Optimal segmented rotor design for the embedded electrical ...

This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg)Nanyang Technological University, Singapore.

Optimal segmented rotor design for theembedded electrical machine for the moreelectric aircraft

Loh, Jenn Yen; Prabhu, Mathivanan Anand; Wang, Shuai; Joshi, Sunil Chandrakant;Viswanathan, Vaiyapuri; Ramakrishna, Shanmukha

2019

Loh, J. Y., Prabhu, M. A., Wang, S., Joshi, S. C., Viswanathan, V., & Ramakrishna, S. (2019).Optimal segmented rotor design for the embedded electrical machine for the more electricaircraft. The Journal of Engineering, 2019(17), 4321‑4324. doi:10.1049/joe.2018.8192

https://hdl.handle.net/10356/104820

https://doi.org/10.1049/joe.2018.8192

© 2019 The Author(s).This is an open access article published by the IET under the CreativeCommons Attribution ‑NonCommercialLicense(http://creativecommons.org/licenses/by‑nc/3.0/)

Downloaded on 07 Oct 2021 00:42:34 SGT

The Journal of Engineering

The 9th International Conference on Power Electronics, Machines andDrives (PEMD 2018)

Optimal segmented rotor design for theembedded electrical machine for the moreelectric aircraft

eISSN 2051-3305Received on 25th June 2018Accepted on 30th July 2018doi: 10.1049/joe.2018.8192www.ietdl.org

Jenn Yen Loh1,2 , Mathivanan Anand Prabhu2, Shuai Wang2, Sunil Chandrakant Joshi1, VaiyapuriViswanathan3, Shanmukha Ramakrishna3

1School of Mechanical and Aerospace, Nanyang Technological University, Singapore 6397982Rolls-Royce@NTU Corporate Laboratory, Nanyang Technological University Singapore 6374603Advanced Technology Centre, Rolls-Royce Singapore Pte. Ltd., Singapore 797575

E-mail: [email protected]

Abstract: This study presents the design and analysis of the segmented rotor design for an electrical machine embedded ontothe more electric aircraft gas turbine engine shaft. The high-speed operational environment imposes high stress levels due tocentrifugal forces; thus, the feasibility of this segmented rotor topology for such application is explored and a suitable design isproposed to overcome mechanical stresses. Finite element structural analysis was performed to compare the mechanicalbehaviour of a different rotor segment fixing structure and the proposed design. Also, electromagnetic analysis is carried out ona high-speed switched reluctance machine to study the impact of a new design on machine electromagnetic performance.

1 IntroductionIn the conventional aircraft, the non-propulsive systems aretypically driven by a combination of different secondary powerunits: hydraulic, pneumatic, electrical, and mechanical. Thesepowers, extracted from the aircraft main engine by individualdiscipline, result in rather complicated and inefficient aircraftsecondary power distribution systems. Due to its complexity andredundancy, there is a trend to adopt a single power type, i.e.electrical power, to drive the non-propulsive aircraft system, anidea with regard to more electric aircraft (MEA) [1].

The more electric engine (MEE) is a companion concept ofMEA. Taking the MEA system approach, it is anticipated that theaircraft electrical power requirement to increase greatly. Thisresults in a need for step change in the on-board electricalgeneration system. MEE extracts the electric power from a seriesof high-speed, high-power density generators which is directlyembedded on to the engine shaft without using a traditionalgearbox. The mechanical design of a rotor for high-speed electricalmachine represents a challenging task, in particular dominated bythe need of a stable, mechanically robust rotor. Also, the embeddedelectrical machine requires high-power density due to the confinedspace inside a gas turbine engine.

To enable this MEE concept, a segmented rotor-type switchedreluctance machine (SRM) is one of the suitable choices as astarter/generator that is to be integrated onto the shaft of an aircraftturbine engine. Despite the segmented rotor SRM can deliverhigher torque [2–4], the mechanical properties of the rotor structurebecome a major consideration at higher rotational speed and power[5].

