Research Journal of Recent Sciences _______________________________________________ISSN 2277-2502 Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 56 Torque Comparison between a Novel Multilayer Switched Reluctance Motor and a Custom one Alireza Siadatan1*, Mohammad Mahdi Mahmoodi and Mohammad Mehdi Nezamabadi1 Department of Electrical Engineering, West Tehran Branch, Islamic Azad University, Tehran, Iran Department of Electrical Engineering, Saveh Branch, Islamic Azad University, Saveh, Iran a_siadatan@wtiau.ac.ir Available online at: www.isca.in,www.isca.me Received 15th January 2014, revised 22nd February 2015, accepted 18th October 2015Abstract In this paper a custom three phase Switched Reluctance Motor (SRM) with six stator and four rotor salient poles (6 by 4) was designed, simulated and analyzed and it’s produced torque, phase inductance, flux linkage and some other characteristics were obtained by the 3D Finite Element Method (FEM). Then a novel 7-layer 4 by 4 Multilayer SRM (MSRM) with the same volume was designed and under the same conditions was simulated and analyzed. In the latter motor, 3 layers are energized at the same time causing more torque and efficiency and less torque ripple. Both motors were analyzed under different situations and results were attained. Finally prototypes of each one in the laboratory were constructed and tested and the results were compared with the simulations. Keywords: Torque, MSRM, FEM. Introduction The SRM is one of the special motor for low cost and simple structure in variable speed drive applications such as cars, home appliances, and industrial1,2. Many types of SRMs are presented. The general classification of SRMs is illustrated in Figure-1. There are two types of SRMs. Linear SRM and the rotary SRM. The linear SRM is divided in single stack SRM and multi stack SRM. Rotary SRM has two types of radial field and axial field SRM. In some radial field SRM the rotor is the outer part and the stator is the inner part. In some others the rotor is rotated inside the stator. The axial field SRM has two types of multi stack with internal rotor and single stack with external rotor3,4. Figure-1 SRM general types Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 57 Various configurations of this machine are proposed in the literatures with specific characteristics, such as; isolated double layer, dual channel, hybrid stator pole, linear8,9, multi-layer10, and disc-type11 machine. C. Lee and R. Krishnan introduced a two-phase SRM using common pole E-core structure which its stator has four small poles and two or four large poles between small poles. Its common poles do not have copper windings and are shared by both phases for positive torque production through the entire operation12. Two new two-phase SRM with modified rotor poles shape and optimized torque have been presented13. Different geometries have been proposed by S. G. Oh, and R. Krishnan with six stator and three rotor poles14. They are designed with different pole arcs and some with variable air gaps. A new hybrid Switched Reluctance Motor/Generator (SRMG) with salient stator pole was introduced after which it has a pair of permanent magnets attached symmetrically between the iron poles of the rotor15. H. Chen and J. J. Gu presented a three-phase SRMG. One is a dual motors drive for the electric locomotive traction; the other is a variable-speed generator system for wind power applications16. The operating characteristics of the SRG excited by a Suppression Resistor Converter (SRC) based on FEM analysis and experiments are examined17. N. Radimov considered the operation of the SRMG as an autonomous three-phase AC generator18. The generator circuit consists of only capacitors and load supplementary to the generator, but does not contain any power supply. M. S. Toulabi et. al. suggested a new hybrid two-phase SRMG which consists of two independent stator and rotor sets where each stator pole has 45º arc length. Each rotor pole is the same as stator pole (45º) in one side and twice as much in the other side19. MSRMs are new SRMs with more than one independent layer. Their advantages especially the improvement in torque per volume parameter makes them so attractive to enter industry. A new three-layer MSRM with eight stator salient poles and 6 rotor salient poles is introduced by Afjei and Toliyat20. This paper presents the characteristics and comparison of two types of SRMs