Performance Analysis of a Soft Magnetic Composite Switched Reluctance Generator

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1 erformance Analysis of a Soft Magnetic Composite Switched Reluctance Generator R.Karthikeyan 1, K.Vijayakumar 2 1 Srivenkateswara College of Engineering ennalur, Sriperumbudur, India 2 Mailam Engineering College,Mailam,India 1 rkar@svce.ac.in Dr.R.Arumugam rofessor / EEE Department SSN College of Engineering Kalavakkam, Chennai, India k.vijaymec@gmail.com Abstract This paper introduces a switched reluctance generator (SRG) made from soft magnetic composite (SMC) material.the soft magnetic composite switched reluctance generator prototype has been fabricated using SOMALOY1000 pre-form blanks utilizing indigenous technology. The machine geometry has been chosen to utilize maximum material of the procured prototyping blanks with the due care taken to ensure that the geometry adheres strictly to the established design conventions for a switched reluctance machine. The performance has been analysed through circuit coupled finite element analysis taking into account mutual flux. The fabrication details are expounded tersely along with the results of real time thermal analysis carried out on the developed prototype. Keywords- Finite Element Analysis (FEA); Mutual flux; Real time thermal analysis; Soft Magnetic Composite (SMC) Switched Reluctance Generator (SRG). A I. INTRODUCTION switched reluctance machine is characterized by simple and rugged structure, low cost, good fault tolerance capability and the ability to work in harsh environment along with the high speed of operation. The switched reluctance generator in recent times being considered in applications such as aircraft power systems, as starter/generator in automotive systems and in wind energy systems. Traditionally laminated steels have been employed in the construction of a switched reluctance machine which lends to it higher weight and considerable eddy current losses. The work reported in this paper delineates the fabrication and performance analysis of a switched reluctance machine made entirely of soft magnetic composite material SOMOLOY1000 to be run as a generator. Soft magnetic composite materials are composed of iron powder particles bonded together by a resin and compacted through powder metallurgy technology. Soft magnetic composite materials are characterized by isotropic ferromagnetic behaviour, three dimensional flux flow, enhanced thermal conductivity, very low eddy current losses and ability to be shaped into any complex structure. Soft magnetic composite materials have been utilized in claw pole machines, permanent magnet machines and transverse flux machines mainly in small size machines as the SMC material suffers from poor mechanical strength and brittleness. Switched reluctance generators of varied geometry have been designed and tested [1] [2].A C Cored stator SRG for wind energy had been developed and tested [1], [2], while in [3] an external rotor SRG had been studied for its improved performance.in [5] a two phase field assisted hybrid switched reluctance generator had been simulated in FEA and tested which the authors claimed to yield higher output power. In this paper a switched reluctance generator made entirely of SMC material SOMALOY1000 had been developed from pre-form prototyping blanks imported from HOGANOS, Sweden. The performance analysis had been carried out through FEA taking into account the mutual flux. The fabrication details are explained tersely along with the presentation of real time thermal analysis results. II. SWITCHED RELUCTANCE GENERATOR A. Construction and Basic riciple In a switched reluctance generator, mechanical energy is converted to electrical form by virtue of proper synchronization of phase currents with rotor position. A switched reluctance machine is operated in the generating mode by positioning phase current pulses during periods where the rotor is positioned such that the phase inductance is decreasing. This occurs immediately after the rotor and stator poles have passed alignment. Normally, in this generator mode, the machine obtains its excitation from the same voltage bus that it generates power to. Typically, a phase is turned on before a rotor pole aligns with that phase, drawing energy from the DC bus to excite the phase. During generation, the switched reluctance generator produces negative torque that tends to oppose rotation, thereby extracting energy from the prime mover [4]. It is the responsibility of a commutator to excite the phases in proper sequence to support continuous energy conversion. Torque in the SRG, as in the switched reluctance motor, is created by the natural tendency of the stator poles to attract the nearest rotor poles. If the phase is excited before the rotor poles come into alignment with the stator poles, the rotor experiences torque in the direction of rotation consistent with operation as a motor. If the phase is excited as the rotor poles move through the aligned position, the rotor experiences torque opposing rotation consistent with generator operation. The control key is to position the phase current pulses precisely timed with the rotor position relative to the phase s stator poles in order to maximize the efficiency and to reduce the stresses on the power electronic converter. The relationship between the idealized inductance profile and phase currents for 1

Owing to machine symmetry only one phase namely phase1 is considered[7].

