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BLDC VS Induction motors

BLDC can come with very small inductance.
Induction motors can come with very small inductance which requires a higher PWM frequency, if you compare both them with controller that may cause different.

There are exceptions to every rule, but the bottom line is a BLDC motor will NOT have any more or less 'noise' than an equivalent induction motor.

You can even get low cost variable frequency drives with low commutation resolution (8 bit and less) that WILL cause more audible noise in EITHER type motor irregardless of PWM frequency! The variables are infinite. The fact remains BLDC does not have to have different noise than an equivalent induction motor.

I have run 1Hp to 250HP PMSM motor as well as the same horse power induction motor. I was required to measure the sound level of each application due environment concern. I found that there is no co-relationship between the noise level between the two, if they are VFD driven. The loudest noise is definitely from the PWM frequency. Out in the field, most of the PWM frequency being use 2.5khz, 5Khz and very seldom to be 8Khz, all of them are in the audible range.

It is interesting that some brands of motor are noisier than the other. That is mostly due to the motor construction itself. A good VPI make a lot of difference. If you have a BLDC motor that is noise at the motor frequency, that is most likely due to the drive configuration - the current phase angle is not optimal. That can be tuned out. I was operating a 100Kw BLDC a few years ago, the humming sound was very loud. We removed that noise completely just by adjusting the current phase angle in relative to the rotor position.

Motor is a very interesting field. Theory and math is important to get you started, but there is nothing like putting your hands on a 500Kw machine that you can feel and hear how it is running.
BLDC with fractional slot concentrated winding are very interesting motors. It has very short end turns, so copper loss is very low. Some of these machines have high cogging torque and some don't. So choose the pole and stator slot combination that give you low cogging torque and don't use the one that give you high cogging - that will solve the problem. In terms of torque ripple, if you do it right, you can eliminate both cogging torque and torque completely.

Recently, I was playing with a 275Kw BLDC. The cogging torque is so low that I can turn it by 1 hand and doesn't feel anything. It is a fractional slot concentrated winding machine. Its rated torque is 1500NM (or about 1150 ft-lb). On the dyno, we see a torque ripple less than 0.1 NM. No vibration or torque pulsation what so ever. So as speak, there are a lot of theories in the BLDC and PM motor, but they need to be tested with the real machine.

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