Why BLDC motors are noiseless compare to Induction motors?

"Noiseless" electrically or audibly?

Audio noise will have a great deal to do with the controllers PWM frequency and it's commutation type (sinesoidal being the most quiet as opposed to block). This also combined with the fact that the rotating magnetic core is not in contact with the coil by the brushes. Graphite brushes are sprung loaded to the comutator plate which causes the brush fire (spark). This creates both electrical and auidable noise. I hope this helps you Vengadessin

Are you asking or telling that?

BLDC motors are no more or less noisy than their induction motor counterparts, either audibly or electrically.

Why some people are smarter than others :-)

There are no "dumb questions". I'll try to speak to this. If you are 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. I will assume that you are 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 you're talking 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 agree there are no dumb questions; but there are dumb answers FWIW....

why would you assume there are more laminations in the induction motor rotor than the BLDC motor can you state the lam thickness in the two motors the OP is conerned about? I see no reason to make such a statement. I suspect if you look into it you will find similar lamination thickness in both; like transformers, you can have thicker or thinner laminations depending on how cheap you want/need to build the machine. To make a global statement that they are significantly different IMHO is wrong.

Of course to compare a 5hp BLDC motor to a 100HP induction motor for audible noise is just wrong on too many levels to even mention, and thus of no value IMHO.

Of course there are lots of small reasons one variable frequency drive may be more noisy than another but that is not the OP question.

BLDC can come with very small inductance which require 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 machine from 2 different vendor to same electrical spec and they will not sound the same.

BLDC can come with very small inductance.



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


So again, NO DIFFERENCE between noise from BLDC or Induction motor. Sorry.


Ya'll now 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. If you don't have experience working with BOTH low inductance, difference size etc etc, etc BLDC AND induction motors, then you should not reply.


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

Mike, 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.

Your arrogant statement regarding the 5Hp BLDC motor to a 100Hp induction motor being "just wrong on too many levels" shows you clearly do NOT understand that a smaller 5Hp BLDC motor will likely have a 20kHz+ PWM frequency compared to the sub 10-12kHz PWM of a 100Hp induction motor.

I would stop adding IMHO to your statements because you are anything but humble, IMHO.

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 brand 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 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.

If your BLDC motor has fractional slot concentrated winding topology (FSCW), vibration and acoustic noise are higher as expected because of the higher cogging torque and torque ripples of this winding technique according to conventional winding technique.

Very well Mr Vengadessin you made the statement that “ BLDC motors are noiseless compare to Induction motor”. I am sure that the engineers would like to help you if you give them more information.
Is this your experience? Give us some examples. What motors are you using? How did you measure the sound levels and the frequency?
For what it is worth, it could be the sound of the forced-cooled-motor fan.

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 machine 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.

Noises are directly related to design and control procedure. in design stage, slot and teeth make noise and in control stage, PWM frequency is more important, I think.