ABSTRACT: Since the introduction of neodymium-iron-boron (NdFeB) magnetic materials brushless permanent magnet (PM) machines have found use in a wide variety of high performance applications. Various compositions of this magnetic material can produce larger flux densities than other PM materials previously used in brushless machines, even under more adverse demagnetization conditions. These properties make NdFeB permanent magnets suitable for variable speed drive and high precision control applications.
Electric and hybrid-electric vehicles are currently being developed in order to better conserve energy. Energy conservation has become a goal for all technological fields. This implies that efficiency has become an important factor in electromechanical machine design.
Although PM machines are high performance devices, there are two torque components that affect their output performance. The first, called ripple torque, is produced from the harmonic content of the current and voltage waveforms in the machine. The second, called cogging torque, is due to the physical structure of the machine.
Cogging torque is produced by the magnetic attraction between the rotor mounted permanent magnets and the stator. It is an undesired effect that contributes to the output ripple, vibration, and noise in the machine.
Cogging torque minimization techniques have been extensively researched. A variety of these minimization techniques are applied to a six pole, eighteen slot, surface mounted, rare earth type, brushless permanent magnet motor in this thesis. The analysis includes variations in the magnet remanence, magnet arc length, single and double pole pair offset, magnetization, eccentricity, stator slot opening, stator tooth shoulder width, yoke notch radii, skew, and overhang.
Significant changes in the peak value of cogging torque occur with variations in the magnet remanence, magnet arc length, slot width, and skew. For a variation of magnet remanence from 1.27 to 1.17 Tesla, the peak-to-peak value of cogging torque decreased from approximately 0.62 to approximately 0.52 Newton meters (approximately ten percent). There is approximately a seventeen percent decrease in the peak-to-peak value of cogging torque for a decrease in magnet arc length from 60 to 55 degrees. A decrease in the peak-to-peak value of cogging torque of approximately seventy percent occurs with a decrease in the slot width from 2.9 to 1.5 millimeters. Skewing either the permanent magnets or the stator teeth by one slot pitch reduces the peak-to-peak value of cogging torque by approximately 50% compared to the non-skewed machine.
The simulation is performed using two and three dimensional finite element analysis software. The finite element method discretizes the model and approximates the solution to the field problem through a system of linear equations. An analysis involving variations in the convergence tolerance, mesh size, and a comparison with two and three dimensional results is also performed in this thesis. In general, the two dimensional software produced more conservative (greater) peak-to-peak values of cogging torque. The two dimensional software is faster and requires less memory but, cannot be used to analyze end effects which can be accounted for by the three dimensional software.
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