Optimal Design of a Six-Phase Permanent-Magnet-Assisted Synchronous Reluctance Motor to Convert into Three Phases for Fault-Tolerant Improvement in a Traction System

Round 1

Reviewer 1 Report

My comments and questions:

1) There are many papers on investigating multiphase PMaSynRMs and their various emergency modes. The novelty of the paper is not clear.

2) The objective function contains the parameter “the cost of materials”. However, there is no information on how this cost was calculated in the paper. Please, add the detailed information on the active material mass (steel, copper, magnets separately) and on the calculation of their cost. Provide the information on the material cost before and after the optimization.

3) Provide the information on power factor before and after optimization.

4) Why are the terms aimed to increase the power factor and to decrease the torque ripple not included in objective function (21)?

5) The efficiency in wide range of speed and torque is important in traction motors. Why is the efficiency only in  a single operating mode included in objective function (21)?

6) In table 5, the information on the torque ripple for 6-phase motor is provided. It is too high torque ripple for a 6-phase motor. It results in high acoustic noise. Even three-phase traction motors have less torque ripple.

7) Which is the constant power range of your drive? Provide the values of the efficiencies, the torque ripples, the currents, and the voltages for the motors before and after the optimization at the most important modes of traction motors.

8) The mechanical power of your motor is 3 kW. To supply the motor, you use two very powerful Simikron invertors. The power utilization factor (the ratio of the mechanical power to the full invertors power) is too low. The drive you suggest is very expensive and loses in terms of basic characteristics to modern competing solutions.

Author Response

Dear professor Dr

First of all, we want to express our thanks for the constructive comments and discussions to improve the quality of our manuscript significantly.

 

• Comments and Suggestions for Authors and answers

Comment#1

• There are many papers on investigating multiphase PMa-SynRMs and their various emergency modes. The novelty of the paper is not clear.

 

Answer:

  Thank a lot, professor for asking the fundamental question of why this paper is necessary. I had an opportunity to think deeply about the contents of my paper. In my opinion, the value of this paper is that it is a basic practical study to commercialize a six-phase motor with excellent fault-tolerant capability, and I present the following reasons:

1. For machine design, using the lumped parameter method(LPM) on basis of the magnetic-circuits with relatively high accuracy, a realistic method that can significantly reduce the design time of the machine was introduced and its accuracy was verified.
2. The phase switching function which is an important advantage of a six-phase motor that can be used in electric vehicle systems(EVs) requiring high fault-tolerance, was realized through experiments and its characteristics were verified. In addition, the possibility for commercialization was paid attention to by presenting two switching modes that can be applied during phase-change.

To highlight the clear direction of the research, the title of the paper was modified as follows:

• Before : Optimal Design of a Six-phase Permanent Magnet assisted Synchronous Reluctance Motor for a Traction System, and for Switching to a Three-phase System
• After : Optimal Design of a Six-phase Permanent Magnet Assisted Synchronous Reluctance Motor to Convert into Three-Phase for Fault-Tolerant Improvement in a Traction System

Comment#2

• The objective function contains the parameter “the cost of materials”. However, there is no information on how this cost was calculated in the paper. Please, add the detailed information on the active material mass (steel, copper, magnets separately) and on the calculation of their cost. Provide the information on the material cost before and after the optimization.

 

Answer:

  The information of a material cost term explained above was included on page 8.

A paragraph was added as follow:

  Equation (21) is composed of efficiency and cost terms, and k1 and k2 were set to 0.6 and 0.4 as weights for each term, respectively. In addition, it was set to derive the optimized shape in which the objective function value is the minimum by the efficiency calculated and the cost value of the initial model as the denominator in the equation. The material cost included in the objective function was set as a per-unit value(p.u/kg) relative to the unit cost per weight of each material constituting the machine. Then, the total cost was calculated by multiplying weight and unit cost per weight corresponding to each material calculated through LPM, where the unit cost per weight was set as the iron to be 1, the copper to be 6, and the PM to be 60.