2 motoring and generating above base speed is depicted in figure 1. The reluctance variation has an important bearing upon the performance and to assess the same flux linkage characteristics for various rotor positions and excitation currents have been obtained. Owing to machine symmetry only one phase namely phase1 is considered[7]. At aligned and/or at higher current level stator and rotor poles begin to saturate as a result of which the secondary effects such as saturation, fringing and leakage non linearity have been introduced. At unaligned position, magnetic saturation is unlikely to occur owing to large reluctance on account of large air gap, the flux linkage is almost linear. The self- inductance is the ratio of each phase flux linkage to the exciting current and as the inductance is directly proportional to flux linkage, the resulting inductance values for phase 1 have same variations under different loads. III. SOFT MAGNETIC COMOSITE SWITCHED RELUCTANCE GENERATOR CHARACTERISTICS Fig.1. Idealized inductance variation and the currents used to support Motor and Generator operation. B. Finite Element Modeling of SMC-SRG In order to evaluate the main parameters of the generator accurately, field analysis [6] has been carried out using the FEA tool MagNet 7.1.The meshed model along with the flux lines plot at the onset of stator - rotor poles overlap is shown in figure 2. The proposed generator s characteristics such as flux linkage, self-inductance, mutual inductance and generated voltage have been obtained through numerical analysis and discussed. For assessing the performance of the generator, its characteristics have been obtained under different loads at 2A through 12A. A. Flux linkage /Self inductance The reluctance variation has cardinal bearing upon the performance; hence data concerning the flux distribution for different rotor positions and excitation currents have been obtained and is shown in figure 3. Fig.3. Flux linkage of phase 1from aligned to unaligned position Fig.2.Meshed machine model of SMC-SRG and Flux lines plot At higher current level or at aligned position stator-rotor poles tend to saturate which begets secondary effects such as fringing and leakage nonlinearities. At unaligned position, phase inductance has a minimum value owing to high reluctance of the large airgap.magnetic saturation is unlikely to occur at aligned position and thus the flux linkage shows a linear behaviour until the onset of overlap. 2

47 30 4.38 4.42 4.57 4.77 4.94 5.05 35 4.78 4.79 4.86 4.95-5.05 5.15 40 4.754 4.75 4.75 4.75 4.77 4.73 45 4.22 4.27 4.22 4.28 4.28 4.29 Average 3.70 3.71 3.71 3.71 3.72 3.73 TABLE II.

3 B. Mutual flux/mutual Inductance In the proposed generator, when phase A is active for sampling, the two other phases are inactive. As a result, mutual flux (or mutual inductance) appears in two other inactive phases as shown in Fig.4 and Fig.5. ϴ(deg) (Lph2/Lph1) 100% I=2A I=4A I=6A I=8A I=10A I=12A Average TABLE II. RATIO OF MUTUAL INDUCTANCE (IN HASE 3) TO SELF INDUCTANCE (IN HASE 1) (Lph3/Lph1) 100% ϴ(deg) I=2A I=4A I=6A I=8A I=10A I=12A Fig. 4. Mutual flux in hase 2 vs. rotor position under different loads Average Fig.5. Mutual flux in hase 3 vs. rotor position under different loads As it is depicted, mutual fluxes in phase 2, and phase 3 are very low and in higher currents it goes up. It is worth mentioning that this feature is very promising in this type of machine than conventional SRG to decrease the losses and improve the efficiency. The ratio of mutual inductance in inactive phases (2 and 3) respect to self-inductance in active phase (1) in different conditions is presented in Table II and Table III, respectively. TABLE I. ϴ(deg) RATIO OF MUTUAL INDUCTANCE (IN HASE 2) TO SELF INDUCTANCE (IN HASE 1) (Lph2/Lph1) 100% I=2A I=4A I=6A I=8A I=10A I=12A These tables depict that with an increase in load current, the average produced mutual inductance in the phase 2 With respect to self-inductance in phase 1 increases from 3.7% to a maximum of 3.7%. Similarity, the average produced mutual inductance in the phase 3 respect to self-inductance in phase 1 are limited to [1.63%-2.13%] range. It is concluded that induced flux in inactive phases in idle mode has very low amplitude. It is originated from material characteristics of the designed SMC-SRG. In motor designs, the inductance ratio (Mutual Inductance/ self Inductance) should be minimized, so the proposed SMC-SRG is usually guaranteed for a welldesigned generator. C. Generated Voltage In order to estimate the shape of the output voltage of the SMC-SRG, some major steps must be considered and applied precisely which are analyzed ahead. The generated voltage in each pole of the proposed generator can be calculated by equation (1); where λ, L, and i are flux linkage, inductance, and current, (1) 3

As shown above the important factor which can estimate the real shape of the output voltage of the SMC-SRG is the waveform of the flux linkage versus rotor position.

