

5. Figures resolution must be improved (especially figure 5)

Author Response

Dear Reviewer,

Authors want to thank the reviewer to help us increasing the paper quality. Some modifications are visible with the “Track Changes” function in the revised paper.

Please, look at the word file enclosed to wath our responses to your comments.

Sincerely yours,

The co-authors,

Author Response File: Author Response.docx

Reviewer 2 Report

The paper proposes an electric powertrain architecture for electrified aircraft that compose of only permanent magnet machines without power electronics. The main motivation of this work is to reduce the aircraft's weight by eliminating power electronics. I have the following comments and findings.
1.    First of all, the application requires the permanent magnet motor to be driven without any control except the gas turbine control. This is a problematic setup as any case of disengagement will result in excessive voltages on the AC bus and circulating currents between the generators and motors. Considering that aviation regulations include strict safety requirements, I do not believe that such a system can find a place in the aviation industry.
2.    It is stated that one of the main contributions of this work is building a transient model of coupled PM synchronous machines, which is not an original contribution. The DQ model of the PMSM is formulated decades ago. What is done in this paper is to expand the model with multiple machines.
3.    In order to couple multiple PMSMs, the parameters of those machines should be tailored to get the best performance and avoid undesired oscillations. Even so, there are numerous operating conditions in an aircraft, even if a specifically designed motor is used, without an active motor control, stability problems may occur. Also, what type of PMSMs are used in this work? Are they interior or surface-mount PMSMs? Are the inductance values given in Table 1 phase inductances? If so, I guess the machines are surface-mount PMSMs, otherwise, the synchronous inductance should be provided.
4.    Although the aircraft require less weight to operate cheaper if the efficiency of the electric system is not maximized with a high-performance motor drive, the total payload decreases, and energy cost increases. This tradeoff is not analyzed. 
5.    The propeller load cannot be modeled as a linear load. It should be proportional to at least the square of the propeller speed (TL = k*omeha^2).
6.    The circuit simulation model given in Figure 5 is blurry.
7.    Some of the variables are not defined such as φ_a_m, φ_a_g, etc. 
8.    Fig 3 should have a, b, and c parts for all the frames presented separately. Also, the font size of these figures is too big.
9.    How the authors linearized the model given in 11 is not explained.
10.    Why two simulation tools are used? They are having nonidentical transient responses. Especially at the beginning of the simulations.
11.    Experimental and simulation results are having different dynamics
12.    Such systems should have an energy storage element like flywheels to damp the oscillations from both the generator and motor sides, which would increase the weight of the system.
13.    Lastly, PMSM performance is affected by thermal variations a lot. The model should contain thermal effects such as the model in https://ieeexplore.ieee.org/document/7873348.

Author Response

Dear Reviewer,

Authors want to thank the reviewer to help us increasing the paper quality. Some modifications are visible with the “Track Changes” function in the revised paper.

Please, look at the word file enclosed to wath our responses to your comments.

Sincerely yours,

The co-authors,

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors acted cooperatively and addressed most of the questions I asked. Although I still have some doubts about the application, the paper seems ready to be published.