Comment#3

• Provide the information on power factor before and after optimization.

 

Answer:

  The information on power factor before and after optimization was included on page 9.

The Information was added as follow:

(a)	(b)

Figure 6. Cross-section of designed a six-phase PMa-SynRM. (a) Initial model before optimization; (b) Optimized model

 

 

 Table 2. Results of design using LPM before and after optimizing process.

Parameters	Before optimization	After optimization
Base speed [r/min]	1800	1800
Rated current [Arms]	10	10
Phase voltage [Vrms]	110.7	97.9
Torque [Nm]	17.7	17.5
Ripple factor [%]	5.1	6.3
Power factor	0.690	0.848
Efficiency [%]	93.4	95.3
Material cost [p.u]	506.1	501.2

 

  Figure 6a shows the shape of the initial model before optimization. It can be shown in Table 2, a phase voltage, power factor, and efficiency were analyzed to be 110.7 Vrms,  0.69 and 93.4% respectively. Figure 6b shows the optimized model which is a model selected as a final candidate through the optimizing process, and a phase voltage, power factor and efficiency were analyzed to be 97.9 Vrms, 0.848 and 95.3% respectively. In addition, it was confirmed that the cost was reduced through Equation (21).

Comment#4

• Why are the terms aimed to increase the power factor and to decrease the torque ripple not included in objective function (21)?

 

Answer:

  The content for the question was included in page 7 and the description is as follow:

1. The six-phase PMa-SynRM was designed as a machine with concentrated windings to minimize the manufacturing cost and reduce the possibility of fault during experiments for the fault-tolerance test. Therefore, the power factor was not included in the objective function (21) because it was not a major evaluation index for machine design.
2. Torque ripple is a performance indicator that could not be calculated in the LPM program, so it could not be included in the objective function (21). Therefore, a pole/slot combination that can minimize torque ripple is selected by calculating the magnitude of the harmonic orders of winding coefficient indirect way. In addition, a final model is selected through the FEM analysis results among the derived candidate models since the torque ripple depends on the machine shape.

A sentence was added as follow:

  In this paper for the purpose of evaluating the fault-tolerance of a machine, the terms of power factor and torque ripple were not included in the objective function.

 

 

Comment#5

• The efficiency in wide range of speed and torque is important in traction motors. Why is the efficiency only in a single operating mode included in objective function (21)?

 

Answer:

  Torque and efficiency whether to be achieved in wide speed ranges were not included in the evaluation index of this study. Since the six-phase motor used in this study was designed with a low-cost concept to test fault-tolerance only, high specifications were excepted and the optimal design was performed based on a single operating point. The abstract has been partially revised to clarify the aims of this study.

The abstract was revised as follow:

  A six-phase motor with a high degree of freedom can be converted into a three-phase motor, so it can be used in a traction system. In addition, when the phase-change technology is applied, it is possible to establish an efficient control strategy tailored to the driving environment of the EVs. Therefore, in this paper, a down-scaled 3 kW permanent-magnet-assisted synchronous motor (PMa-SynRM) capable of phase switching was designed, and its driving states in controlled fault-modes were analyzed through experiments. The PMa-SynRM selected for this study is a machine that has good fault-tolerance capabilities and inexpensive than IPMSM of same performance; it was designed using the lumped-parameter method (LPM) having a fast calculating speed and a genetic algorithm. In addition, the effectiveness of the optimal design was verified by comparing the analytical results of the FEM and LPM. Lastly, a phase switching experiment was conducted to analyze the steady-state and transient-state characteristics, and the results are presented.

Comment#6

• In table 5, the information on the torque ripple for 6-phase motor is provided. It is too high torque ripple for a 6-phase motor. It results in high acoustic noise. Even three-phase traction motors have less torque ripple.

