Document Type


Date of Award


Degree Name

Master of Science in Electrical Engineering - (M.S.)


Electrical and Computer Engineering

First Advisor

M. C. Leu

Second Advisor

Andrew Ulrich Meyer

Third Advisor

Anthony D. Robbi


A study of a three-phase, pulse-width modulated, velocity controlled brushless DC motor speed control system is presented. Previous works on brushless DC drive systems could not explain global instability, due to linear approximations. A complete three-phase system model is presented here, along with a simplified single-phase model, which is similar to a conventional DC motor model. The models incorporate saturation nonlinearity. The brushless DC drive system is analyzed and simulated. Special emphasis is laid on the study of the limit-cycle phenomenon, due to saturation nonlinearity. Global instability is accounted for and relevant parameter regions are established for global stability. The effects of system parameters, including velocity controller coefficients, frequency and amplitude of dither, and time delay in PWM control on the systems performance are investigated and verified. The behavior of current in the phase windings is also studied.

Comparison between the simulation results from the single-phase model and from the three-phase model indicates that the single-phase model is sufficient to approximate the performance of the system within the normal range of operation. However, the results deviate from that predicted by the three-phase model beyond the normal range of operation.

Experiments are performed to validate the results of analysis and simulation. The measurements are carried out with the help of a computerized data acquisition and control system. Comparison between the simulation and experimental results shows good agreement. The limit-cycle predictions are also verified.



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