performance comparison of double and single rotor permanent magnet machine

8/10/2019 Performance Comparison of Double and Single Rotor Permanent Magnet Machine

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PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 3/2011 133

Peter PIŠEK1, Bojan ŠTUMBERGER2,1, Tine MARČIČ1, Peter VIRTIČ3

TECES, Research and Development Centre for Electric Machines (1),University of Maribor, Faculty of Electrical Engineering and Computer Science (2),

University of Maribor, Faculty of Energy Technology (3)

Performance comparison of double and single rotor permanent

magnet machine Abstract . This paper presents the performance comparison between a special double rotor permanent magnet synchronous machine and theadequate conventional single rotor permanent magnet machine. The here presented double rotor machine is primary meant for use in hybrid electricvehicles to improve the fuel economy and to reduce emissions of the whole traction system. Finite element analyses are employed to determine the

performance of each machine, where special attention was paid to the iron loss and electromagnetic torque calculation.

Streszczenie. W artykule zaprezentowano porównanie dział anie maszyny synchronicznej z magnesem trwał ym i specialnym podwójnym wirnikemoraz konwencjonalnej maszyny z magnesem trwał ym i jednym wirnikiem. Prezentowany tutaj silnik z podwójnym wirnikiem by ł pierwotnie pomy ślanydo wykorzystania w hybrydowych pojazdach elektrycznych w celu poprawienia gospodarki paliwami i redukcji emisyjności gazów z cał ego systemutrakcyjnego. Do opisu dział ania każ dej z maszyn wykorzystano metod ę elementów skończonych a specjalną uwag ę skierowano na straty w ż elazie imoment elektromagnetyczny. ( Porównanie dział ania maszyn z magnesem trwał ym jedno- i dwuwirnikowych )

Keywords: double rotor electric machine, permanent magnets, finite element analysis, hybrid electric vehicles.Słowa kluczowe: maszyna elektryczna dwuwirnikowa, magnesy trwałe, metoda elementów skończonych, elektryczny pojazd hybrydowy

IntroductionWith ever increasing oil prices and concerns on natural

environment, the hybrid electric vehicle (HEV) has becomeone of the most celebrated concepts for present andnearest future transportation. One of the key components ofHEVs is a highly efficient traction system. One of the mostinteresting solutions for hybrid vehicles traction systempresents the use of a double rotor permanent magnetsynchronous machine (DRPMSM) commonly called as fourquadrant transducer [1-3] or dual mechanical port machine[4-6]. Due to double rotor structure and double winding, itenables the vehicle’s internal combustion engine to operateat optimal efficiency, thus improving fuel economy and

reducing emissions of the whole traction system.In the present paper one of the possible solutions ofsuch a DRPMSM is presented and compared to aconventional single rotor permanent magnet (PM) machinestructure. The purpose of this paper is to evaluate theperformance of the DRPMSM which employs two movingparts in order to be directly connected on one side to theHEV’s wheels and on the other side to the HEV’s internalcombustion engine. Furthermore, the performances of theDRPMSM are directly compared to the traditional PMmachine characteristics which can be used only as atraditional motor/generator in the HEVs traction system. Thehere employed machines can be directly comparedbecause they differ only in the moving parts design. Theyare made of equal materials.

Methods and compared machines topologiesFig. 1 and Fig. 2 show cross sections of the two

analyzed machine topologies. The machine on Fig. 1 is theso called DRPMSM and is built of three active parts; astator, inner wound rotor and outer PM rotor between them.The machine topology on Fig. 2 has only one rotor andcould be regarded as a conventional PM machine. It hasthe same stator, PM, flux barrier and stack dimensions asthe DRPMSM. Since there is no inner wound rotor, the PMrotor is extended to the shaft.

All results presented in the paper are obtained fromfinite element (FE) analyses by employing the FE package Ansys. The nonlinearity of the magnetic material is taken

into account using the single-value B-H characteristic asshown in Fig. 3, given by the manufacturer of electrical steel

M235-35A (for the frequency of 200 Hz), which is used in

both FE models investigated in this paper. All calculationsare made considering constant PM rotor speed of 3000rpm. Permanent magnets 33UH used in the FE models areconsidered to have the ideal radial magnetization directionand the remanence adequate to the PM temperature of150°C.

