Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 11 http://apc.aast.edu I. INTRODUCTION In recent years, there has been an increasing interest in replacing conventional fossil fuels for electrical energy generation with renewable energy resources to reduce pollution in an attempt to achieve a green environment. With the transportation sector being a major source of carbon dioxide emissions, researchers are oriented towards electrifying the transportation section [1]. Different electrical machines, including DC, synchronous, and induction machines could be used for propulsion [2], [3]. Conventionally, the first-choice traction machine is the permanent magnet synchronous machine (PMSM) due to its high torque density, and wide constant torque/ speed range. However, escalated prices and restricted resources of rare-earth materials forced the market to look for a suitable, magnet-free alternative [4]. The switched reluctance machine (SRM) is the dark horse in this race [5]. It has a simple, robust structure, with low cost. Being a double-salient machine with concentrated winding on stator poles, and with neither windings nor permanent magnets on the rotor poles made its design and geometry simpler. Despite the SRM advantages, it suffers from two main drawbacks, namely, vibration and low power density when compared with other traction machines like the PMSM [6] - [8]. Recently, there have been decent attempts to improve the SRM power density [9], [10]. Improving the efficiency and the power density of the SRM to compete with the PMSM was achieved [11] - [13]. The SRM has been studied extensively in recent decades [14]. A new SRM trend explores a higher number of rotor poles than stator poles, as presented in [15]. The new motor concept (N s N r ). Due to the extra space available in the stator slot area, windings with a higher number of turns and thicker cross-sectional area can be deployed [16]. Also, the rotor pole number increase minimizes torque ripple, which is vital for electric vehicle (EV) applications [17]. However, since the interpolar rotor airgaps are narrower in the new motor design, there is an increase in the unaligned inductance value when compared to that of the conventional SRM. The increase of unaligned inductance reduces the energy conversion area, and hence the produced torque. In addition, the process of current build-up in the stator winding will be slower due to the decrease in the unaligned inductance value [18]. Hence, to restore the speed of current build-up, a DC link voltage with higher magnitude is required. Also, the salient rotor structure increases the windage loss, especially at high speeds. Rotor ribs are proposed in [19], [20], to mitigate the SRM windage loss. Film magnetic material is inserted between rotor poles; hence, producing a cylindrical rotor shape. Although the windage loss is significantly reduced, the torque density is reduced, and the torque ripple increased, in addition to the mechanical constraints imposed in fabricating the thin magnetic material. Power Density Improvement Due To Rotor Flux Screens In An Srm With A Higher Number Of Rotor Poles Than Stator Poles Ali A. Abdel-Aziz1, Khaled H. Ahmed1, 2, Ahmed M. Massoud3 and Barry W. Williams1 1 Departement of Electronic and Electrical Engineering, University of Strathclyde, Glasgow, G1 1XQ, UK 2Department of Electrical Engineering, Alexandria University, Alexandria, 21544, Egypt 3 Department of Electrical Engineering, Qatar University, Doha, P.O. Box 2713, Qatar Emails: ali.hassan-abdelaziz-ali@strath.ac.uk, khaled.ahmed@strath.ac.uk, ahmed.massoud@qu.edu.q, barry.williams@strath.ac.uk ABSTRACT This paper st udies the performance of screened switched reluctance motors (SRMs) with a number of rotor poles higher than the number of stator poles. Flux (conduct ing) screens are electrically conduct ing, non-magnet ic materials like aluminum or copper. These screens fill the interpolar rotor air gaps decreasing the unaligned inductance, and thereby increasing the output torque. In addit ion, flux screens result in a c ylindrical rotor str uct ure which minimizes windage losses especially at high speeds. The paper invest igates the effect of the flux screens thickness and material on the SRM performance including output torque, power and phase current. A modified flux t ube approach for est imat ing the unaligned inductance of screened SRM is proposed. Finite element analysis results for different