Modeling and Control of Halfwave Rectified Rotor Brushless Synchronous Motor
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Date
2020-08
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Addis Ababa University
Abstract
In this thesis, modeling and control of the halfwave rectified rotor brushless synchronous
motor is presented. The motor has a range of promising performance characteristics that
are suitable for electric vehicle (EV) traction drive applications. Its mechanical structure
is simple and robust with brushless means of field excitation capable of variable field
flux control over a wide range of operating speed and torque. To use the motor for such
application, accurate modeling and characterization of the motor is needed. Two different
modeling paradigms were used to study the behavior of the motor.
The finite element method (FEM) was used to first verify the principle of operation
and next to confirm the analytical model calculations. COMSOL Multiphysics was used
to perform the two dimensional (2D) FEM analysis. Besides the FEM model, a simplified
analytical model and Simscape based model convenient for MATLAB/Simulink simula
tions and control system design purposes were also built using the magnetic equivalent
circuit concept. The analytical model developed in this thesis uses a closed form ana
lytical expression for the field winding current, ifd, derived from observing FEM model
behavior. This enabled a more complete description of basic motor characteristics that
allowed determination of motor capabilities, limits and design of controllers which were
not possible without the use of the such closed form expressions. Comparison of the re
sponses of those models was performed. Field oriented control (FOC) based architecture
was used to design and implement a velocity control system for both the FEM based and
Simscape based models.
The results show that the three models agree as long as effects due to the inductance
variation of the FEM model because of iron cores saturation (motor load) and stator slots
are taken into account. The three models showed a logarithmic relationship between the
bias frequency fb and motor torque, field winding current and flux. The field winding
flux was shown to be linearly dependent on the amplitude of the field excitation current
component If of the stator current for a fixed bias frequency fb. The decoupled nature
of field flux and torque currents allowed implementation simple PI based speed and cur
rent controllers. In terms of field winding flux linkage variation, the analytical showed a
1.7%, Simscape a 14.8% and FEM a 18.4% of maximum deviation from the ideal con
stant flux. As long as the basic motor characteristics such as flux, torque and efficiency
are considered, the models can be used interchangeably.
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Keywords
2D FEM, variable flux, Simscape, halfwave rectified rotor excitation, brushless