Part (c) of figure 3 shows the 20th and 24th harmonics
being reduced when microphone is used for a sensor. After
the first 2 steps, which show a similar response as the first
two sub plots, the reduction in amplitude is slower than for
previous sensors. The 24th harmonic requires five steps to
converge. After 10 steps, the amplitude has been reduced to
2.4% of its initial value. The 20th harmonic begins to
converge after 4 steps, however the level of reduction of its
amplitude is less than for the other two feedback types.
After 10 steps it has been reduced to 31% of its initial value.
This is due to the fact that the torque ripple at the 20th
harmonic has been reduced to a level where the sound being
produced is no longer detectable by the microphone even
though torque ripple is still detectable by the torque sensor.
The mechanical system is no longer being excited by the
torque ripple. For this reason, the microphone is a good
choice of sensor if the aim is to reduce acoustic noise
generated by a PMSM.