Ten Con 2

Torque-to-Weight Ratio Improvement with Soft Magnetic Composite Material in High Speed

Switched Reluctance Motor K. Vijayakumar*, R. Karthikeyan*, G.K.Sathishkumar#, R. Arumugam**, Member IEEE,

*Research scholar, # ITC InfoTech, Bangalore, ** Retired Professor Department of Electrical Engineering, Anna University, Chennai, India. [email protected], [email protected]

Abstract- In aircraft electric motor drive fuel delivery system,

the motor must be characterized by high power density, reliability, less size, less weight and high speed [1]. The high speed operation, fault tolerance, high power density makes switched reluctance motor an ideal candidate for high speed aerospace applications. This paper investigates the application potential of soft magnetic composite material (SOMALOY 500) characterized by three dimensional ferromagnetic behavior, very low eddy current loss, flexible machine design and assembly in high-speed switched reluctance motor. Two configurations (i) all sheet metal (ii) all Soft magnetic composite have been studied using finite element analysis to obtain their magnetic characteristics. The weight of active parts has been evaluated for the two configurations to determine torque-to-weight ratio. The study reveals that the all soft magnetic composite configuration albeit suffers from poor average torque, digress in torque-to-weight ratio from conventional model is not very much which makes it a viable alternative in high speed aerospace applications.

Keywords - Finite element analysis (FEA), Inductance profile, Soft magnetic composite (SMC), Switched reluctance motor (SRM).

I. INTRODUCTION

The low cost, high speed, high power density, fault tolerance and reliability of switched reluctance motor make it a viable alternative in aircraft electric-motor-driven fuel pump system. Recently soft magnetic composite materials find rampant applications in electrical machines [3] and these materials are characterized by three dimensional isotropic ferromagnetic behaviors, very low eddy current loss, flexible machine design and assembly and a prospect for greatly reduced production cost. This paper investigates the use of soft magnetic composite material in switched reluctance motor designed for high-speed aerospace applications from a study of its static magnetic characteristics. Two configurations viz (i) All laminated sheet steels (ii) All Soft Magnetic Composites (SMC) have been extensively studied using finite element analysis based software tool. The study concludes that all SMCs configuration albeit its poor average torque in comparison with conventional motor, promises to possess acceptable torque-to-weight ratio which makes it a suitable candidate in high speed aerospace applications along with better thermal and mechanical characteristics.

II. SWITCHED RELUCTANCE MOTOR

Fig. 1. Geometric Model of 6/4 Switched reluctance motor. A switched reluctance motor is an electrical machine in which the torque is developed by the tendency of the rotor to occupy a position so as to minimize the reluctance of the magnetic path of the excited stator phase winding. The switched reluctance motor is a doubly salient but singly excited machine wherein the stator carries the winding while the rotor is simply made of stacked silicon steel laminations. This lends to a simpler geometry for switched reluctance motors as evidenced from the two dimensional (2D) CAD model of a 6/4 switched reluctance motor shown in Fig. 1.

The switched reluctance motor considered for this study is a 6/4 model designed for high-speed applications whose specifications are given in Appendix and whose projected model is shown in Fig. 2.

Fig. 2. Projected Model of 6/4 Switched Reluctance Motor.

Authorized licensed use limited to: Nanchang University Trial User. Downloaded on March 16, 2009 at 00:55 from IEEE Xplore. Restrictions apply.

III. SOFT MAGNETIC COMPOSITE MATERIALS IN ROTATING ELECTRICAL MACHINES

Electrical steel lamination is the most commonly used core material in electrical machines. Electrical steels are typically classified into grain-oriented electrical steels and non-oriented electrical steels. Typical applications for grain-oriented steels are power transformer cores whereas non-oriented steels are broadly used in different kinds of rotating electrical machines. Electromechanical steels currently used in the manufacture of electrical machines posses high induction of magnetic saturation (Bs~2T), low coercive force (Hc< 100A/m), and they are characterized by low total losses [2] and [4]. Electrical sheet steels have been the dominant choice for the soft iron components in electrical machines subject to time varying magnetic fields. The new soft iron powder metallurgy materials can be considered as an alternative for magnetic core of the electrical machines. The basis for soft magnetic composites is bonded iron powder developed by Hoganas of Sweden [5] as shown in Fig. 3. The powder is coated, pressed into a solid material using a die, and heat-treated to anneal and cure the bond.