 

Answer:

  A small error related to the assign area was contained in the rotor sketch of the FEM modeling. I updated all FEM analytical results presented in this paper after correcting the error. As a result, the torque ripple was reduced overall, and it was reduced from 11.3% to 6.3% at a rated current. However, the analytical results still show a high torque ripple, which is analyzed as an effect of concentrated winding. But, in the experimental results, the torque ripple at rated current is measured as 0.7% due to the 1st low-pass filter characteristics of the mechanical system, which is about 98% lower than the analytical results.

A mistake was corrected and the graph was updated as follow:

 

Figure 9. Torque curves of six- and three-phase from FEM at 480 r/min and 10 A.

 

 

Table 6. Torque characteristic according to currents in FEM at 480 r/min.

Parameters		2A	4A	5A	6A	8A	10A
Six-phase	Max. torque [Nm]	3.4	6.8	8.5	10.4	14.2	18.1
	Min. torque [Nm]	2.9	6.1	7.7	9.5	13.1	17.0
	Mean torque [Nm]	3.1	6.4	8.1	9.9	13.6	17.5
	Difference of min/max [Nm]	0.5	0.7	0.8	0.9	1.1	1.1
	Ripple factor [%]	16.1	10.9	9.9	9.1	8.1	6.3
	Power factor	0.992	0.965	0.951	0.934	0.903	0.869
	Efficiency [%]	95.0	95.9	92.0	90.9	88.9	87.0
Three-phase	Max. torque [Nm]	1.8	3.5	4.3	5.2	6.9	8.7
	Min. torque [Nm]	1.4	2.8	3.6	4.4	6.1	7.8
	Mean torque [Nm]	1.6	3.2	4.0	4.8	6.5	8.2
	Difference of min/max [Nm]	0.4	0.7	0.7	0.8	0.8	0.9
	Ripple factor [%]	25.0	21.9	17.5	16.7	12.3	11.0
	Power factor	0.997	0.983	0.974	0.963	0.969	0.924
	Efficiency [%]	90.7	87.0	84.9	82.8	79.2	75.9

 

Comment#7

• Which is the constant power range of your drive? Provide the values of the efficiencies, the torque ripples, the currents, and the voltages for the motors before and after the optimization at the most important modes of traction motors.

 

Answer:

The constant power range is from 1800 r/min to 4500 r/min, but the optimal design was performed based on a single operating point. Therefore, the most important mode proposed in analytical results is a rated operating point at 1800 r/min, 10 Arms. The current, the voltage, the torque, the torque ripple, and the efficiency are presented in Table 2.

The change was done as follow:

Table 2. Results of design using LPM before and after optimizing the process.

Parameters	Before optimization	After optimization
Base speed [r/min]	1800	1800
Rated current [Arms]	10	10
Phase voltage [Vrms]	110.7	97.9
Torque [Nm]	17.7	17.5
Ripple factor [%]	5.1	6.3
Power factor	0.690	0.848
Efficiency [%]	93.4	95.3
Material cost [p.u]	506.1	501.2

 

Comment#8

• The mechanical power of your motor is 3 kW. To supply the motor, you use two very powerful Simikron invertors. The power utilization factor (the ratio of the mechanical power to the full invertors power) is too low. The drive you suggest is very expensive and loses in terms of basic characteristics to modern competing solutions.

 

Answer:

  Thanks for the good point on power utilization. Two inverters are used as versatile equipment which has robustness in a laboratory rather than the industrial. In addition, The overall system efficiency does not be evaluated because only a study for phase switching was conducted to confirm the fault-tolerance for the six-phase machine. In the future, when designing an optimally designed six-phase EVs machine with actual specifications, we will evaluate the overall system efficiency including the inverters.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors, 

1/ Please add more details (a paragraph or 2) to describe the machine fault behavior, what can potentially trigger the fault described, loss of excitation from the inverter or short circuit in the windings.... The information between 52 and 54 is not enough.

2/ Please add more details in section 3.5 on the fault tolerant operating modes, and the simulation setup to help explain table 6.

3/ Please add more details in section 5, figure 16 on how the loss of 1 inverter was implemented in the experiment. 