screen cases confirm the effect iveness of conduct ing screens in improving the torque, hence power capabilit y, of switched reluctance motors. Index-words: Electric vehicles, Finite element analysis, Flux screens, Power densit y, Renewable energ y, Switched reluctance motor, Torque ripple. Received on, 11 January 2023 Accepted on, 03 May 2023 Published on, 30 May 2023 http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 12 http://apc.aast.edu Segmented rotor SRM (SRSRM) with cylindrical rotor design was investigated [21]. However, it suffers from complexity in manufacturing and mechanical weaknesses [22]. In addition, the SRSRM has a longer end winding, which deteriorates the electrical loading of the motor [23]. Also, this motor is not suitable for applications requiring motors with short lamination stack length [24]. The concept of flux screens was proposed in [25]. Interpolar rotor air gaps are filled with materials like copper or aluminium which are electrically conducting materials with non-magnetic properties. When the rotor rotates with any speed, voltage is induced in the conducting screens, resulting in the flow of eddy currents. A magnetic field is set up by the flow of eddy currents, which opposes the stator main magnetic field. The result is a decrease in the unaligned inductance value. In [25], the impact of utilizing rotor conducting screens on a three-phase 6/4 SRM was investigated. The deployment of rotor conducting screens for three different SRMs was investigated in [26] – [28], where the screened machines produced higher torque levels than unscreened machines with equivalent volume. However, there was no attempt to investigate the SRM performance when varying the thickness or the material of the conducting screen. Also, the low number of rotor poles increases the torque ripple, which is undesirable for EV applications. In this paper, the utilization of rotor conducting screens for an SRM with a higher number of rotor poles than stator poles (N r >N s ) is investigated. The increased rotor pole number reduces torque ripple. The deployment of rotor conducting screens reduces the unaligned inductance value, hence allowing faster current build-up resulting in higher power per unit volume. Also, as with any SRM, filling the spaces between rotor poles mitigates the windage loss, especially at higher rotor speed. A detailed procedure using the flux tube method for calculating the effective value of unaligned inductance for screened SRMs is presented. Finally, the performance of screened SRM with different screen thicknesses and materials is assessed. The paper is organized as follows. Section ΙΙ sheds light on the concept of utilizing rotor conducting screens for SRMs. A method, based on flux tube approach, is discussed in section III to calculate the unaligned inductance value for screened SRM. Supporting 2D and 3D finite element analysis (FEA) results are presented in section IV. II. SRM WITH ROTOR CONDUCTING SCREENS The SRM structure is simple having salient stator and rotor poles. Only the stator poles have concentrated winding, where each two opposite poles are connected in series forming a phase. In the unaligned position, the flux linkage-current (λ-i) characteristics is linear as the core reluctance is much smaller than the air gap reluctance. On the other hand, when the SRM is in the aligned position core reluctance cannot be ignored resulting non-linear (λ-i ) characteristics, as shown in Figure 1. The conversion area OAB, which is the increase in co- energy when the rotor moves from the unaligned to the aligned position, controls the developed torque. Reducing the effective unaligned inductance increases the conversion area, hence increasing motor output torque. Equation (1) defines the torque. (1) where θ is the rotor position and W f ' is the co-energy λ i λ λ 2 1 A BC D Aligned position θ = θ 2 Unaligned position θ = θ 1 O A* Fig. 1. Flux linkage-current (λ-i) characteristics of SRM A new family of SRMs with a higher number of rotor poles than stator poles was presented as in literature. A three- phase 6/10 SRM and a four-phase 8/14 SRM are examples of the new concept. The new SRM can be operated using the traditional asymmetric half-bridge converter [29] and is characterized by reduced torque ripple since Nr>Ns, which results in an increased number of strokes per revolution whence increased phase overlap. Figure 2 shows a three-phase 6/10 SRM. Where, D sh , d, D are the shaft diameter, rotor diameter, and outer stator diameter, respectively. h s , h r are the stator and rotor pole heights, respectively. b sy , b ry are the stator and rotor back iron, respectively.