which are 6 by 4 conventional SRM and a 7-layer 4 by 4 MSRM. These motors have the same iron volume. In section II, the specifications of the 6 by 4 SRM is explained. In section III the specifications of 7-layer MSRM are presented. In section IV, these motors are simulated and different parameters of them are obtained by 3D FEM and the simulation results are compared to each other. In section V, the motors are constructed and the operational results of them are compared to each other. The paper is finished by conclusion in section VI. The Conventional 6 By 4 Srm Specifications The conventional 6 by 4 SRM has six stator and four rotor salient poles. The motor shape is shown in Figure 2. The arc of the stator pole is 29 degree and the arc of the rotor pole is 32 degrees. The motor specifications are presented in table 1. The stator and rotor poles are made up of M-27 laminations of non-oriented silicon steel whose static B-H curve is shown in Figure 3. The 6 by 4 SRM proto type is fabricated in laboratory which is shown in Figure-4. Figure-2 6 by 4 SRMTable-1 The Specifications of the 6 by 4 three-phase SRM Nominal Power 1hp Nominal voltage 60 V Nominal speed 3000 rpm Stack length 175 mm Stator outer diameter 182.1mm Stator inner diameter 148.7mm Stator pole arc 29 Rotor outer diameter 148.4mm Rotor shaft diameter 20mm Rotor pole arc 31 Air gap between stator and rotor pole 0.25mm Number of winding turns per pole 80 turns Each phase resistance 1.5 Seven-Layer 4 By 4 Msrm Specifications The 7-layer 4 by 4 MSRM includes four independent stator and rotor salient poles in each layer. The windings of stator poles in each layer are in series making up a phase. So this type of motor has the advantages of the less maintenance requirement, having no isolation problem, producing more torque with less torque ripple and working smoother. Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 58 Figure-3 B-H curve of the used M-27 laminationsFigure-4 The fabricated 6 by 4 SRMThe Rotor of each layer is shifted some degrees in comparison to other layers which is calculated by Equation-1:  \n   !" #$%$ \r ' \r()*+,\r (1) The motor shape is shown in Figure-5. The arc of the stator pole is 44 degree and the arc of the rotor pole is 46 degrees. The specifications of this type of MSRM are presented in Table-2. The stator and rotor poles are made up of M-27 lamination of non-oriented silicon steel just like other presented SRMs. The 7-layer 4 by 4 MSRM proto type is also built in laboratory and illustrated in Figure-6. This type of motor is also designed in a way that a number of layers (phases) can be switched on at the same time. In this MSRM three layers are switched on each time. The phase switching algorithm of the 7-layer MSRM is presented in Table-3. Figure-5 7-layer 4 by 4 MSRM in magnetTable-2 The specification of the 7-layer 4 by 4 MSRMNominal Power 1hp Nominal voltage 60 V Nominal speed 3000 rpm Each layer Stack length 25 mm Stator outer diameter 182.1mm Stator inner diameter 148.7mm Stator pole arc 44 Rotor outer diameter 148.4mm Rotor shaft diameter 20mm Rotor pole arc 46 Air gap between stator and rotor pole 0.25mm Number of winding turns per pole 100 turns Each phase resistance 1.1 Figure-6 The fabricated 7-layer 4 by 4 MSRM Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 59 Table-3 The phase switching algorithm in the 7-layer MSRM Ph1Ph2Ph3Ph4Ph5Ph6Ph7 C1On On On off off off off C2off On On On off off off C3off off On On On off off C4off off off On On On off C5off off off off On On On C6On off off off off On On C7On On off off off off On CX = Case X PhX = Phase X The Presented Motors Analyses By 3d Fem After motor designing, it should be simulated in order to obtain an accurate model of the motor. Different parameters of motors are obtained during simulations which are so important. The presented motors in this paper are simulated in Magnet Infolytica Package in which 3D FEM is used21. Magnet can also simulate the two dimensional model but 3D simulation results are more accurate although it requires more time. To obtain the different parameters of SRM, the motor phase is exited in different positions of rotor. Electric vector potential (T) is used solve the problems which is calculated by: -\r./0(2) By using Maxwell's equation which is: ./1\r-\r./0 (3) And Then, ./21304\r (4) And also the vector (H-T) which can be declared as:1\r03.