A NEW METHOD FOR REDUCING MAGNETIC RADIAL FORCEIN SWITCHED RELUCTANCE MOTOR

K. Vijayakumar1*, R. Karthikeyan,2 S. Kannan3, S. Rajkumar4 and G.K. Sathishkumar5

1*Assistant professor, Department of electrical & electronics engineering, Mailam EngineeringCollege, Mailam and k_vijaymec@yahoo.com.

2Assistant professor, Department of electrical & electronics engineering, Shri VenkateswaraCollege of Engineering, Sriperumpudoor, Chennai and vijipreethi@rediffmail.com.

3Final year student, Department of electrical & electronics engineering, Mailam EngineeringCollege, Mailam.

4Lecturer, Manakula vinayar Institute of Technology, Madagadipet, pondicherry.5 Design Engineer ITC Infotech, Bangalore.

Abstract--This paper investigates the influence ofrotor with feeble-magnetic portions (groove or holein a rotor) on electromagnetic radial forcecharacteristics of 6/4 switched reluctance (SR)motor. Three different configurations viz (i)Conventional rotor (ii) rotor with circular hole and(iii) rotor with triangular hole SR motor have beenstudied using finite element analysis tool ANSYS10.0 to obtain their force and torquecharacteristics. The study reveals that the rotorwith hole provision has lesser radial force levelwhich makes it a viable alternative in high speedapplications.

Index Terms--Finite Element Analysis (FEA),Switched Reluctance Motor (SRM), Torque ripple,Radial force.

I. INTRODUCTION

The proliferation of power electronicstechnology with advances in material technologyhas led to the development of special machinesfor myriad applications in recent times. One ofthe such special machines, the SwitchedReluctance Motor (SRM) [1] is gainingrecognition in the electric drives market due toits simple and rugged construction, low expectedmanufacturing costs, fault tolerance capability,high efficiency and high torque to inertia ratio.Despite many advantages, the SRM has somedrawbacks. It requires an electronic control andshaft position sensor, a huge capacitor is neededin the dc link and the double salient structurecauses acoustic noise and torque ripple. Thispaper presents the analysis of magnetic radialforce and torque characteristics of a 6/4-switchedreluctance motor. The investigation is focused onthe influence of groove or hole formed in aprojected pole of the rotor on the radial magneticof a 6/4 switched reluctance motor.

II. SWITCHED RELUCTANCE MOTOR

Fig. 1. The 2D CAD model of a 6/4 SRM.

A switched reluctance motor is an electricalmachine in which the torque is developed by thetendency of the rotor to occupy a position so asto minimize the reluctance of the magnetic pathof the excited stator phase winding. The SRM isa doubly salient but singly excited machinewherein the stator carries the winding while therotor is simply made of stacked silicon steellaminations. This lends to a simpler geometry forSRM as evidenced from the two dimensional(2D) CAD model of a 6/4 switched reluctancemotor shown in Fig. 1.

III. THE 2D NUMERICAL FINITE ELEMENTANALYSIS

The time stepped FEA is the most accuratemethod available to obtain the magneticcharacteristics such as torque and forcecalculation in an electromagnetic device. In thispaper a two-dimensional FEA has been carriedout on the three configurations using FEApackage ANSYS 10.0. [3].

The studied configurations of conventionalrotor SR motor, rotor with circular hole SRmotor and rotor with triangular hole switchedreluctance motor shown in Fig. 2, 3 and 4.

Fig. 2. The 2D CAD model of a 6/4 conventional SRM.

Fig. 3. The 2D CAD model of a 6/4 SR motor with circularhole formed in a projected rotor.

Fig. 4. The 2D CAD model of a 6/4 SR motor with triangularhole formed in a projected rotor.

IV. RADIAL FORCE OF SWITCHED-RELUCTANCE MOTOR

The switched reluctance motor is a doublysalient construction, with salient poles on boththe stator and the rotor. Phase windings arewound on the stator poles, each coil beingwound around a single pole or tooth. The SRmotors generate torque by utilizing magneticattractive force that acts between the rotor andthe stator when the coils are magnetized.However, conventional SR motors normallysuffer vibration problems. The magneticattractive force acting between the stator and the

rotor in generally circumferential directionsquickly increases as the rotor rotates during acycle of the electric current distribution, and thenabruptly terminates at the time of switching thecurrent distribution. This on-off cycle ofmagnetic attractive force causes the rotor and thestator to considerably vibrate in generallycircumferential directions [4]-[5].