βs , βr are the stator and rotor pole arcs, respectively. L g is the air gap length, L is the stack length, and N is the number of turns per phase. β r sβ rh sh shD ryb bsy D d lg Fig. 2. Model of 6/10 SRM http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 13 http://apc.aast.edu For a high number of rotor poles, the available interpolar rotor spaces are narrower when compared with the traditional SRM. Thus, the unaligned inductance is high. Copper or aluminum, which are electrically conducting but non-magnetic materials, are utilized to fill in the interpolar rotor air gaps as shown in Figure 3. Whence, this material is referred to as a conducting screen. When the rotor rotates with any speed, voltage is induced in the conducting screens, resulting in the flow of eddy currents. A magnetic field is set up by the flow of eddy currents, which opposes the stator main magnetic field. The result is a decrease in the unaligned inductance. III. FLUX TUBE APPROACH FOR UNALIGNED INDUCTANCE CALCULATION OF SCREENED-SRM The unaligned inductance effective value is of paramount importance to predict the SRM performance [30]. FEA is an intuitive choice for designing and testing electrical machines. Nevertheless, in the early design stage any change in the SRM geometry, turns number or conduction period, will dictate a new model to be built and simulated. which is a time-consuming process. Hence, starting with a mathematical model is a wise choice which compromises both accuracy and time [31]. This section establishes the unaligned inductance calculation method of screened-SRM using the flux-tube approach [32]. It is worth mentioning that, at the aligned position the screens do not alter the aligned inductance. Five flux paths are used to describe the flux magnetic path at the unaligned position as illustrated in Figure 3. Figure 4 demonstrates the magnetic equivalent circuit for calculating the screened-SRM unaligned inductance. Where, R sp ,R g ,R rp , R sy and R ry are the reluctances of the stator pole, air gap, rotor pole, stator back iron, and rotor back iron, respectively. Flux path reluctances are calculated based on the machine geometrical dimensions. In the unaligned position, core reluctance is insignificant when compared to air gap reluctance. Hence, a linear flux linkage – current (λ - i) characteristics curve is obtained. Flux path 1 Flux path 2 Flux path 3 Flux path 4 Flux path 5 Fig. 3. Flux paths in unaligned position Generally, reluctance could be expressed by [33], [34]. (2) where l is the flux magnetic-path length, A is the cross- sectional area, and μ 0 and μ r are the permeability of air and core material relative permeability, respectively. The reluctances for the flux paths are derived in detail in the next subsections. 𝑹𝑹𝒔𝒔𝒔𝒔𝒔𝒔 𝑹𝑹𝒔𝒔𝒔𝒔𝟓𝟓 𝟒𝟒� + - 𝟓𝟓 𝟖𝟖� 𝑵𝑵𝑵𝑵 + - 𝝋𝝋𝟏𝟏 𝝋𝝋𝟐𝟐 𝝋𝝋𝒔𝒔 𝝋𝝋𝟒𝟒 𝝋𝝋𝟓𝟓 𝑹𝑹𝒈𝒈𝟓𝟓 𝟒𝟒� 𝑹𝑹𝒔𝒔𝒔𝒔𝟓𝟓 𝟒𝟒� 𝑹𝑹𝒔𝒔𝒔𝒔𝟒𝟒 𝟐𝟐� 𝑹𝑹𝒈𝒈𝟒𝟒 𝟒𝟒� 𝑹𝑹𝒔𝒔𝒔𝒔𝟒𝟒 𝟒𝟒� 𝑹𝑹𝒈𝒈𝒔𝒔 𝑹𝑹𝒓𝒓𝒔𝒔𝒔𝒔 𝑹𝑹𝒔𝒔𝒔𝒔𝒔𝒔 𝟐𝟐� 𝑹𝑹𝒓𝒓𝒔𝒔𝒔𝒔 𝟐𝟐� 𝑹𝑹𝒔𝒔𝒔𝒔𝟐𝟐 𝑹𝑹𝒈𝒈𝟐𝟐 𝑹𝑹𝒓𝒓𝒔𝒔𝟐𝟐 𝑹𝑹𝒔𝒔𝒔𝒔𝟐𝟐 𝟐𝟐� 𝑹𝑹𝒓𝒓𝒔𝒔𝟐𝟐 𝟐𝟐� 𝑵𝑵𝑵𝑵 𝟖𝟖� 𝑹𝑹𝒔𝒔𝒔𝒔𝟏𝟏 𝑹𝑹𝒈𝒈𝟏𝟏 𝑹𝑹𝒓𝒓𝒔𝒔𝟏𝟏 𝑹𝑹𝒔𝒔𝒔𝒔𝟏𝟏 𝟐𝟐� 𝑹𝑹𝒓𝒓𝒔𝒔𝟏𝟏 𝟐𝟐� + - 𝑵𝑵𝑵𝑵 𝟒𝟒� Fig 4. Magnetic equivalent circuit A. Flux Paths 1, 2 and 3 The first three flux tubes are similar. Hence, their analysis is combined in this subsection. Figure 5 shows the first magnetic flux path where flux flows through the rotor back iron, the stator back iron, the rotor pole, the stator pole, and finally, the interpolar rotor air gap. Five reluctances are used to describe the magnetic flux path. The magnetic-flux path length and cross-sectional area are calculated as follows: A B O C DE θ θ θ 3 2 1 Stator pole Rotor pole Conducting screen Shaft Flux path 1 Fig. 5. Flux path 1 http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 14 http://apc.aast.edu 1. Rotor Back Iron Reluctance, R ry1 , R ry2 , R ry3 For rotor back iron, the average magnetic flux path length and the cross-sectional area are defined by (3) and (4), respectively. (3) (4) where, x =1, 2, 3 represents flux paths 1, 2 and 3, respectively. 