4 CS2 CS4 CS5 D6 D4 D2 I1 WL D5 D1 D3 CS1 CS3 CS6 Output Voltage(V) respectively. Since current, i in each pole is kept constant so equation (1) can be rewritten as: e e e ind ind ind L i ( Li) (2) where, ω is generator angular speed in rad/sec and θ is the rotor position. As shown above the important factor which can estimate the real shape of the output voltage of the SMC-SRG is the waveform of the flux linkage versus rotor position. Therefore, all of flux waves must be estimated by a suitable equation. In this study a fourth order equation for each waveform has been approximated by least square method to achieve this purpose and the generated voltage is shown in figure Output voltage for SMC-SRG Fig.8. Current through the capacitor i=12a i=10a Rotor osition(deg) Fig.6.Output voltage of SMC-SRG As shown in Fig.6, the generated voltage in different conditions is desirable. As a result, the ratio of generated voltage to machine weight makes this machine suitable for small size application. Transient analysis carried out with ideal switches in the asymmetric half bridge converter demonstrated the voltage generation capability of the designed SRG as portrayed in figures 7, 8 and 9. D7 i=4a i=2a i=8a i=6a IV. Fig.9. Generated voltage in SMC-SRG FABRICATION OF SOFT MAGNETIC COMOSITE SWITCHED RELUCTANCE GENERATOR Three pieces of standard preform material (SOMOLOY1000) each for stator and rotor have been procured form HOGANAS of Sweden and to fabricate stator and rotor these three pieces have to be bonded together using epoxy glue and then subjected to the painstaking process of milling as the modern machining methods like CNC machining, Wire cutting process applicable for conventional lamination machine are not be suitable for machining the SMC material. After the milling process, the stator and rotor are subjected to the surface grinding process for a smoother finish. Fixation of Shaft and rotor position sensors follows this and finally stator winding has been wound and the entire assembly has been encased in a metal casing with the stator winding terminals brought out for external power and control electronics connection. Glimpses of the fabrication process have been illustrated in Fig.10 and 11. Coil#3 T1 T2 Coil#2 T1 T2 Coil#1 T1 T2 R1 10 C Fig.7. ower Converter Model (a) (b) (c) 4

The logic circuit for generator operation is shown in figure13 while figures 14 and 15

Fabrication process of SMC1000 based SRG (a) Bonded material blanks ( b) Machining process

machine structure with shaft and (e) Winding with end plates and bearings. (e) Fig.13.

hase current during transition from motor to generator mode (c ) (d) Fig.11.

mounted for rotor position sensing (b) SMC-SRG coupled to a prime mover (c) The DC motor as

motoring mode of operation has been derived as shown in figure 12.

hase voltage during the motor to generator mode transition A.

type thermocouples at various points such as stator body, winding, casing, stator poles in

The thermo couple outputs have been processed by Agilent 349704A Data Acquisition Unit

5 The logic circuit for generator operation is shown in figure13 while figures 14 and 15 depict the practical results obtained. (d) Fig.10. Fabrication process of SMC1000 based SRG (a) Bonded material blanks ( b) Machining process of stator (c) stator and rotor component of SMC1000 switched reluctance machine (d)complete machine structure with shaft and (e) Winding with end plates and bearings. (e) Fig.13. Logic circuit for Generator mode of operation (a) (b) Fig.14. hase current during transition from motor to generator mode (c ) (d) Fig.11. Assembly of the entire machine (a) Slotted disk fitted to the shaft with photo sensors mounted for rotor position sensing (b) SMC-SRG coupled to a prime mover (c) The DC motor as prime mover (c) The prototype SMC-SRG in its entirety The logic circuit corresponding to motoring mode of operation has been derived as shown in figure 12. Fig.12. Switching logic circuit for motoring mode Fig.15. hase voltage during the motor to generator mode transition A. Real Time Thermal Analysis Thermal analysis in real time has been carried by embedding T type thermocouples at various points such as stator body, winding, casing, stator poles in the machine and at the power converter. The thermo couple outputs have been processed by Agilent A Data Acquisition Unit consisting of A data logger and an Agilent 34901A 20- channel multiplexer interfaced with Agilent Bench Link Data Logger software loaded in a high speed computer system when the machine was run for one hour continously.the test setup is depicted in figure 16, while the result of the real time thermal analysis is furnished in figures 17 and 18. 5

Fig.16 (a) T type thermo couples embedded in machine and converter Fig.