(5) Note that, is a magnetic scalar potential. And, Since: ./5\r3676 (6) And then: ./5\r./89 ;./0\r3676 \r36 03.6?6 ;3.96 ; (7) The final two scalar equations used by Magnet are: 036?6 ;\r3.96 ; (8) And \r(9) The simulation is done between unaligned to full aligned position of rotor. In unaligned position where a stator pole is opposite to the rotor pole, the reluctance of the phase winding has its maximum value which causes the inductance of the phase to be in minimum value. Figure-7 shows the inductance of the phase winding versus rotor position. In fully aligned position where the stator and rotor poles are aligned completely, there is only small air gap between the stator pole and rotor pole which causes the reluctance and the inductance of the phase winding to be minimum and maximum respectively. The rotor in each simulation is rotated between these two regions. Each position is defined as a problem for the software and at end the parameters in each component like flux, flux path, flux density, losses, forces and torques are obtained. The problems can be solved for different values of phase excitation current. Figure-7 The phase inductance versus rotor position Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 60 (a) (b) (c) Figure-8 6 by 4 simulation in: a) unaligned, b) half aligned and c) full aligned position 6 by 4 SRM analyses: Figure-8 illustrates the simulation of 6 by 4 SRM in unaligned, half aligned and fully aligned position of rotor. This result is obtained for a phase excitation dc current of 3A and the phase winding of 150 turns. The flux completes its path through the yoke, stator pole, air gap, rotor pole and then another stator pole. As it's observed, the flux density is much higher in the exited pole of the stator in comparison with other poles. Usually the stator pole is thinner than rotor pole in SRMs which causes the stator pole to be in higher value of saturation than rotor poles. The maximum value of flux density is about 1.49 Tesla which is occurred in fully aligned position of rotor. The simulation is performed for other value of phase excitation dc currents. Flux curve versus rotor position is shown in Figure-9 for different value of currents. The rotor is rotated from unaligned position (-30 deg) to aligned position (0 deg) and again to unaligned position with the next stator pole (30 deg). As it's illustrated the flux has higher value when the phase excitation current is higher. It's starts to increase in the start of alignment position and has its maximum value in fully aligned position. The generated torque in the rotor component is also obtained in different values of current for the presented 6 by 4 SRM which is shown in Figure-10. As it’s illustrated, the torque starts to increase in the start of alignment position and have its maximum value in fully aligned position region and then starts to decrease. Higher value of excitation current increases the value of the generated torque in rotor. The maximum value of torque is about 1.8 N.m. for the phase excitation current of 6 A. Figure-9 6 by 4 fluxes versus rotor position curve for different value of phase excitation current Seven-Layer 4 by 4 MSRM Analyses: The field simulation of the 7-layer 4 by 4 MSRM is presented in Figure-11 for unaligned, half aligned and fully aligned position of rotor. As it is shown, three phases are excited in this MSRM. The phase excitation current is 3 A. The maximum value of flux density in fully aligned position of rotor is about 1.43 Tesla. Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 61 Figure-10 6 by 4 generated torque versus rotor position curve for different value of phase excitation current The simulation is done for other value of excitation currents. Flux curve of this MSRM versus rotor position is presented in Figure-12 for different value of phase currents. The rotor is rotated from unaligned position (0 deg) to aligned position (12.86 deg). As it's illustrated, the flux starts to increase in the start of alignment position and has its maximum value in fully aligned position. The generated torque of rotor is depicted in Figure-13 in different position of rotor. As it's illustrated, the torque starts to increase in the start of alignment position and have its maximum value in fully aligned position region and then decreases. The maximum value of torque is about 5.1 N.m. in the phase excitation current of 6 A. (a) (b) (c) Figure-11 7-layer 4 by 4 MSRM simulation in: a) unaligned, b) half aligned and c) full