4/ In the experimental results, please add more information on when the phase switching from 6-phase to 3-phase was implemented, how the control performed after the phase switching was implemented? Figures 17 and 18 are very busy to look at.

Thank you very much.

 

Author Response

Dear professor Dr

First of all, we want to express our thanks for the constructive comments and discussions to improve the quality of our manuscript significantly.

 

• Comments and Suggestions for Authors and answers

Comment#1

• Please add more details (a paragraph or 2) to describe the machine fault behavior, what can potentially trigger the fault described, loss of excitation from the inverter or short circuit in the windings.... The information between 52 and 54 is not enough.

Answer:

  Two paragraphs regarding a reference and its contents were added on page 2 to explain the fault situation or conditions.

Paragraphs were added and modified as follow:

  When the amount of permanent magnet (PM) is large, there is the risk of PM demagnetization during field weakening control to suppress back electromotive force due to high-speed rotation, and excessive current may be induced from high flux linkage on a phase short-circuit phase [15,16,17].

  In traction systems, reliability, safety, and high power density must be ensured [1,17]. If the operation cannot continue to a fault in the inverter or in the motor occurs on the train, cars that use an existing three-phase traction system can lead to increasing the possibility of secondary accidents. Some typical types of faults are a case in which current does not flow to a phase due to an open winding of the motor, and a case in which excessive current flows through a faulty short-circuit phase.

  In order to overcome the low reliability and safety problems of such a three-phase traction system, the author in [17] proposed a triplin redundant nine-phase PMa-SynRM with improved fault-tolerance and suggested solutions for two fault scenarios, especially the fault current due to short-circuit was analyzed as the worst case of fault scenario. However, although the proposed motor is a nine-phase system composed of three windings, it is essentially a three-phase system and has no additional advantages other than the merits of continuous operation in case of faulty condition.

  Therefore, a multi-phase machine was proposed as an alternative that can operate continuously during a failure that occurs in the motor or inverter and improves power density [2-7, 18-30].

 

 

Comment#2

• Please add more details in section 3.5 on the fault tolerant operating modes, and the simulation setup to help explain table 6.

Answer:

A description of the simulation setup was added to Section 3.5 on page 13.

Sentences were added and corrected as follow:

  Torque characteristics were analyzed for two cases assuming a fault state using FEM and the analysis was performed at 480 r/min with the UVW2 phase opened as shown in Figure 1. The first is constant-current(CC) mode, and since the magnitude of the phase current did not change, it was a control mode in which the output torque decreased when switching from six- to three-phases. The second is constant-torque(CT) mode, which is a control mode that maintains the output torque by doubling the phase current when switching from six- to three-phases. Therefore, 5 A which does not exceed the rated current of 10 A, was selected for the load current when switching to three-phase winding in the constant-torque mode, and additionally, the simulation was conducted under 4 A to confirm the variation of torque ripple. In constant-torque mode, there is a risk of exceeding the rated current of the system, so appropriate control scenarios and safeguards must accompany it. Table 6 shows an example of the operation of a six-phase motor that can be converted into three-phases one within a rated phase current of 10 A, and analysis was performed assuming that a phase current of 5 A flowed in the six-phase motor.

Table 7. Performance of each case of operation modes for six- and three-phase.

Parameters	6PH_5A	3PH_5A	3PH_10A	6PH_4A	3PH_4A	3PH_8A
Operation mode	Initial	CC	CT	Initial	CC	CT
Max. torque [Nm]	8.5	4.3	8.7	6.8	3.5	6.9
Min. torque [Nm]	7.7	3.6	7.8	6.1	2.8	6.1
Mean torque [Nm]	8.1	4.0	8.2	6.4	3.2	6.5
Difference of min/max [Nm]	0.8	0.7	0.9	0.7	0.7	0.8
Ripple factor [%]	9.9	17.5	11.0	10.9	21.9	12.3
Power factor	0.951	0.974	0.924	0.965	0.983	0.969
Efficiency [%]	92.0	84.9	75.9	95.9	87.0	79.2

Comment#3

• Please add more details in section 5, figure 16 on how the loss of 1 inverter was implemented in the experiment.