2 Segmented rotor designIn a segmented rotor, the soft magnetic iron rotor segments areconstrained in a non-magnetic rotor hub through a root fixingstructure, as illustrated in Fig. 1. This rotor configuration is ofteneliminated for high-speed application because of its susceptibilityto mechanical problems. The rotor suffers from high stresses in thecontact regions due to centrifugal force, particularly when operatedat high speeds. In order to mitigate a high stress level in the rotor,gas turbine rotor blade root fixing designs are adopted insegmented rotor application. The root-fixing designs used in thesegmented rotor are mainly (a) dovetail, (b) fir tree, and (c)straddle T [6], as shown in Fig. 2.

In MEE application, the available space for the rotor structureof the embedded electrical machine becomes extremely limited dueto the large shaft diameter of the aircraft gas turbine engine. Therelatively large minimum inner diameter compared to themaximum outer diameter of the machine results in a thin non-magnetic rotor hub structure, which is not participating in aidingthe electromagnetic performance but to provide mechanical supportto the rotor segments.

2.1 Segmented rotor analysis

Structural finite element analysis (FEA) is carried out in ANSYSWorkbench to evaluate the stress distribution and rotor segmentdisplacement from the centrifugal force in the rotor structuresdesigned with different fixing mechanisms attached to thin non-magnetic hub. In this 2D analysis, a 10 pole-segmented rotor wasmodelled into 1/20th, as shown in Fig. 3, considering the symmetryof the geometry. Frictionless supports were applied to thesymmetry boundaries. This allows the body to move freelytangentially to the boundary but prohibits any perpendicularmovement.

Fig. 1  Segmented rotor

Fig. 2  Rotor segment root-fixing designs

J. Eng.This is an open access article published by the IET under the Creative Commons Attribution -NonCommercial License(http://creativecommons.org/licenses/by-nc/3.0/)

1

As both rotor segments and hub are not mechanically bondedwith each other, the FEA involves contact modelling, in which theforces are transmitted from one body to another in the contactregion by means of normal compressive stresses and shear stressesif friction is present [7]. The contact constraints are imposed byusing augmented Lagrange formulation, and frictionless contact isassumed between the two bodies in this contact analysis. Themodelled segmented rotor is operated at a maximum rotationalspeed of 20,000 rpm. Table 1 summarises the modelling conditionsand material properties of the segmented rotor used in this study.

Principal stresses are used as the failure criterion in this study,as it was suggested that von Mises equivalent stress would bemisleading [8] due to the compressive stress concentration at theedge of the contact. Accurate modelling of compressive contactstress would require extremely high mesh density (∼1 μm), it iscomputationally costly, and is not within the scope of this study.However, relative high mesh density (0.01 mm) is applied at thecontact region, as shown in Fig. 3, so that the effect of the stressconcentration reduces rapidly and does not affect the global stressdistribution in the segmented rotor.

The results of segmented rotor static structural analysis areshown in Figs. 4 and 5. Maximum principal stresses of thesegmented rotors are reported, which represent the tensile stress inthe body that could lead to immediate material failure. It isobserved that all rotor structures experience high stress levels thatare closed or beyond the material limits, making the designunacceptable. This mainly attributed to the thin non-magnetic rotorhub which limits the size of fixing structure and induces asubstantially high stress level.

3 Stress reducing rotor designIn order to mitigate the stress levels in the segmented rotorstructures in embedded machine application, a new design isproposed. This rotor design holds the magnetic segments fullyembedded onto the non-magnetic rotor hub, with the toothgeometries on the sides of the rotor segment, as shown in Fig. 6.This design offers a larger contact area between the rotor segmentsand the hub, which in turn reduces the stress concentration andprevents the structure from mechanical failure. In addition, thedesign also inherently renders a smooth cylindrical surface whichhelps in the reduction of windage losses.

Figs. 7 and 8 show the stress distribution and displacement ofthe toothed segment rotor structure. The rotor segment has themaximum stress level of 360.3 MPa, while the rotor hub registers

Fig. 3  Fine mesh density near contact region

Table 1 Finite element analysis settingFEA Model Setup ValueRotational speed, rpm 20,000Maximum global mesh size, mm 0.5Mesh size in contact region, mm 0.01Contact type FrictionlessRotor HubMaterial Inconel 718 [9]Young's Modulus, GPa 205

Density, kg/m3 8190

Yield strength, MPa 1100Rotor SegmentMaterial Iron-Cobalt [10]Young's modulus, GPa 210