2. Stator Back Iron Reluctance, R sy1 , R sy2 , R sy3 Equations (5) and (6) define the average magnetic flux path length and the cross-sectional area for the stator back iron. (5) (6) 3. Rotor Pole Reluctance, R rp1 , R rp2 , R rp3 For the first three flux paths, the flux travels through the entire rotor pole height. Hence, the average magnetic flux path length is given by (7), where (8) defines the cross- sectional area. (7) (8) 4. Stator Pole Reluctance, R sp1 , R sp2 , R sp3 For path 1, the flux is assumed to leave the stator at the pole tip. For paths 2 and 3, the flux is assumed to leave the stator at ⅒ and ¾ of the stator pole height from the top, respectively. Hence, the length of the flux path is defined by (9). The width of the flux path determines the area. For flux path 1, the area is assumed to have width ½ β s +¼ h s . For paths 2 and 3, the entire flux flows throughout the pole height at a width ½ h s and ⅛ h s , respectively. The area is then defined by (10). (9) (10) 5. Air Gap Reluctance, R g1 , R g2 , R g3 The average magnetic flux path length for the air gap is the arc BC as illustrated in Figure 5 and defined by (11) lgx=BC=½(EB+EC) θ2 rad (11) When calculating the cross-sectional area involving large air gaps, fringing cannot be neglected. Hence, the area is considered to be the sum of the rotor area defined by (8) and the stator area given by (10). Finally, the inductance for each flux path is calculated using: (12) B. Flux Path 4 Flux path 4 is demonstrated in Figure 6. θ 3 θ 4 O A B Flux path 4 Stator pole Shaft C D E Fθ 2 θ 1 Fig. 6. Flux path 4 In this magnetic flux path, the rotor is not involved as the flux lines cross from one stator pole to the adjacent through the air gap and then returns back through the stator back iron. Hence, the three reluctances, R g4 , R sp4 , and R sy4 , could be calculated as follows: 1. Air Gap Reluctance, R g4 The arc BC represents the average flux path length in the air gap as given by (13) lg4=BC=(OB)θ4 rad (13) The cross-sectional area is defined by: (14) 2. Stator Pole Reluctance, R sp4 The flux is assumed to leave the stator pole at ¾ h s of the pole height from the top. Therefore, the average magnetic flux path length and the cross-sectional area are defined by (15) and (16), respectively for the stator pole reluctance. http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 15 http://apc.aast.edu (15) (16) 3. Stator Back Iron Reluctance, R sy4 The arc EF is the magnetic flux path length for the stator back iron reluctance, which is defined by lsy4=EF=(OE)θ3 rad (17) On the other hand, the cross-sectional area is expressed by: Asy4= Lbsy (18) The flux does not link the entire number of turns per phase, N. It only links ⅜ of the turns number. Therefore, the fourth flux path inductance, L u4 is given by: (19) C. Flux Path 5 Figure 7 illustrates flux path 5. Flux path 5 Stator pole C B Fig. 7. Flux path 5 The flux leaves the stator pole to enter the stator back iron, passing through the air gap. The flux path is assumed to represent the perimeter of a quarter circle with center at point A and radius of a quarter the stator pole height ¼ h s . The reluctances are calculated as follows: 1. Air Gap Reluctance, R g5 The arc BC represents the length of the magnetic flux path and defined by (20). While, (21) defines the area. (20) (21) 2. Stator Pole Reluctance, R sp5 For the stator pole reluctance, the flux path length is defined by (22), and the area is expressed by (23). lsp5=¼(hs+bsy) (22) (23) 3. Stator Back Iron Reluctance, R sy5 The mean flux path length and the cross-sectional area for the stator back iron reluctances are defined by (24) and (25), respectively. (24) Asy5= Lbsy (25) The flux links only ⅛ the turns per phase N. Therefore, , which represents flux path 5 inductance is given by: (26) Finally, the effective value of unaligned inductance for the screened- SRM is calculated by summing all the flux path inductances as expressed by (27). (27) IV. SIMULATION RESULTS In this section, the performance of the screened-SRM is investigated statically and dynamically using FEA. The static test gives an insight on the effective value of unaligned inductance to validate the proposed mathematical approach. On the other hand, the dynamic test studies the effect of varying the thickness and material of the screen on the developed torque and the current build-up process. A three-phase, 6/10 SRM with the specification in Table 1 is used for analysis. The increased number of rotor poles allows for more space to accommodate the stator winding (since pole arcs of the new SRM are narrower than the pole arcs of a conventional three-phase SRM). According to the selected specification, the