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OPERAION AND WORKING OF SRM MOTOR WITH TEN ROTOR POLES

49

CHAPTER 1 INTRODUCTION

1.1 INTRODUCTION SRM is a doubly- salient, singly-excited motor. It has salient poles on both the stator and rotor, but only one member (usually the stator) carries windings. The rotor has no windings, magnets, or cage winding, but is built up from a stack of salient-pole laminations. When a stator coil is energized, the rotor will move to the lowest magnetic reluctance path. The reluctance of the motor varies with the position of the rotor. It has desirable features including simple construction, high reliability and lower cost. These inherent properties of SRM make it a viable candidate for various general purpose adjustable speed applications.Fig.1 shows its typical structure. It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient machine.

Fig.1 6/4 SRM

1.2 BASIC STRUCTURE Fig.2 Cross sectional view The stator is made up of silicon steel stampings with inward projected poles. All these poles carry field coils. The field coils of opposite poles are connected in series such that their MMFs are additive in nature. The rotor has no windings, magnets and cage windings but it is built from a stack of salient pole laminations. It is preferred to variable speed dc motor and Induction motor. Due to the absence of rotor windings, SRM is very simple to construct, has a low inertia and allows an extremely high-speed operation. The conventional SR machine has 6 stator and 4 rotor poles this configuration has disadvantages such as torque ripple, acoustic noise.1.3 OPERATION OF SWITCHED RELUCTANCE MOTOR The rotor is aligned whenever the diametrically opposite stator poles are excited. In a magnetic circuit, the rotating part prefers to come to the minimum reluctance position at the instance of excitation. While two rotor poles are aligned to the two stator poles, another set of rotor poles is out of alignment with respect to a different set of stator poles. Then, this set of stator poles is excited to bring the rotor poles into alignment. The Fig.3 shows the block diagram of SRM. Fig 3 Block Diagram of SRM A torque is produced when one phase is energized and the magnetic circuit tends to adopt a configuration of minimum reluctance, i.e. the rotor poles aligned with excited stator poles in order to maximize the phase inductance. As the motor is symmetric, it means that the one phase inductance cycle is compromised between the aligned and unaligned positions or vice versa. Fig.4 Principle of Operation The stator pole axis AA and rotor pole axis aa are in alignment. Since the SRM works on Variable reluctance path it searches for minimum reluctance path. At this position inductance of B winding is neither maximum nor minimum. Next the phase B is energized. Due to this the rotor develops a torque because of variable reluctance and existence of variation in inductance. The direction of this torque is such that BB and bb try to get aligned. If this torque is more than the opposing load torque and frictional torque then the rotor starts rotating. Similarly phase winding C is energized and the same process continues. Thus the rotor rotates.

1.4 CONCLUSION Thus in this chapter the basics of SR machine has been discussed. The structure, the principle of operation and the advantages of the SR machine have been discussed.

CHAPTER 2 THEORY OF SRM 2.1 INTRODUCTION This chapter contains characteristics of Switched Reluctance motor. In this section we discuss about the torque producing mechanisms and torque production rate of the SRM. 2.2 TORQUE PRODUCTION MECHANISM The instantaneous torque for any phase is expressed in terms of the rate of change of co energy with respect to rotor position at some constant current, as given by Te =at i=constant (1) where Te -Electromagnetic Torque. Wm -rate of change of co energy. - rate of change of rotor position angle. The electromagnetic torque is produced due to the continuous excitation of the phase windings A, B, C. Due to the energization of the phase windings the stator poles and rotor pole come into alignment. Hence flux lines will flow from stator to rotor and according to Faradays law when a current carrying conductor is placed in a magnetic field it experiences a force. So the rotor experiences a force and it is driven into motion.

TORQUE EQUATION The torque equation of a switched reluctance motor is given by2L/ (2)Where Te - Electromagnetic Torque in N-m L - Inductance in Henry - Excitation current in A . L/-Rate of change of inductance with respect to rotor position .

2.3 Reasons for Torque Ripple Torque pulsations are most significant at the commutation instants when torque production mechanism is being transferred from one active phase to another. Toque ripple is produced due to the geometry of the SRM involving the shape of the rotor and stator, the area of the air gap and length of the stator and rotor polar arc . Hence torque ripple is produced due to the following two reasons Geometry Switching devices in the input side%Ripple factor = (Tmax-Tmin) /Tavg * 100 (3)The average torque produced is given by the formula (4)where Ti Instantaneous Torque in N-m 2.4 METHODS OF TORQUE RIPPLE MINIMIZATION The torque ripple can be minimized by various methods. By changing the geometry of the machine and by designing new converters for proper switching of the phase windings of the SRM.

2.4.1 REVIEW OF PREVIOUS METHODS FOR TORQUE RIPPLE MINIMIZATION The concept of optimization of the geometry of the SRM has-been addressed by several researchers [1], [2]. In addition to varying the combination of statorrotor poles, several design adjustments have been proposed to improve the efficiency of the machine and minimize losses. Horst illustrated the stepped rotor construction for 12/4 and 6/2 and multiple teeth per pole [8].The stator and rotor teeth ratios have been defined as 6(n):2(n),6(n):4(n), 6(n):8(n), 8(n):6(n), and 8(n):10(n), where n is a positive integer from one to four. Gilman [3] described the pulsed duration modulated control technique with reference to 4/6 SRM with stepped rotor construction. Kolomeitsev [4] illustrated a 6/8 SRM with longitudinally different cross sectional shapes for stator and rotor poles. He also explained bifurcated stator poles with 2n teeth and 2n+2 rotor poles, where in an integer, through an example of 6/14 SRM. Kalpathi et al. showed a 6/14 SRM, closely resembling two teeth per pole configuration, to illustrate method of torque ripple reduction. Morinigo [5] showed a 6/8 SRM with rotor laminations stacked in a particular spiral pattern to deliver constant torque from three-phase sinusoidal voltages. The torque ripple and the acoustic noise for the above mentioned configurations is higher when compared to the new proposed 6/10 SR machine. The average torque of the 6/10 SRM is higher than the available configurations hence the torque ripple is lesser. The rotor poles are displaced from each other by an angle of 36 whereas in 6/4 machine they are displaced by 90.So the stator pole and rotor pole come into alignment more quickly than the conventional SR machine.2.4.2 SCOPE OF THE PROJECT The aim of the project is to model a new configuration of SR machine with more number of rotor poles than stator poles by designing a new pole design formula given by Nr=2*Ns-2Where Nr -No of rotor poles Ns No of stator poles In order to reduce the torque ripple and improve torque production capability this new configuration is proposed. In addition to this a New Material Type is suggested to enhance the thermal capabilities and reduce the losses. A new Soft composite magnetic material is suggested. Somaloy 500 is used for simulation purposes and it is compared with various other materials. 2.4.3 CONTRIBUTIONS A new configuration of SR machine is modeled with higher number of rotor poles. A new 6/10 configuration with Somaloy 500 material is presented in the project.

2.5 MAGNETIZATION CURVE In an SRM, reluctance (and, hence, inductance) is a function of excitation current and rotor position. At an aligned position or at higher current levels, the ferromagnetic material in the rotor and stator poles begins to saturate. Due to secondary effects such as saturation, fringing, and leakage, nonlinearities are introduced in the relationship between inductance, current and rotor position. At an unaligned position, phase inductance has a minimum value due to high reluctance offered by large air gap. Magnetic saturation is unlikely to occur at an unaligned position, and hence, the flux linkage shows a linear behavior until the start of overlap, labeled as 100 rotor position. At a fully aligned position where rotor poles completely overlap with stator poles, magnetic field density tends to saturate at hig