Density, kg/m3 8120

Yield strength, MPa 680

Fig. 4  Maximum principal stress in(a) Dovetail, (b) Straddle T, and, (c) Fir-tree design

Fig. 5  Rotor segments displacement in(a) Dovetail, (b) Straddle T, and, (c) Fir-tree design

Fig. 6  Toothed segment design for the embedded electrical machine

2 J. Eng.This is an open access article published by the IET under the Creative Commons Attribution -NonCommercial License

(http://creativecommons.org/licenses/by-nc/3.0/)

the maximum stress level of 456.2 MPa. These stress levels arewell below the material limits, which ensure safe operation at highspeed. Fig. 9 shows the comparison between all the rotor structuredesigns.

In the dovetail design, the rotor hub has a maximum principalstress of 805.6 MPa, with a safety factor of 1.365. This value isconsiderably lower than 2, which is a common practice used inaerospace application. Compared with the dovetail design, it isobserved that the maximum principal stress of the rotor hub in boththe straddle T and fir-tree designs increase substantially, eventhough both designs have been proved to have lower stresses. Thisis mainly due to the thin rotor hub used in this application has alimited space, making the complex geometries unsuitable.

While the maximum principal stress of the rotor segment in thetoothed segment design is slightly lower than others, the stress inthe rotor hub is significantly reduced, with 43% lower than that inthe dovetail design. The toothed segment design delivers a safetyfactor of 2.41. Moreover, the displacement of the rotor segments inthe toothed segment design is the lowest among all the rotorstructure design, 20% lower than that in the straddle T design. Thisoffers another advantage to this design as minimal rotor segmentdisplacement is desirable in the electrical machine.

3.1 Electromagnetic impact

The impact of the toothed segment design on machineelectromagnetic performance was analysed using JMAG finiteelement software. The analysis was performed between toothedand non-toothed segments as shown in Fig. 10. The results of theelectromagnetic analysis are shown in Figs. 11–13. It was foundthat the inclusion of the structural tooth on rotor segment has aminimal effect on the machine electromagnetic performance.

In Fig. 11, the i−Ψ curves of the two segmented rotor SRM areshown. It is observed that in both aligned and unaligned rotorpositions, the toothed rotor segment has a higher flux linkagecompared to its counterpart. This is due to the inclusion of thetooth structure which results in the reduction in the overall distancebetween the rotor segments, l:

dℜ = l(r)μoLdr

dΨ = Nidℜ = Ni

μoLdrl(r)

(1)

From (1), we can see that the reluctance ℜ between segments isreduced which in turn increases the flux linkage Ψ. The machinepower conversion is proportional to the area enclosed by the fluxlinkage at aligned and unaligned rotor positions at a phase current.

Fig. 7  Maximum principal stress in the toothed segment design

Fig. 8  Displacement in the toothed segment design

Fig. 9  Comparison of maximum principal stress and displacement ofvarious segmented rotor structure designs

Fig. 10  2D finite element model of SRM with(a) Non-toothed and, (b) Toothed rotor segment design

Fig. 11  Impact on flux linkage – excitation current characteristic

Fig. 12  Induction profiles comparison between the toothed and non-toothed rotor segments

J. Eng.This is an open access article published by the IET under the Creative Commons Attribution -NonCommercial License(http://creativecommons.org/licenses/by-nc/3.0/)

3

With the maximum operating phase current of 100 A, the areaOAB is only 2% lesser than the area OPQ.

Fig. 12 depicts the inductance profiles over a half rotor pitch. Itis observed that the difference in inductance between both rotorstructures, at the operating region of the rotor position of 7°−15°, isrelatively unchanged as the excitation current increases.Correspondingly, for the same operating region, the average statictorque of toothed rotor segments is slightly smaller compared to itscounterpart, as depicted in Fig. 13. Table 2 summarises themaximum difference in inductance and average static torque overthe operating region of the rotor position 7°−15°.