Fig. 1. Double rotor PM synchronous machine (DRPMSM)

Fig. 2. Single rotor PM synchronous machine

Stator winding

Rotor

PM



134 PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 3/2011

B [T]

0 0.50 1.00 1.50 2.36

Fig. 3. B-H characteristic of electrical steel M235-35A given by themanufacturer for the frequency of 200 Hz

FE analysis resultsFE computations for both machines are made using

single mesh during rotor rotation. It means that the mesh ofthe whole FE model (Fig. 4) is generated only once. Sincethe mesh can stay unchanged, the remesh error iseliminated and besides, it is time saving. Another benefit,

when single mesh is used, is obtained at iron losscalculation. Since each mesh element remains equal duringrotation, the flux density characteristic over one electricalperiod can be obtained for each element. Furthermore thehigher harmonic components of flux density can becalculated for each element separately, and for this reasonthe iron losses can be accurately determined. The ironlosses in this paper have been calculated using theapproach shown in [7] where improved Bertotti’s iron lossmodel is proposed and coefficients for M235-35A steelgrade are identified.

Fig. 4. Double rotor PMSM model mesh

Fig. 5. Zoomed outer air-gap mesh divided in 3 layers and with slotopenings

Fig. 6 through Fig. 9 present no-load conditions for bothconsidered machines. It is clearly seen that the DRPMSM,due to inner rotor slots, contains less iron in the

construction than the single rotor machine and therefore atequal PM excitation achieves significantly higher fluxdensity values in the inner rotor region. Furthermore, higherflux density values in the inner rotor cause higher total ironlosses of the DRPMSM in comparison to the total ironlosses of the single rotor machine.

Fig. 6. No-load flux density distribution in DRPMSM

Fig. 7. No-load flux density distribution in single rotor PM machine

Fig. 8. Comparison of no-load air gap flux density

Since the DRPMSM has two air-gaps, the air-gap fluxdensities achieve lower values than the one in the case ofsingle rotor machine with single air-gap as seen in Fig. 8.

Consequently, the back-EMFs of DRMPSM have lowervalues as shown in Fig. 9.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0 4000 8000 12000 16000 20000

B

[ T ]

H [A/m]

M235‐35A

(200Hz)

‐0,8

‐0,6

‐0,4

‐0,2

0,0

0,2

0,4

0,6

0,8

0 60 120 180 240 300 360

f l u x d e n s i t y [ T

]

electrical angle [°]

SINGLE ROTOR ‐ air gap

DOUBLE ROTOR ‐ outer air gap

DOUBLE rotor ‐ inner air gap



PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 3/2011 135

Fig. 9. Comparison of line-to-line back-EMF at speed of 3000 rpm

Because both of the stators have the same geometryand because the DRPMSM exhibits much smaller magneticflux density value in the outer air-gap, its stator iron loss issmaller than the stator iron loss from the single rotormachine as is presented from the iron loss calculationresults from Table 1.

Table 1. Comparison of iron losses at no-load condition (3000 rpm)

Machine topology DRPMSM Single rotor PMSM

Stator [W] 60 76

PM rotor [W] 58 55

Wound rotor [W] 35 /

Total iron losses [W] 153 131

Two load cases, stator half and stator full load, wereadditionaly investigated where the electromagnetic torqueand iron losses have been compared. At full load conditionthe stator winding in both compared machines is suppliedwith current density of 20 A/mm

2 (adequate for a water

cooled stator housing) and in case of DRPMSM withadditional 7.5 A/mm

2 of current density in inner rotor

winding. The electromagnetic torque is calculated using theVirtual Work method considering the supply currents asbeing perfectly sinusoidal. Moreover, the electromagnetic

torque is calculated at the appropriate torque angle (torqueangle is the angle between the stator/inner rotormagnetomotive force and the PM rotor direct axis), wherethe maximum torque per ampere is achieved.

The electromagnetic torque of the DRPMSM in the Fig.10 and Fig. 11 is the torque produced on the outer PM rotoras the result of the combined interaction of the statorcurrents with outer PM rotor and also the inner rotorcurrents with outer PM rotor.

Fig. 10. Electromagnetic torque comparison at stator half loadconditions; DRPMSM – stator current density of 10 A/mm

2, inner

rotor current density of 7.5 A/mm2; Single rotor PM machine –

stator current density of 10 A/mm2

Due to the additional currents in the inner rotor, the

DRPMSM in case of stator half-load conditions achieveshigher average torque value than the single rotor PMmachine as shown in Fig. 10.

Table 2. Comparison of iron losses at stator half-load condition(3000 rpm)


Stator [W] 173 228

PM rotor [W] 80 115



In case of stator full-load conditions (Fig. 11) it is clearthat the single rotor PM machine reaches approximately the

same average value of the electromagnetic torque than theDRPMSM. From the Fig. 10 and Fig. 11 and consideringthe fact that the DRPMSM has less iron material, we couldconclude that the DRPMSM core gets obviously moresaturated at the same stator current density levels of bothmachines and with additional inner rotor current.