Fig. 3. Iron powder after (a) coating and (b) compression.

The B-H characteristics of M19 silicon steel and SMC material SOMALOY 500, shown in Fig. 4, reveals that although the SMC has inferior relative permeability when compared with lamination steel it still posses the following desirable characteristics [3].

a. Reduced copper volume as a result of increased fill factor

and reduced end winding length and reduced copper loss as a result of the reduced copper volume,

b. Reduced high frequency tooth ripple losses since the SMC has essentially very low eddy current losses,

c. Potential for reduced air gap length as a result of the tight tolerances maintained in manufacturing SMC material,

d. Modular construction allows the possibility of easy removal of an individual modular unit for quick repair or replacement,

e. Stator is easily recyclable since the stator can again be compressed back into powered form with pressure and the copper windings readily removed.

Fig .4. B-H characteristics of laminated steel and SMC.

IV. THE 2D NUMERICAL FINITE ELEMENT ANALYSIS

The time stepped finite element analysis is the most accurate method available to obtain the magnetic characteristics in an electromagnetic device. In this paper a two-dimensional finite element analysis has been carried out on the two machines depicted in table I using FEA based CAD package MagNet 6.22.1

TABLE I STUDIED STRUCTURES

Configuration -1 Configuration -2

Laminated sheet steel stator

(M19)

Soft Magnetic Composite (SMC)

stator

Laminated sheet steel rotor

(M19)

Soft Magnetic Composite (SMC) rotor

The following assumptions are made in determining the magnetic field distribution inside the motor using finite element analysis [6]. a. The outer periphery of the stator can be treated as a zero

vector potential line with the magnetic field outside the stator is negligible.

b. Magnetic materials of stator and rotor are isotropic and the magnetization curve is single valued.

c. Magnetic vector potential (A) and current density (J) has only z-directed components.

d. End effects are neglected.

The fine mesh has been employed to enhance the accuracy of the results that has been obtained .The meshed structure of the model is depicted in Fig. 5.


Fig. 5. Fine mesh for the two dimensional CAD model of the machine. The flux lines plot obtained at the aligned position is shown in Fig. 6.

Fig. 6. Flux lines plot of two-dimensional model at aligned position.

IV. STATIC MAGNETIC CHARACTERIZATION CONFIGURATION-1 (VS) CONFIGURATION-2

The saturation region of operation of configuration - 2 has been determined by a comparison of the self-inductance variation of an excited phase as a function of rotor position with current level variation from 5A to 20A as shown in Fig. 7.

Fig. 7. Self-inductance profile for configuration – 2. The weight of active parts of the two configurations has been estimated from geometrical and the material data as shown in

table II. The computational results show that there is a 4% difference in weight of the all SMC material configuration in comparison with all sheet steel configurations which has a profound impact in machines of bigger size thus resulting a substantial saving in the usage of material for all SMC configuration, which leads to lower production cost.

TABLE II MASS OF ACTIVE PARTS OF MOTOR CONFIGURATIONS

Parts

Formula

Volume

(3m )

Weight (kg) Configuration

1

Weight (kg) Configuration

2 Yoke of

rotor 2 2

1 0( ) stkr r Lπ − 2.94

X10 -5 0.233 0.215

Poles of rotor . . .r r r stkN t d L

1.32 X10 -5 0.100 0.096

Total rotor (Yoke +Poles) rotor

4.26 X10 -5 0.323 0.311

Shaft of rotor

2. .stk shL rπ 9.74

X10 -6 0.074 0.074

Yoke of stator

2 23 2. .( )stkL r rπ −

9.27 X10 –5 0.704 0.677

Poles of stator . . .s s s stkN t d L

4.49 X10 -5 0.341 0.328

Total stator (Yoke +Poles) stator

1.38 X10 -4 1.046 1.006

Winding

2 hc⋅ bc⋅ Lstk⋅ Ns⋅

3.88X10-

5

0.348

Total motor (Stator + Rotor) 18 X10 -5 1.718 1.666

% Difference in Weight between configurations with conventional machine (ie. Configuration 1)

4%

The aligned and unaligned inductances for two configurations has been computed and listed in table III.