Answer:

A description for the loss of one inverter was added to section 5 on page 17.

Sentences were added as follow:

  In Section 4, the load characteristics in the steady-state were analyzed in detail. Next, it is necessary to check how changes in voltage, current, and torque appear when switching from six- to three-phase operation. Here, the data obtained through transient analysis during phase switching can be used as a reference when designing the controller and inverter of the fault-tolerant system. The phase switching experiment was conducted by changing the control program for each mode.

  In order to switch from six- to three-phase as shown in Figure 16, the input and output of INV2 must be limited to zero in the controller. Here, the output variable of SVPWM, which is the input of the inverter, was set to zero, and the three-phase switch of INV2 is turned off using the trip function to prevent malfunction. At this time, according to the state of the Clarke transformation matrix of the fed-back current, it was divided into constant-current and a constant-torque mode. In a normal six-phase operation, a six-phase Clarke transformation matrix is applied and multiplied by a scaling factor of 1/3, as shown in Equation (22).

Comment#4

• In the experimental results, please add more information on when the phase switching from 6-phase to 3-phase was implemented, how the control performed after the phase switching was implemented? Figures 17 and 18 are very busy to look at.

Answer:

  The phase switching time and the results of control were indicated in each figure.

Figures 17 and 18 were changed as follow:

 

Figure 17. Transient-state of constant-current mode at 480 r/min and 5 A.

 

Figure 18. Transient-state of constant-torque mode at 480 r/min and 5A.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors write: “However, the analytical results still show a high torque ripple, which is analyzed as an effect of concentrated winding.”

Please explain in more detail why the results obtained using the FEM and analytical model differ so much? Are the torque ripple results obtained with the FEM model using the Maxwell stress tensor and the integral of the air-gap flux density the same?

“But, in the experimental results, the torque ripple at rated current is measured as 0.7% due to the 1st low-pass filter characteristics of the mechanical system”.
I consider this answer of the authors unsatisfactory. This phrase only shows that the authors do not know how to measure the motor torque ripple and do not distinguish between the motor torque ripple and the mechanism torque ripple. These measurements with mechanism a do not characterize the properties of the motor in any way. Is it possible to provide the measurements of the torque ripple according to doi.org/10.3390/en13112886?

“The constant power range is from 1800 r/min to 4500 r/min, but the optimal design was performed based on a single operating point. Therefore, the most important mode proposed in analytical results is a rated operating point at 1800 r/min, 10 Arms.”
Since traction motors run above rated speed most of the time, I find this answer by the authors unsatisfactory. Please also provide the optimized and non-optimized motor performances for the 4500 rpm, 3 kW operation point. 

Author Response

Review report#1-2

 

 

Dear professor Dr

  We express our deep appreciation and respect for your fine reviews. Thanks to your point, we were able to find out a problem of torque sensor regarding resolution in the torque ripple analysis and revise the paper properly. Thanks again for your reviews.

The description for the problem is as follows:

Since the motor used in this study is a 6-phase 10-pole motor, torque including the 12th pulsation appears in one cycle of the electrical angle (72 degrees) and has a 60th pulsation in one rotation. In addition, Since the data sampling of the torque sensor is performed 30 times per one rotation of the rotor shaft and this low resolution is not enough to measure the normal torque pulsation. Therefore, the torque ripple presented as an experimental result is incorrect information because the torque sensor cannot measure the torque of peak-to-peak properly during one rotation of the rotor shaft.

 

Comments and Suggestions for Authors and answers

Comment#1

• “However, the analytical results still show a high torque ripple, which is analyzed as an effect of concentrated winding.”
  • Please explain in more detail why the results obtained using the FEM and analytical model differ so much? Are the torque ripple results obtained with the FEM model using the Maxwell stress tensor and the integral of the air-gap flux density the same?

