8 Switched Reluctance Motor

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 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. The causes of the torque ripple include the geometric structure including doubly salient motor, excitation windings concentrated around the stator poles and the working modes which are necessity of magnetic saturation in order to maximize the torque per mass ratio and pulsed magnetic field obtained by feeding successively the different stator windings. The phase current commutation is the main cause of the torque ripple. The torque ripple can be minimized through magnetic circuit design in a motor design stage or by using torque control techniques. In contrast to rotating field machines, torque control of switched reluctance machines is not based on model reference control theory, such as fieldoriented control, but is achieved by setting control variables according to calculated or measured functions. By controlling the torque of the SRM, low torque ripple, noise reduction or even increasing of the efficiency can be achieved. There are many different types of control strategy from simple methods to complicated methods. In this book, motor design factors are not considered and detailed characteristics of each control method are introduced in order to give the advanced knowledge about torque control method in SRM drive.