3.2 Eddy current losses in the rotor hub

In the toothed rotor segment design, the rotor hub inevitablyintrudes the region near the air gap and experience time-varyingmagnetic fields. This leads to the induction of the eddy current andhence losses in the rotor hub. To estimate the eddy current losses inthe rotor hub, a simple 3D FEA was carried out with only one coilexcited (Fig. 14). It was found that the considerable large eddycurrent was induced in the rotor, flowing between the rotorsegments, as shown in Fig. 15. These eddy current losses wereclearly unacceptable as it will deteriorate the machine efficiencyand performance. Moreover, the additional rotor-coolingmechanism is not favourable in this application due to the limitedspace in the aircraft engine and increasing the complexity of themachine. Therefore, it is necessary to reduce the eddy currentinduced in the rotor hub. Slitting circumferential lines on the rotorhub near the airgap region where most of the eddy current areinduced or rotor hub lamination as in soft magnetic material wouldlikely be the solution of high eddy current losses. The effectivenessof the proposed methods will be examined in the future work.

4 ConclusionIn this paper, toothed rotor segment design has been proposed toreplace the conventional rotor segment fixing designs due to aharsh operating environment in the aircraft gas turbine engine.Structural analysis has been carried out, and the results show thatthe toothed rotor segment has superior performance in terms ofstresses and rotor displacement. The electromagnetic impact of thetoothed rotor segment to the electrical machine performance hasalso been investigated. While the inclusion of the structural toothon the rotor segment has a minimal impact on the machineelectromagnetic performance, the major limitation is the eddycurrent induced in the rotor hub near the airgap region. Possiblesolutions have been proposed to reduce the eddy current losses andthe effectiveness will be evaluated in the future work.

5 AcknowledgmentsThis work was conducted within the Rolls-Royce@NTU CorporateLab with support from the National Research Foundation (NRF)Singapore under the Corp Lab@University Scheme.

6 References[1] Provost, M.J.: ‘The more electric aero-engine: a general overview from an

engine manufacturer’. Int. Conf. on Power Electronics, Machines and Drives(Conf. Publ. No. 487), Sante Fe, NM, USA, 2002, pp. 246–251

[2] Mecrow, B.C., Finch, J.W., El-Kharashi, E.A., et al.: ‘Switched reluctancemotors with segmental rotors’, IEE Proc., Electr. Power Appl., 2002, 149, (4),pp. 245–254

[3] Mecrow, B.C., El-Kharashi, E.A., Finch, J.W., et al.: ‘Preliminaryperformance evaluation of switched reluctance motors with segmental rotors’,IEEE Trans. Energy Convers., 2004, 19, (4), pp. 679–686

[4] Mecrow, B.C., El-Kharashi, E.A., Finch, J.W., et al.: ‘Segmental rotorswitched reluctance motors with single-tooth windings’, IEE Proc., Electr.Power Appl., 2003, 150, (5), pp. 591–599

[5] Hall, R., Jack, A.G., Mecrow, B.C., et al.: ‘Design and initial testing of anouter rotating segmented rotor switched reluctance machine for an aero-engine shaft-line-embedded starter/generator’. IEEE Int. Conf. on ElectricMachines and Drives, San Antonio, TX, 2005, pp. 1870–1877

[6] Harman, R.T.C.: ‘Gas turbine engineering’ (The MacMillan Press, Londonand Basingstoke, 1981)

[7] Nandish, R.V., Paul Vizhian, S., Gopinath, M., et al.: ‘Elastic contact stressanalysis of dovetail attachment in turbine engines’, Int. J. Res. Mech. Eng.Technol., 2013, 3, (2), pp. 166–170

[8] Widmer, J.D.: ‘Segmental rotor switched reluctance machines for use inautomotive traction’, PhD Thesis, Newcastle University, 2013

[9] Inconel 718 Technical Data, High Temp Metals Inc.[10] Vacodur S Plus, Soft Magnetic Cobalt-Iron Alloys - Vacoflux and Vacodur,

Vacuumschmelze GmbH & Co. KG

Fig. 13  Static torque comparison between the toothed and non-toothedrotor segments

Table 2 Impact on machine electromagnetic performanceExcitationcurrent, A

Max. difference ininductance, mH

Percentage changein average static

torque20 0.082 2.8940 0.090 2.9060 0.124 2.4280 0.137 1.23100 0.133 0.65

Fig. 14  3D finite element model of SRM

Fig. 15  Eddy current in the rotor hub

4 J. Eng.This is an open access article published by the IET under the Creative Commons Attribution -NonCommercial License

(http://creativecommons.org/licenses/by-nc/3.0/)