Answer:

  A paragraph was added to explain the torque ripple on lines from 358 to 364 with an additional description including the reference.

A paragraph was added as follow:

In Table 6, most RF has a large value, which is analyzed as the effect of concentrated winding. In general, the air-gap flux density distribution is a square waveform or trapezoidal waveform in the concentrated winding, which has large spatial harmonics are included, causing a large torque ripple. To reduce the torque ripple, optimization methods of rotor and stator shapes was proposed [33,34], but in this study, the optimization of design variables to reduce the torque ripple of FSCW (fractional slot concentrated winding) did not be considered.

• “But, in the experimental results, the torque ripple at rated current is measured as 0.7% due to the 1st low-pass filter characteristics of the mechanical system”.
  • I consider this answer of the authors unsatisfactory. This phrase only shows that the authors do not know how to measure the motor torque ripple and do not distinguish between the motor torque ripple and the mechanism torque ripple. These measurements with mechanism a do not characterize the properties of the motor in any way. Is it possible to provide the measurements of the torque ripple according to doi.org/10.3390/en13112886?

Answer:

I thought the experimental mechanism system was the main cause of the low torque ripple before I realized that the measured low torque ripple was caused by the resolution of the torque sensor. However, this idea has not been validated and should not be inserted into the paper. It's an honor to learn from your review. Therefore, all the contents of the LPF characteristics of the machine were deleted from the paper, and the contents of the torque measurement process in the experiment and torque sensor characteristics were added on lines from 412 to 425.

The torque ripple measurement method you provided is currently not feasible in our system. To measure torque ripple in our experimental equipment, we need to use another torque sensor that can receive data in analog form.

Sentences were added as follow:

The torque sensor used in this study is a type that can measure only the average torque 30 times per rotation of the rotor shaft. Therefore, information on torque ripple cannot be provided through experimental results. (lines from 411 to 414)

The experiment proceeded as follows. First, the test speed is made through speed control in the servo motor. At this time, the six-phase motor operates in generating mode, and when the load current is not applied, the back electromotive force can be measured. Here, when a load current is applied to the motor, it becomes a motoring mode, and torque is generated by a rotating force in the opposite direction. (lines from 420 to 424)

 

• “The constant power range is from 1800 r/min to 4500 r/min, but the optimal design was performed based on a single operating point. Therefore, the most important mode proposed in analytical results is a rated operating point at 1800 r/min, 10 Arms.”
  • Since traction motors run above rated speed most of the time, I find this answer by the authors unsatisfactory. Please also provide the optimized and non-optimized motor performances for the 4500 rpm, 3 kW operation point.

Answer:

  The information of motor performances at the speed of 4500 rpm and descriptions were added in Table 2 and on lines from 274 to 280 respectively.

The information was added as follow:

At the maximum speed of 4500 r/min, the torque decreased by 49% and 54% before and after optimization, respectively, and the power factor showed a tendency to increase. However, it was analyzed that RF increased to 12.1% before optimization and decreased to 4.9% after optimization. In addition, the line to line voltage exceeded 120 Vrms in the case of the initial model, but the line to line voltage of the optimized model was interpreted as less than 120 Vrms. Also, it was found that the efficiency was higher after optimization.

Table 2. Results of design using LPM before and after optimizing the process.

Parameters	Before optimization		After optimization
Speed [r/min]	1800	4500	1800	4500
Rated current [Arms]	10	10	10	10
Line to Line voltage [Vrms]	135.6	128.9	120.0	116.8
Torque [Nm]	17.7	9.1	17.5	8.1
Ripple factor [%]	5.1	12.1	6.3	4.9
Power factor	0.690	1.0	0.848	0.960
Efficiency [%]	93.4	94.2	95.3	95.1
Material cost [p.u]	506.1	506.1	501.2	501.2

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

I think the authors have responded to all my comments satisfactorily.

A more detailed study of the torque ripple of the machine in question is expected in future papers.