TABLE III COMPARISON OF THE UNALIGNED AND ALIGNED INDUCTANCES FOR THE TWO CONFIGURATIONS

Configuration 1 2

Aligned inductance (H) 0.016 0.012

Unaligned inductance (H) 0.005 0.005

The variation in aligned inductance between configurations 1 and 2 at aligned positions amounts to 25% showing the effect of material non-linearity and permeability on the inductance profile. The static torque characteristic at the current density of 8A/mm2 has been estimated for the two configurations and is shown in Fig. 8, along with the comparison table IV. For the two configurations the differences are wide as for as static torque curve shapes as well as their maximum torque levels are concerned. The superior permeability of lamination material renders configuration-1, the most powerful machine with the


level of torque obtained, while the torque-to-weight ratio is 23% higher than configuration-2, the all SMCs.

TABLE IV: COMPARISON OF STATIC TORQUES

Configuration 1 2

Effective Torque (N.m) 2.18 1.59

Torque/weight (N.m/Kg) 1.59 1.20

Difference in T/W ratio (N.m/Kg) from

conf.1 % 23.35

Fig. 8. Static Torque Characteristics of the two configurations. In the context of high-speed applications, apart from the output obtained, the configuration-2 outweighs in terms of thermal capability, manufacturability, design modularity and cost. The use of soft magnetic composite material with better permeability level will compensate for the lacuna observed.

VI. CONCLUSION

The study has led to the determination of aligned and unaligned inductances for the two configurations. The variation of aligned inductance between configuration-1 and configuration-2 is rather significant (45% lower for configuration-2) as also the level of average torque obtained (23% lesser for configuration-2). The considerable reduction is due to the poor permeability of soft magnetic composite material. On the other hand the torque-to-weight ratio of configuration-2 when compared with configuration-1 is limited to only 23%, which gives it an edge over configuration-1 in the high-speed aerospace applications regime.

APPENDIX

Main Dimensions of the 6/4 Configuration.

Stator diameter (Ds) = 90mm Rotor diameter (Dr) = 45mm Stack length (Lstk) = 45mm Overall length (Le) = 73.6mm Stator pole width (ts) = 11.9mm Rotor pole width (tr) = 12.4mm Stator pole depth (ds) = 14mm Rotor pole depth (dr) = 5.9mm Stator yoke thickness (ys) = 8mm Rotor yoke thickness (yr) = 8.3mm Airgap (g) = 0.5mm No. of stator poles (Ns) = 6 No. of stator poles (Nr) = 4 Stator pole arc (ßs) = 30 degree Rotor pole arc (ßr) = 32 degree Shaft diameter (Dsh) = 16.6mm Power rating = 0.5hp Turns per phase = 110 Speed (N) = 8000rpm Rated current = 15A

REFERENCES [1] A. V. Radun, “High-power density switched reluctance motor drive for aerospace applications,” IEEE Trans. Ind. Applicat., vol. 28, no. 1, pp. 113-119, Jan/Feb. 1992. [2] A.G. Jack, “Experience with the Use of Soft Magnetic Composites in Electrical Machines”, International Conference on Electrical Machines, Istanbul, Turkey, 1998, pp. 1441-1448. [3] Guo, Y.G., Zhu, J.G., Watterson, P.A., and Wu, W., “Design and Analysis of a Transverse Flux Machine with Soft Magnetic Composite Core”, The 6th International Conference on Electrical Machines and Systems, Beijing, China, Aug. 2003. [4] M. Persson, P. Jansson, A. G. Jack, B. C. Mecrow, “Soft Magnetic Composite Materials – Use for Electrical Machines”; 7th International Conference on Electrical Machines and Drive at Durham, England, Sep. 1995. [5] “The Latest Development in Soft magnetic Composite Technology from Hoganas Metal Powders”, Hoganas Manual, 1999-2003. [6] R. Arumugam, D. A. Lowther, R. Krishnan and J. F. Lindsay, “Magnetic field analysis of a switched reluctance motor using a two dimensional finite element model,” IEEE Trans. Magn., vol. MAG-21, no. 5, pp. 1883-1885, Sep. 1985.


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