The initial temperature of SMC-SRM is taken as ambient which is the bottom curve in figure 17.

The results of the analysis show that, the rotor temperatures are low compared with that of the stator.

The stator temperature is high due to added copper losses on the stator and the lower convection coefficients.

6 Fig.16 (a) T type thermo couples embedded in machine and converter Fig.16 (b) The entire real time thermal analysis setup The real time thermal analysis has been carried out to estimate the temperature rise over a period of time. The initial temperature of SMC-SRM is taken as ambient which is the bottom curve in figure 17.The temperature rise in machine parts while the temperature variation in the asymmetric half bridge power converter is depicted in figure 17 and figure 18 respectively. The results of the analysis show that, the rotor temperatures are low compared with that of the stator. Rotor temperature is low due to lower electrical losses and the higher convection on its surface. The stator temperature is high due to added copper losses on the stator and the lower convection coefficients. It is also found that stator inner region is hotter than stator outer region due to lower convection coefficient on inner region of stator. Overall analysis of the results are indicative of the fact that the thermal behaviour of a switched reluctance machine made of soft magnetic composite material has better precedence over conventional SRM when the machine is expected to perform in harsh environment and for long hours. V. CONCLUSION In this paper, a switched reluctance generator made of soft magnetic composite material SOMOLOY 1000 called SMC- SRG was introduced and analyzed numerically. As the first priority of this paper, it is shown that the inductance ratio (mutual inductance/ self-inductance) of the generator which refers to a well-designed generator is minimized. The flux waveforms of the phases were estimated via fourth order polynomial function, and the actual output voltage waveform of has been obtained. The analysis findings carried out in FEA model confirmed the well designed generator. Based on this study and through analysis results it was understood that in high speed of the performance it is better for users to utilize the proposed SRG. Furthermore, thermal capability and acceptable physical vibration in the machine were remarked modally as the other advantages of proposed prototype and could encourage users to employ these interesting generators in various conditions of application. REFERENCES [1]. C. Desai, M. Krishnamurthy, N. Schofield et al., Novel Switched Reluctance Machine Configuration With Higher Number of Rotor oles Than Stator oles: Concept to Implementation, IEEE Transaction on Industrial Electronics, vol. 57, no. 2, pp ,2010. [2] E. Afjei, and H. Torkaman, Comparison of Two Types of Hybrid Motor/Generator, in 20th International Symposium on ower Electronics, Electrical Drives, Automation and Motion (SEEDAM),isa, Italy, 2010, pp [3] H. Torkaman, and E. Afjei, Determining Degrees of Freedom for Eccentricity Fault in SRM Based on Nonlinear Static Torque Function, COMEL:The International Journal for Computation and Mathematics in Electrical and Electronic Engineering, vol. 30, no. 2,pp , [4] C. H. Yu, and T. C. Chen, Novel sensorless driving method of SRM with external rotor using impressed voltage pulse, IEE roceedings -Electric ower Applications vol. 153, no. 5, pp , Fig.17. Real time thermal analysis result on the machine [5] E. Afjei, and H. Torkaman, Comparison of Two Types of Dual Layer Generator in Field Assisted Mode Utilizing 3D-FEM and Experimental Verification, rogress in Electromagnetics Research B,IER, vol. 23, pp , [6] M. D. Hennen, and R. W. De Doncker, Comparison of Outer- and Inner-Rotor Switched Reluctance Machines, in 7th International Conference on ower Electronics and Drive Systems, EDS, 2007, pp Fig.18. Real time thermal analysis result on the power converter [7]. J. Holik, D. G. Dorrell, and M. opescu, erformance Improvement of an External-Rotor Split-hase Induction Motor for Low-Cost Drive applications Using External Rotor Can, IEEE Transactions on Magnetics, vol. 43, no. 6, pp ,

COMPARISON OF THREE NOVEL TYPES OF TWO- PHASE SWITCHED RELUCTANCE MOTORS USING FINITE ELEMENT METHOD

Progress In Electromagnetics Research, Vol. 125, 151 164, 212 COMPARISON OF THREE NOVEL TYPES OF TWO- PHASE SWITCHED RELUCTANCE MOTORS USING FINITE ELEMENT METHOD H. Torkaman 1, * and E. Afjei 2 1 Young