| Switched reluctance (SR) motors are inexpensive and reliable, because salient poles on both stator and rotor simplify their construction. However, these same features give rise to a naturally pulsating torque, which has limited the SR motor's usefulness as a servo. Earlier work has shown that torque ripple can be reduced, and servo performance achieved, by a control method in which the phase currents are made to track well-chosen reference currents. The waveform of the reference currents depends on the phase torque-angle characteristics. Reduced torque-ripple, and thus improved servo performance, depends on how accurately the reference currents can be tracked.; This thesis presents improvements to the design and control of SR motors to reduce torque ripple. A torque ripple index, chosen to account for the ripple waveshape, is used to compare the performance of alternative designs for the motor and its control. The control is improved by choosing reference currents that reduce the bandwidth and current ratings required of the drive circuit, for a given motor design. The motor design is improved by determining the choice of lamination geometry that results in the best overlap of the phase torque-angle characteristics, and therefore the lowest rate of change of the reference currents, for the given control. Torque-angle characteristics are predicted by numerical calculation of the magnetic fields. Improvement of the motor design is demonstrated by measurements using prototype motors constructed especially for this purpose. Improvement of the electrical control is demonstrated by measurements of the instantaneous torque when a commercial motor is driven by our new commutation scheme. Considered together, these improvements can reduce SR motor torque ripple to levels comparable with other popular servos. |
