special electrical motor(switched reluctance motor)

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This is for those who want to know about switched reluctance moto basics........r

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  • 1.Regency Institute of Technology, Yanam 1 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Unit-3 Switched Reluctance Motor Introduction Switched Reluctance Motors (SRM) have inherent advantages such as simple structure with non winding construction in rotor side, fail safe because of its characteristic which has a high tolerances, robustness, low cost with no permanent magnet in the structure, and possible operation in high temperatures or in intense temperature variations. The origin of the reluctance motor can be traced back to 1842,but the reinvention has been possibly due to the advent of inexpensive, high-power switching devices. The torque production in switched reluctance motor comes from the tendency of the rotor poles to align with the excited stator poles. The operation principle is based on the difference in magnetic reluctance for magnetic field lines between aligned and unaligned rotor position when a stator coil is excited, the rotor experiences a force which will pull the rotor to the aligned position. However, because SRM construction with doubly salient poles and its non-linear magnetic characteristics, the problems of acoustic noise and torque ripple are more severe than these of other traditional motors. The torque ripple is an inherent drawback of switched reluctance motor drives. Construction of a switched reluctance motor (SRM): Switched reluctance motor (SRM) drives are simpler in construction compared to induction and synchronous machines. Their combination with power electronic controllers may yield an economical solution . The structure of the motor is simple with concentrated coils on the stator and no coils or magnets on its rotor. SRM is a type of synchronous machine. It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine. Stator windings on diametrically opposite poles are connected in series or parallel to form one phase of the motor. Several combinations of stator and rotor poles are possible, such as 6/4 (6 stator poles and 4 rotor poles), 8/4, 10/6 etc. The configurations with higher number of stator/rotor pole combinations have less torque ripple. The Switched Reluctance Motor drives present several advantages as high efficiency, maximum operating speed, good performance of the motor in terms of torque/inertia ratio together with four-quadrant operation, making it an attractive solution for variable speed applications. The very wide size, power and speed range together with the economical aspects of its construction, will give the SRM place in the drives family. The performances of switched reluctance motor strongly depend on the applied control. Fig.1 Switched reluctance motor configurations

2. Regency Institute of Technology, Yanam 2 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Operation of a switched reluctance motor (SRM): The reluctance motor is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized. 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. 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. This elementary operation can be explained by Fig.2. In the figure, consider that the rotor poles r1 and r1 and stator poles c and c are aligned. Apply a current to phase a with the current direction as shown in Fig.2a. A flux is established through stator poles a and a and rotor poles r2 and r2 which tends to pull the rotor poles r2 and r2toward the stator poles a and a, respectively. When they are aligned, the stator current of phase a is turned off and the corresponding situation is shown in Fig.2b. Now the stator winding b is excited, pulling r1 and r1 toward b and b, respectively, in a clockwise direction. Likewise, energizing phase c winding results in the alignment of r2 and r2 with c and c, respectively. Accordingly, by switching the stator currents in such a sequence, the rotor is rotated. Similarly, the switching of current in the sequence of acb will result in the reversal of rotor rotation. Since the movement of the rotor, hence the production of torque and power, involves a switching of currents into stator windings when there is a variation of reluctance, this variable speed motor is referred to as a switched reluctance motor (SRM). Consideration of this basic operation highlights that the operating direction has nothing to do with the current polarity. Direction of rotation is determined by the sequence of the energized phases, and the frequency of this switching sequence dictates the speed of the rotor. Fig.2 Operation of switched reluctance motor (a) Phase c aligned and (b) Phase a aligned Torque: The torque production in the switched reluctance motor can be explained using the elementary principle of electromechanical energy conversion. In the case of a rotating machine, the incremental mechanical energy in terms of the electromagnetic torque and change in rotor position can be written as: Wm = Te --------------(1) where Te is the electromagnetic torque and the incremental rotor angle. Therefore, 3. Regency Institute of Technology, Yanam 3 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept the electromagnetic torque can be obtained by: -----------------------(2) For the case of constant excitation (i.e., when the mmf is constant), the incremental mechanical energy is equal to the change of magnetic coenergy, Wf: --------------------------(3) By the theory of electromagnetic field, if no magnetic saturation exists, the coenergy at any position in the motor can be expressed by: ---------------------------(4) where L(, i) is the stator inductance at a particular position, and i the stator phase current. Hence, the electromagnetic torque is: ------------(5) Equation (5) has the following implications: 1. The torque is proportional to the square of the current and hence, the current can be unipolar to produce unidirectional torque. This is a distinct advantage in that only one power switch is required for the control of current in a phase winding and thereby makes the drive economical. 2. Since the torque is proportional to the square of the current, it has a good starting torque. 3. Because the stator inductance of a stator winding is a function of both the rotor position and stator current, thus making it nonlinear, a simple equivalent circuit development for SRM is not possible. 4. A generation action is made possible with unipolar current due to its operation on the negative slope of the inductance profile. As a result, this machine is suitable for four-quadrant operation with a converter. 5. Because of its dependence on a power converter for its operation, this motor is an inherently variable-speed motor drive system. SRM Configurations: Switched reluctance motors can be classified as shown in Fig.3. The initial classification is made on the basis of the nature of the motion (i.e., rotating or linear). Fig.3 Classification of SRM Equivalent Circuit: An elementary equivalent circuit for the SRM can be derived neglecting the mutual inductance between the phases as follows. The applied voltage to a phase is equal to the sum of the resistive voltage drop and the rate of the flux linkages and is given as: 4. Regency Institute of Technology, Yanam 4 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Static and Dynamic torque production: Static torque production: Consider the primitive reluctance motor in Fig. 7.6(a). When current is passed through the phase winding the rotor tends to align with the stator poles; that is, it produces a torque that tends to move the rotor to a minimum-reluctance position. FIG. 7.6. Elementary reluctance motor showing principle of torque production: (a) primitive motor; (b) field energy and coenergy. 5. Regency Institute of Technology, Yanam 5 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept If magnetic saturation is negligible, then the relationship between flux linkage & current at the instantaneous rotor position is a straight line whose slope is the instantaneous inductance L. Thus Dynamic torque production : Under normal operating conditions at speed, the energy exchanges, both incremental and total, can be determined by integrating the voltage equation and developing the conversion loop in the ,i diagram. The necessary time-stepping procedure was developed by Stephenson and Corda (1979) and only the outline of their method is described here. The voltage equation is integrated in the form through one time-step, giving a new value of . If the speed is assumed constant, the integration can be done with respect to rotor angle 9. Otherwise the rotor angle must be determined by a simultaneous integration of the mechanical equations of motion, as is normal in such simulations. At the end the time-step, and are both known, and the current i can be determined from the magnetization curve for that rotor angle. To minimize this computation Stephenson and Corda used a set of polynomials to represent the magnetization curves at a number of rotor angles between the aligned and unaligned positions, and then applied an interpolation procedure at the end of each time-step to determine the current from the fiux- linkage at the particular rotor position. The instantaneous torque can be determined from the difference-approximation to the partial derivative of co energy at constant c

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