In a switched reluctance motor, the projectedpoles of a rotor have feeble-magnetic portionseach having a shape that extends radially inwardapproximately in the rotational direction from aposition relative close to a radially outer end ofthe projected pole and to an end of the projectedpole facing in a counter-rotational direction, thatis, a direction opposite to the rotational direction,or a shaped that has a side extending radiallyinward approximately in the rotational directionfrom a position relatively close to a radially outerend and an counter-rotational direction-facingend of the projected pole and that extendsradially inward approximately in the counter-rotational direction. The feeble-magneticportions are weaker in magnetism than thematerial of the rotor and reduce the magneticattraction force occurring at the time ofswitching of coil energization, thereby reducingnoises while ensuring generation of a desiredamount of torque.

Thus, the switched reluctance motor of theinvention reduces the magnetic attractive forcebetween the stator pole portions and the rotorpole protrusions in the radial direction at the timeof switching the current distribution to the coils,compared with the conventional SR motor [2]-[3]. The invention also avoids a large amount oftorque reduction.

The torque and radial force characteristics ofthree rotor configurations as shown in Fig. 5 and6.

TORQUE PROFILES OF THREE ROTORCONFIGURATIONS

-150

-100

-50

0

50

100

150

0 10 20 30 40 50 60 70 80 90 100

ROTOR POSITION (in DEGREES)

TORQ

UE (i

n N-

mm

)

CIRCULAR HOLEDROTORTRIANGULARHOLED ROTORCONVENTIONALROTOR

Fig. 5. Torque angle characteristics of three rotorconfigurations.

ROTOR POSITION Vs MAGNETIC RADIAL FORCE

0100200300400500600700

0 10 20 30 40 50 60 70 80 90 100

ROTOR POSITION (in DEGREES)

MA

GNE

TIC

E R

ADI

AL F

ORC

E(in

N-m

m)

CONVENTIONALROTOR SRMCIRCULAR HOLEDROTOR SRMTRIANGULARHOLED SRM

Fig. 6. Magnetic radial force characteristics of three rotor

configurations.

The analysis of the torque and radial forcecharacteristics of three rotor configurationsindicates that there is indeed a reduction intorque ripple and magnetic attraction forcebetween the stator pole and rotor pole portions inthe radial direction when compared with theconventional configuration.

V. CONCLUSION

In this paper an in-depth study of groove orhole formed in a projected pole of the rotor onthe radial magnetic force characteristics of a 6/4switched reluctance motor with threeconfigurations viz (i) conventional rotor SRmotor (ii) rotor with circular hole SR motor and(iii) rotor with triangular hole SR motor has beenconducted. The studied results shows that agradual decrease in the magnetic radial force ofrotor with hole configurations. Study has lead tothe rotor with hole provision has better magneticradial force characteristic.

VI. REFERENCES

[1] R. Arumugam, D. A. Lowther, R. Krishnan and J.F. Lindsay, “Magnetic field analysis of aswitched reluctance motor using a twodimensional finite element model,” IEEE Trans.Magn., vol. MAG-21, no. 5, pp. 1883-1885, Sep.1985.

[2] R. Rabinovici, “Torque ripple, vibrations, andacoustic noise in switched reluctance motors”,HAIT Journal of Science and Engineering B,Volume 2, Issues 5-6, pp. 776-786, July 2005.

[3] “switched reluctance motor with reduced radialforce”, United States Patent, Dec. 1998.

[4] C.Y.Wu, C.Pollock, “Analysis and reduction ofacoustic noise and vibration in the switchedreluctance drive”, IEEE Trans. on IndustryApplications, Vol.31, No.1, January/February,1995, pp.91~98.

[5] S. Colby, F.Mottier, T.J.E.Miller, “Vibrationmodes and acoustic noise in a 4-phase switchedreluctance motor”, Conference Record of the1995 IEEE Industrial Application Society, 30thIAS Annual Meeting, Vol.1, Orlando, USA, Oct.8-12, 1995, pp.441~447.