copper current density is less than 5A/mm2, thus special cooling is not required. For a fair comparison between unscreened and screened SRMs, the same firing angles are applied in both cases. TABLE I. SRM SPECIFICATION Parameter Value No. of phases m 3 Stator/rotor poles 6/10 Number of turns per pole N 60 Phase winding resistance R 0.8Ω DC link voltage 500V Rated power 6kW Base speed 1500rpm http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 16 http://apc.aast.edu Axial length 240mm Shaft diameter 40mm Rotor outer diameter 120mm Rotor yoke thickness 30mm Ratio of rotor pole arc to pole pitch 0.335 Stator inner diameter 122mm Stator outer diameter 200mm Stator yoke thickness 25mm Ratio of stator pole arc to pole pitch 0.21 A. Static Analysis The proposed mathematical approach for unaligned inductance calculation is validated by a 3D FEA model. Table II shows good agreement between the proposed analytical method and the 3D FEA model. TABLE II. UNALIGNED INDUCTANCE Analytical 3D FEA 4.66mH 5.05mH The inductance profile of the SRM under test is plotted in Figure 8 for both the screened and unscreened (original) SRMs. Fig. 8. Inductance profile Results are obtained using 3D FEA model. As expected, the flux screens have insignificant effect on the aligned inductance value. On the other hand, the unaligned inductance value decreased from 8.68mH in case of unscreened SRM to reach 5.05mH for the screened SRM. The 40% reduction in the effective unaligned inductance value will increase the conversion area (area OAB in Figure 1) resulting in more developed torque, hence increasing the torque/power density of the SRM. B. Dynamics Analysis The dynamic performance of the screened-SRM is presented in this sub-section using 2D FEA where the process of current build-up along with the developed torque are demonstrated. Conventional asymmetric half- bridge with two diodes and two switches per phase is used. The SRM operates in a single-pulse mode; that is, the dc-link voltage is applied for the whole dwell period. Then a negative voltage is applied at the end of the dwell period for rapid current commutation, as implied in Figure 9. Fig. 9. The voltage applied on phase A The dynamic performance at rated conditions is investigated, and Figure 10 compares the results of unscreened and 6mm thick Cu screened SRMs. Figure 10a shows the phase current waveforms for the unscreened and screened SRMs. The reduction in the effective unaligned inductance alters the current response of the SRM with conducting screens. Utilizing the same dc-link voltage, the screened motor accelerates the current build-up. The rms phase current increases from 17.7A for the unscreened SRM, to 21A for the screened case. Figure 10b compares the torque profile in both cases. The screened motor is able to develop more torque as a result of a higher current and lower unaligned inductance. The average torque of the unscreened motor is 38.6Nm. This value increases by 34% to 50.77Nm for the screened SRM, hence improving SRM torque/volume. Thus, the deployment of conducting screens improves the Nm/A by 10%, which reflects on the motor power factor [5]. http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 17 http://apc.aast.edu A B Fig. 10. Performance comparison of unscreened and screened SRM: (a) phase current waveforms and (b) developed torque waveforms. Figure 11 shows the torque/speed characteristics of the unscreened and screened SRM. The screened SRM offers superior torque over the entire speed range. Fig. 11. Torque/speed characteristics Below the motor base speed, current chopping control (CCC) is used. In this control technique, the motor develops its rated torque, and the speed is controlled from zero up to base speed by controlling the phase currents. Above base speed, the phase currents cannot be controlled (chopped) anymore, and the motor enters the single pulse mode. In this mode, the speed of the motor is controlled by adjusting the turn on and turn off angles. Hence, this mode is referred to as advance angle control (AAC). Above the base speed, the motor cannot produce its rated torque. However, controlling the turn on/off angles allows the motor to operate at constant power. C. Effect of Screen Material and Thickness This subsection investigates the screened-SRM performance when screens with different materials and thicknesses are utilized. Figure 12 shows SRM current and torque waveforms using two different