Graduate Thesis Or Dissertation

Prediction of brushless DC drive performance considering non-sinusoidal parameter variations and asymmetrical operating conditions

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  • A highly flexible numerical simulation model for the performance prediction of brushless dc drives is presented, which is capable of investigating unbalanced and asymmetrical operating conditions as well as drives with non-ideal parameters. Numerical speed-up procedures allow for the investigation of pulse-width modulated (PWM) drives using a personal computer. Experimental verification shows excellent correlation between predicted and measured quantities. The simulation is used to investigate the performance of several brushless dc motors, two of which are prototypes intended for aerospace actuator applications. Inverter control options examined include sixstep continuous and discontinuous as well as pulse-width modulated strategies. A brushless dc motor with trapezoidal induced voltages and complex magnetic geometry is the subject of a sensitivity study to determine the differences in performance predictions obtained using the "exact" numerical simulation developed, a reference frame approach, and an equivalent circuit model. The results are used to draw conclusions about drive modeling and design issues. The numerical drive simulation is used to investigate speed control obtained from variations of pulse-width modulation index and commutation advance for a drive powered by a sine-coded PWM inverter. The resulting effects on drive performance characteristics are discussed in terms of modeling and design strategies. For an adjustable speed application requiring constant torque output, the simulation is used as a design tool to provide for excellent drive efficiency well into the field weakening region. The potential to readily analyze unbalanced drive systems is exploited in the simulation of failures in a brushless dc drive supplying a fan load. Faults addressed in detail include open-circuited motor windings, failure of a rotor position sensor and a defect in the commutation control logic. Experimental data shows excellent agreement with simulated waveforms. Finally, a use of the simulation model to investigate the possibility of fault tolerant drive development is proposed. It is conceived that "intelligent" controllers have the potential to use remedial control strategies to optimize drive performance under fault conditions. A design example uses the numerical simulation to investigate several strategies to restore required torque output after a base drive failure has disabled one of the six inverter power switches.
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