Fig. 11. Electromagnetic torque comparison at stator full loadconditions; DRPMSM – stator current density of 20 A/mm

2, inner

rotor current density of 7.5 A/mm2; Single rotor PM machine –

stator current density of 20 A/mm2

As presented in Fig. 12, the single rotor PM machineachieves lower flux density values at full-load condition in

rotor area and higher flux density values in stator whereconsequently produces higher iron losses as shown inTable 3 in comparison to DRPMSM.

Fig. 12. Full-load flux density distribution in single rotor PMmachine

Table 3. Comparison of iron losses at stator full-load condition(3000 rpm)


Stator [W] 267 321

PM rotor [W] 98 152



‐250

‐200

‐150

‐100

‐50

0

50

100

150

200

250

0 60 120 180 240 300 360

l i n e

‐ t o

‐ l i n e b a c k

‐ E M F [ V ]

electrical angle [°]

SINGLE ROTOR ‐ Stator winding

DOUBLE ROTOR ‐ Stator winding

DOUBLE rotor ‐ Rotor winding

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90

E l e c t r o m a g n e t i c t o r q u e [ N m ]

mechanical angle [°]

DRPMSM ‐Tavg=62.8 Nm

Single rotor PMSM ‐ Tavg = 56 Nm

100

102

104

106

108

110

112

114

116

118

0 10 20 30 40 50 60 70 80 90

E l e c t r o m a g n e t i c t o r q u e [ N m ]

mechanical angle [°]

DRPMSM ‐Tavg=112.5Nm

Single rotor PMSM ‐ Tavg = 112.9Nm



136 PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 3/2011

ConclusionIn the paper a DRPMSM, primary used for specific

application in HEVs, is presented and its performance iscompared to a conventional single rotor PM machine. Dueto less complex construction and on the basis of thepresented results, the single rotor PM machine is stillpreferable in applications, where double rotor constructionis not needed.

AcknowledgmentOperation part financed by the European Union,European Social Fund. Operation implemented in theframework of the Operational Programme for HumanResources Development for the Period 2007-2013, Priorityaxis 1: Promoting entrepreneurship and adaptability, Maintype of activity 1.1.: Experts and researchers for competitiveenterprises.

REFERENCES[1] Nor dl un d E ., Sa da ra ng an i C., Four-quadrant energy

transducer, IAS 2002, Pittsburgh, USA, October 2002[2] Zhen g P., L iu R., Wu Q ., Zhao J. , and Yao Z. ,

Magnetic coupling analysis of four-quadrant transducer usedfor hybrid electric vehicles, IEEE Trans. Magn., Vol.43, pp.2597-2599, June 2007

[3] Ar sha d W. M. , Nor dl und E . And T hel in P, New drivetopologies for hybrid-electric vehicles, International Conferenceon Electrical Engineering and Technology, August 2003

[4] X u L ., A new breed of electric machines - basic analysis andapplications of dual mechanical port electric machines,Proceedings of ICEMS 2005, Vol. 1, pp. 24-31, Nanjing, Sept.27-29, 2005

[5 ] Tao F. , Xuhu i W. , J ingwei C. , and Xizheng G. ,Permanent magnet dual mechanical port machine design forhybrid electric vehicle application, Industrial Technology, IEEEICIT 2008

[6 ] Tao F. , Xuhu i W. , J ingwei C. , and Xizheng G. ,Comparison of two different structure of permanent magnet

dual mechanical port machine, IEEE Vehicle Power andPropulsion Conference, September 2008, China

[7] Jacobs S. , H ectors D. , Henr o t te F . A e t a l .,Magnetic material optimization for hybrid vehicle PMSM drives,Inductica 2009 Conference, Germany

Authors: Peter Pišek, Dr. Tine Mar č i č , TECES, Pobreška cesta 20,SI-2000 Maribor, Slovenia;

Assist. Prof. Dr. Bojan Štumberger, University of Maribor, Faculty ofElectrical Engineering and Computer Science, Smetanova ulica 17,SI-2000 Maribor, Slovenia;

Assist. Prof. Dr. Peter Virti č , University of Maribor, Faculty ofEnergy Technology, Dislocated unit Velenje, Trg Mladosti 2, SI-3320 Velenje, Slovenia;E-mail: [email protected].

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performance comparison of double and single rotor permanent magnet machine

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