screens. The first screen is 3.5mm thick copper, while the second screen is 10 mm thick aluminum (filling the whole interpolar rotor air gaps). A B Fig. 12. Performance of SRM using different screen materials: (a) current waveforms and (b) torque waveforms The electrical conductivity of copper is 5.98×107 S/m with density 8960 kg/m3. The electrical conductivity of aluminum is 3.5×107 S/m with density 2600 kg/m3. The SRM with a 3.5 mm copper screen is able to deliver the same output torque as the 10 mm aluminum screen. This establishes that electrical conductivity plays an important role in the behavior of the induced eddy current. Figure 13 compares SRM performance when different thickness copper screens are deployed, where increasing the screen thickness results in more developed torque. From this study, it is concluded that the thickness and material of the screen affect the SRM performance. Using a film screen with low conductivity results in higher resistance to the induced voltage, hence the eddy current is smaller. Increased resistivity does result in a reduced eddy current decay time constant. http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 18 http://apc.aast.edu A B Fig. 13. Performance of SRM using different screen thicknesses: (a) current waveforms and (b) torque waveforms The deployment of rotor conducting screens does not alter motor physical volume (as opposed to material volume). Hence, the increase in output power will directly reflect the increase in power density (kW/litre), as shown in Figure 14a. The specific power (kW/kg) (which is an important factor in most applications) is compared for the unscreened and screened SRMs in Figure 14b, showing an improvement in specific power when conducting screens are used. The penalty for reducing torque ripple and improved torque and power output is screen eddy losses, which reduce machine efficiency. A B Fig. 14. Performance comparison of unscreened and screened SRMs: (a) output power and (b) specific power VII. CONCLUSION The paper investigated the effect of utilizing rotor conducting screens to enhance the torque capability of SRM with a higher number of rotor poles than stator poles. The rotor pole increase reduces torque ripple. However, since the interpolar rotor gaps are narrower in the new motor design, the unaligned inductance is significantly higher when compared with conventional SRM, thus reducing the conversion area. Filling the rotor interpolar gaps with electrically conducting, non- magnetic material reduces the effective unaligned inductance as a result of the opposing flux generated by eddy currents in the screens. A detailed derivation of the effective value of unaligned inductance for screened SRM was presented and validated using FEA. A 40% reduction in the unaligned inductance and a 34% increase in output torque was recorded using copper screens. Also, a 10% increase in the Nm/A, hence power factor improvement, was achieved. The effect of using conducting screens of different materials and thicknesses on SRM performance was presented. The SRM with copper screens is able to http://dx.doi.org/10.21622/RESD.2021.07.2.043 Journal of Renewable Energy and Sustainable Development (RESD) Volume 9, Issue 1, June 2023 - ISSN 2356-8569 http://dx.doi.org/10.21622/RESD.2023.09.1.011 19 http://apc.aast.edu deliver more output torque. On the other hand, aluminum is lighter and cheaper. Increasing screen thickness increases torque production. Finally, screened SRMs provided better power density and specific power than unscreened SRM. Some interesting topics could further be investigated as: • The performance of SRM with rotor conducting screens has been reported only for motoring mode. The research could be extended to cover the generating mode. • Given the simple coil winding arrangement in the SRM, and water cooling, advanced manufacturing techniques may offer higher copper density (slot fill factor) improvement possibilities, for example with square section conductors. References [1] Z. Wang, T. W. Ching, S. Huang, H. Wang, and T. Xu, “Challenges Faced by Electric Vehicle Motors and Their Solutions,” IEEE Access, vol. 9, 2021, doi: 10.1109/ACCESS.2020.3045716. [2] E. Sayed et al., “Review of Electric Machines in More-/Hybrid-/Turbo-Electric Aircraft,” IEEE Transactions on Transportation Electrification, vol. 7, no. 4. 2021. doi: 10.1109/TTE.2021.3089605. [3] L. 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