aligned position Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 62 Figure-12 7-layer 4 by 4 MSRM flux versus rotor position curve for different value of phase excitation current Figure-13 7-layer 4 by 4 MSRM generated torque versus rotor position curve for different value of phase excitation currentComparing the Simulation Results of The Proposed Motors: The flux values in the flux curves starts to increase in start of alignment position and has its maximum value in fully aligned position of the rotor. The flux of the 6 by 4 SRM at 3 A, is about 0.185 Wb in fully aligned position, is much higher than 7-layer 4 by 4 MSRMs (0.05 Wb in fully aligned position). The rotor generated torques in these motors were presented for variation of rotor position from unaligned to fully aligned position. For example for the phase excitation current of 6A as it was illustrated, the generated torque in the rotor of the 7-layer 4 by 4 MSRM (=about 5.1 N.m in fully aligned region) is about double of the generated torque of 6 by 4 SRM. The 6 by 4 generated torque value in the same region is about 1.9 N.m. By comparing the torque curves, it can be observed that MSRMs generate more torque in their rotor rather than 6 by 4 conventional SRM. The 7-layer 4 by 4 MSRM has less torque ripple, because three layers are always excited which means the torque is continually generated. In 7-layer 4 by 4 MSRM the torque is about 3.9 N.m in start of alignment position. It increases to about 5.1 N.m and then decreases to about 3.9 N.m again. The 6 by 4 SRM generates 1.1 N.m torque at first. The value of torque is then increases to about 1.9 N.m and falls down to 0.6 N.m. Operational Results of the Constructed Srms The presented 6 by 4 SRM and 7-layer 4 by 4 MSRM are then constructed and tested in laboratory. Operational Results of the 6 by 4 SRM: The operational results of the 6 by 4 three-phase SRM are as follows. Three opto-counter sensors and a plate are coupled on the motor shaft and the motor is tested. The supply voltage of the motor is 60 volts and the driver circuit which is an asymmetric bridge converter uses 12 volts power supply. The motor is tested under different load and speed conditions and its operational results are obtained. The command pulses of three phases which are obtained from the opto-counters are shown in Figure-14. Figure-15 shows the generated torque of the motor versus the operational speed. In the speed of about 3200 rpm, the minimum value of torque which is about 0.039 N.m is generated. In the speed of about 461 rpm, the maximum generated torque of about 2.35 N.m is produced. Figure-15 Command pulses of three phasesThe motor current versus generated torque is illustrated in Figure 16. In the maximum speed where the minimum value of torque is generated (0.039 N.m), the motor current is about 1.59 A. In the minimum speed of the motor where the maximum torque is generated (2.35 N.m), the motor current is about 5.47A. Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 63 Figure-16 Torque versus speed in 6 by 4 SRMFigure-17 Current versus torque in 6 by 4 SRMThe motor efficiency versus generated torque is shown in Figure 17. In the minimum value of torque (0.039 N.m), the motor efficiency is about 4.4%. In the maximum generated torque (2.35 N.m), the motor has the efficiency of about 34.5%. The maximum value of efficiency is about 61% which is in the speed of about 1611 rpm and the generated torque of about 0.617 N.m. Operational Results of Seven-layer 4 by 4 MSRM: Seven opto-counter sensors and a plate are coupled on the motor shaft and the motor is tested. The supply voltage of the motor is 60 volts and the driver circuit which is an asymmetric bridge converter uses 12 volts power supply. The motor is tested under different load and speed conditions and its operational results are obtained. The command pulses of four phases (four layers) which are obtained from the opto-counters are shown in Figure-18. Figure-18 Efficiency versus torque in 6 by 4 SRMFigure-19 Command pulses of four phases in 7-layer MSRM Figure-19 shows the generated torque of the motor versus the operational speed. In the speed of about 3300 rpm, the minimum value of torque which is about 0.0391 N.m is generated. In the speed of about 216 rpm, the maximum generated torque of about 5.3 N.m is produced. The motor current versus generated torque is illustrated in Figure 20. In the maximum speed where the minimum value of torque is generated (0.0391N.m), the motor current is about 2.13 A. In the minimum speed of the motor where the maximum torque is generated (5.3 N.m), the motor current is about 8.24A. The motor efficiency versus generated torque is shown in Figure 21. In the minimum value of torque (0.0391 N.m), the motor efficiency is about 10.7%. In the maximum generated torque (5.3 N.m), the motor has the efficiency of about 24.2%. The maximum value of efficiency is about 72.2% which is in the speed of about 722 rpm and the generated torque of about 3.23 N.m. Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 64 Figure-20 Torque versus speed in 7-layer MSRM Figure-21 Current versus torque in 7-layer MSRM . Figure-22 Efficiency versus torque in 7-layer MSRM Comparing the Operational Results of The Proposed Motors: The operational results of the presented SRMs are compared in this section. The speed-torque specifications of the these SRMs were illustrated in Figure-9, 12 which show that by increasing the number of independent layers of the SRM (with a same iron volume), the generated torque of the motor increases. It was also shown that the torque ripple is reduced by increasing the number of layers. The maximum generated torque of the 6 by 4 SRM is about 2.35 N.min the speed of about 461 rpm. In 7-layer 4 by 4 MSRM, the maximum generated torque is about 5.3 N.m in the speed of about 216 rpm. As shown in the speed-efficiency specifications of the SRMs by increasing the number of independent layers of the SRM (with a same iron volume), the efficiency of the motor increases. The maximum efficiency of the presented 6 by 4 SRM which is obtained in laboratory is about 61% that is obtained in the speed of about 1611 rpm and the generated torque of about 0.617 N.m. The maximum value of efficiency obtained from the 7-layer 4 by 4 MSRM is about 72.2% which is in the speed of about 722 rpm and the generated torque of about 3.23 N.m. Conclusion This paper presented the characteristics of two different types of SRM with the same iron volume. They were 6 by 4 SRM and 7-layer 4 by 4 MSRM. The 6 by 4 SRM which was a three phase motor, had six stator and four rotor salient poles. The MSRMs had four stator and four rotor salient poles in each layer. These motors were numerically analyzed at first and their electromagnetic parameters were obtained by FEM analysis in Magnet software which is a powerful 3D simulator in electromagnetic problems. The different positions of the rotor were determined as the problems of the simulations. Then the rotor was rotated from unaligned to fully aligned position and the motor parameters like flux, flux density, electromagnetic force, torque etc were obtained for each problem of the motor. The motors were then fabricated and tested in laboratory under different load conditions. In the 7-layer MSRM, three layers were switched on each time. More than one layer excitation at a moment caused the MSRMs produced more torque with less torque ripple rather than custom 6 by 4 SRM. The different parameters of motors were illustrated in figures which were in close agreement with the experimental results obtained in laboratory. The torque of the rotor in MSRMs was about double of the 6 by 4 produced torque. The 6 by 4 SRM had the maximum torque of about 2.35 N.m while the presented 7-layer 4 by 4 MSRM had the maximum torque of about 5.3 N.m. The efficiency of the MSRMs were also higher than custom SRM. The maximum efficiency of the presented custom 6 by 4 SRM was about 61% while the presented 7-layer 4 by 4 MSRM had the efficiency of about 72.2%. Research Journal of Recent Sciences ____________________________________________________________E-ISSN 2277-2502Vol. 5(2), 56-65, February (2016) Res.J.Recent Sci. International Science Community Association 65 ReferencesMiller T.J. (1988), Switch Reluctance Motor Drive, Ventura, CA; Intertec Communications Inc. Vijayakumar K., Karthikeyan R., Paramasivam S., Arumugam R. and Srinivas K.N. (2008). Switched Reluctance Motor Modeling, Design, Simulation, and Analysis: A Comprehensive Review, IEEE Transactions on Magnetics, 44(12), 4605-4617. Krishnan R. (2001). Switched Reluctance Motor Drives, Modeling, Simulation, Analysis, Design and Applications, CRC Press. Krishnan R. (2001). Electric Motor Drives, Modeling, Analysis and Control, Printice Hall. Torkaman H., Afjei E. and Toulabi M.S. (2012). 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