Avoiding (acoustic) noise from the electric motor

[Twinbee]

It's very hot in the UK at the moment, and so I went out and bought a semi-industrial 20" pedestal fan for my living room. It certainly does its job in cooling, but unfortunately, it also sounds like a jet engine.

This got me wondering. I assume it uses a standard brushless electric motor to avoid friction, so where is the noise coming from? (I know that it's not coming from the blades whirling, because I removed them, and just heard the engine noise).

Is it possible to engineer an electric motor to avoid the noise completely, and would room-temperature superconductors (if and when they become available) immediately solve the problem?

Searching the web produced surprisingly few answers, but it would be profound for humanity if we could create a silent engine.

 

[russ_watters]

It sounds like a jet engine even with its blades removed? That's very odd. Usually, the vast majority of the noise is airflow noise, not motor noise.

Electric motors are extremely quiet compared to the alternatives - it is rare that you can even hear the motor in a machine run by a motor. I don't see anything profound for humanity in making them quieter.

 

[Twinbee]

I generalized a bit. The fan I removed the blades from was a smaller 9" fan. There was a definite noise coming from the motor (even if the motor is spinning faster because it doesn't have to turn anything). And it was a lot worse (at least in timbre, but the volume was pretty bad too) than the smooth 'pink-noise' sound you get from the wind passing through the blades alone.

I could remove the blades from the 20" industrial fan, though it may be tricky as it was in one piece when I bought it. Also, the sound is very much a mechanical drone sound, and I doubt rushing air would produce anything nearly like it. It sounds like what you hear from the Orbiter fairground ride

Am I right in saying these kind of fans use the brushless DC electric motor design?

 

[russ_watters]

No, they all use AC induction motors.

 

[Twinbee]

Right. Are there any components causing friction in AC induction motors to produce noise?

 

[russ_watters]

Not really, no. There is no metal to metal sliding contact anywhere in a quality motor and the shaft is supported by bearings. The only noise you should hear from a properly functioning and greased motor is a soft hum due to the induction fields flipping back and forth (like a transformer hum).

I have a 9" oscillating desk fan too and just turned it on - the motor noise is barely audible behind the fan noise and the fan noise isn't even all that loud.

 

[Twinbee]

Thanks for the resposnes. Just to follow this up for anyone who might be interested. I took the blades off the smaller fan again for a second listen. There's no doubt that at least a portion of the bassier end of the sound spectrum is generated by the motor (also, if you see on Amazon, plenty of people are complaining about the noise of their older fans in comparison to their newest bought one). I'm guessing the join to the shaft could be at least partly responsible (it has to be supported somewhere, and even quality bearings may succumb under high speeds).

However, the 'droning mechanical' noise I mentioned previously I think comes from the blades after all. There was a little of that with the smaller fan, but I can imagine how steel blades in the bigger fan could produce a much more exaggerated sound.

This leaves me wondering how they could make the blades quieter too. I think they're mostly loud because the wind is suddenly chopped up at the last moment. They should make a better design that enables the air to more gradually increase speed. Perhaps a helical design with more depth to it (like a large scale version of computer fans)? Or more fan blades?

 

[KalamMekhar]

https://www.youtube.com/watch?v=dBS1NRsYuF8

Blue Edge rotor blades reduce rotor noise.

 

[Skrambles]

You could be getting the vibrations from poor alignment and/or poor support of the motor. Have you tried taking it completely out of the fan body and running it?

 

[Twinbee]

Skrambles: I'm unable to open it up to test that.

KalamMekhar, thanks for that video. I thought of having a small slant (well, curved piece) at the edge just like what's shown in the video. It seems so obvious to prevent the sharp part of the blade from chopping up the air so harshly. I find it pretty amazing something like that isn't already in use on fans/helicopter blades generally. Can they really all be missing that? Maybe I'm missing something, and there's more to it than that?

Another factor I just don't get is why almost every fan manufacturer decides to use a flat blade design, when a deeper pitch/depth to the blade would quieten the whole thing down. Correct me if I'm wrong, but although it requires more power to rotate a deeper blade, this is offset against the amount of air that's being pushed. Equivalently, the RPM is slower and quieter, but would push just as much air compared to a flatter blade design. I'm guessing that the ideal depth for a blade to be would be the diameter of the blade, or the radius maybe.

But hey, for all I know, a very deep cylindrical blade/helical design might be even better in terms of quietness.

Wind farm blades are usually pretty flat too. Why also can't they use deeper pitch blades?

 

[russ_watters]

For axial (bladed) fans, you are correct: a twisted and cambered blade is substantially more efficient than a flat blade.

Wind farm blades are usually pretty flat too. Why also can't they use deeper pitch blades?

No, they aren't. They only appear that way because they are so large. The purpose of twist on a blade is so that all of the air being pushed has a uniform speed. So since a point twice as far out on the blade is moving twice as fast, it needs to have half the angle of attack. Obviously, this makes the twist very severe toward the center of the hub but almost imperceptable far out toward the edges of a high aspect ratio (large span, narrow chord) blade).

Wind turbine blades are also cambered/shaped like airfoils.

 

[Twinbee]

Can you think of any reason why the fan manufacturers go for a flatter blade design? (Vornado being one of the exceptions it would seem).

Even ceiling fans seem to suffer from this problem. For example:

Interesting about the wind farm fan blades. They appear at least very stick-like. Do you know how well they would perform if they used a large scale version of the deep pitch design shown at the top of the image below: