NONLINEAR CONTROL OF SWITCHED RELUCTANCE MOTORS

Abstract

Switched reluctance (SR) motors have salient poles on both stator and rotor. The stator consists of concentric windings while there are no windings or permanent magnets on the rotor. All these features make SR motors cheaper and rugged. However, the doubly salient feature gives rise to pulsating torque and hence the SR motor's usefulness for servo applications is limited. Earlier work has shown that torque ripple can be reduced, and servo performance can be enhanced, by using a suitable nonlinear control method. The nonlinear control technique produces the required currents which compensate SR motor's inherent nonlinearities and hence constant torque and constant speed can be achieved.

This thesis aims to investigate various nonlinear control schemes such as feedback linearisation control and variable structure control with sliding mode for speed control of the SR motor drive. A performance comparison of conventional proportional-integral (PI) control and the above mentioned nonlinear control algorithms is carried out. In the first part of the thesis the author proposes a full order mathematical model of the SR motor which takes magnetic saturation into account. Subsequently, a four-quadrant electronic commutator with excitation advance is elaborately designed to fit both one-phase-on and two-phase-on operating modes. Two-phase-on operating mode is necessary to gain optimum torque performance.

In the second part of the thesis the design of a fixed gains conventional proportional-integral controller for speed control of the SR motor is provided. Two low cost implementable nonlinear controllers based on feedback linearising control and sliding mode control algorithms are presented for high performance speed regulation of the SR motor drive. Instead of decoupled control as proposed by previous researchers [30][3 l] which requires m voltage regulators (choppers) for am-phase motor, the author proposes to use only one voltage regulator to feed all phases of the motor. This is required so that the cost of the drive system is affordable. Multiphase-excitation scheme is adopted in controller design for the purpose of exploiting higher power as well as minimising torque ripples.

Simulation studies of a 400W, 4-phase SR motor with a step angle of 15° for proportional-integral, feedback linearisation, and sliding mode controllers have been carried out. The results so obtained confirm that the performance of the nonlinear controllers shows improvements over that of proportional-integral controller. For example, nonlinear controllers have faster dynamic response and much smaller speed ripples at rated torque. Finally, hardware implementation and software programming of proportional integral, feedback linearisation, and sliding mode controllers have been successfully accomplished by using TMS320C50 Digital Signal Processor. Satisfactory experimental results have been obtained which confirm the simulation results.