noise

[weknowthewor]

why do ac generators and motors generate noise and not dc???

[Externet]

You have not tried a DC motor of the same size of the AC ones you say emit noise. They do too. Do not compare your mom's blender to a battery operated toy.

Miguel

[YT2095]

they Both make noise, brushless motors less so (for obvious reasons).

only diff is you get your AC from the Mains 9 times outa 10, anf the wire carrying that works as an antenna for Back EMF spikes.

that`s all

[gcol]

I wonder if the answer is really so simple. If you stall an AC motor, you get (just before it burns out) a much louder hum than when it is turning freely. Stall a DC motor, and it is silent. Even in a large DC motor, the only noise I have ever heard is from the brushes against the commutator. I think the noise (a hum) in AC motors is the result of mechanical vibration caused by magnetic flux quickly changing and vibrating coils and laminations. It may be possible to build an AC motor robust enough so that it does not hum.

[YT2095]

there is such a motor it a brushless type too, it`s found in old record players and the motor for the fan on some microwaves. they`re practicaly Silent and quite fast but have little torque.

[gcol]

So there you have it. High mass relative to power. Can't beat a heavy mass to kill off those pesky vibrations. That was a practical lesson learned even before the wheel, I reckon.

[YT2095]

these are the sort: http://www.mbelectronics.com/images%5Citems%5CFan%20motor.jpg

http://www.applianceblog.com/archives/freezermotor891337.jpg

[gcol]

It may be possible to build an AC motor robust enough so that it does not hum.

 

Can't believe I really said that. I have of course come accross many such, and much larger, from my days in power-hungry valve computers when germanium diodes were still in nappies. A lifetime ago, and memory fades. Was going to write something else, but can't remember it now......

[YT2095]

lol, I still have several of the old thermionic diodes here (perfect working order too), and a few Selenium "diodes"/Rectifiers, And the old wax paper condensers

[ironizer]

DC motors don't have many parts douching each other. If you get a magnetic bearing, the only thing you will hear is air getting pushed around. AC motors have brushes, and those make hella noise/sparks and what not.