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Why BLDC motors are noiseless compare to Induction motor?

If referring to the acoustic noise generated at or around the PWM frequency of the inverter, then you may notice a very slight difference between "BLDC" and an induction motor. Assume that it's referring to a surface permanent magnet machine for the BLDC and a squirrel cage induction machine. The SPM machine has laminations in the stator that will generate audible noise at a frequency consistent with the inverter's PWM frequency. The same is true for the induction machine. However, the induction machine also has laminations within the rotor, which also generate audible noise at a frequency consistent with the inverter's PWM frequency. There are more laminations in an induction machine. This may account for some of the difference.

I also don't know what the relative power difference is between the BLDC and the induction machine. If it's about a 5Hp BLDC and a 100Hp induction machine, then you can bet that the PWM frequency of the BLDC is likely above the audible range and the PWM frequency of the induction inverter is well within the audible range.

These are just a few reasons that you may find subtle differences between the two. There are many factors and more information is needed to really help understand your specific situation.
I also believe there are simply sophomoric and unprofessional answers. My statement is based on the general rule that there is greater surface area between laminations of squirrel cage induction machine then there are in BLDC machines. Of course, if you want to state that you have a thin lamination on a long stack length design for a BLDC then there may be an argument that such a motor design when compared to a typical induction machine of the same power has a similar surface-to-surface lamination area. It is these laminations moving due to eddy currents at the PWM frequency that causes the audible noise.

BLDC can come with very small inductance which requires a higher PWM frequency, if you compare both them with controller that may cause different.
If you build 2 motor using exact mechanical shapes and electrical parameter they should be very close. You can build 2 induction machines from 2 different vendors to same electrical spec and they will not sound the same.

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