Automotive Engineering - May 2021 - 22
Model-based analysis
of e-motor designs
Schematic of the permanent-magnet machine.
Average rotor torque variation with initial rotor angle over the
angular span of two rotor poles.
Magnetic field distribution for iron thicknesses of (from left) 1mm, 2mm and 3mm.
Surface plots of the magnetic flux density for the three thickness values can be
used to inspect the iron utilization and correlate them with the corresponding
torque waveforms.
In the plot of the average torque curve, we can observe two maxima: The initial angular position corresponding to the positive maximum will result in counter-clockwise rotation (with proper stator coil
excitation sequence), and the initial angular position corresponding
to the negative maximum will cause a clockwise rotation (with corresponding stator coil excitation sequence).
22 May 2021
By observing a surface plot of magnetic flux density,
we can gain a lot of insight into the performance of
the machine. The color legend in the three magnetic
field plots above indicate the magnetic flux density
values. If we see a large difference of color in different
portions of the yoke, it indicates that the yoke thickness may be reduced to even out the flux distribution
in the yoke. This will result in better utilization of the
iron for setting up the magnetic flux. If some portion
of the yoke forms a bottleneck for the magnetic flux, it
could cause the magnetic flux density value to move
into the non-linear region of the hysteresis curve.
Sometimes, a specific part of the yoke will consistently show a weak field, which implies that, for
torque production, it is underutilized. If a consistent
bottleneck forms at a certain part, we should probably widen it. If hotspots (tiny areas with large color
difference compared to the surroundings) are observed in any region of the yoke, they indicate regions of a very high local magnetic flux density. Over
time, these hotspots will cause local overheating,
thermal expansion, and persistent localized mechanical stresses - all will considerably reduce the functional life of the machine. These may be mitigated by
AUTOMOTIVE ENGINEERING
ALL IMAGES: COMSOL
Assessing iron usage and losses
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