Naked Science Forum

Non Life Sciences => Technology => Topic started by: thedeadman on 17/06/2010 10:30:04

Title: Why do electric motors make sounds?
Post by: thedeadman on 17/06/2010 10:30:04

Waseem asked the Naked Scientists:
   
I want know that why electric motors make sound during work.

What do you think?

Title: Why do electric motors make sounds?
Post by: graham.d on 18/06/2010 10:56:55

I don't know for sure but there are a few candidates. I would guess that in many motors the brushes on the commutator are the most significant source of noise. Another area would be the bearings. Another source of noise maybe the alternating fields flexing the windings of the electromagnets. Physically the motor rotates in air so turbulance related to the frequency of rotation (and harmonics) will also contribute as well as vibration if everything is not perfectly balanced.

All of these noise sources could be reduced by careful design - think of a CD player for example - but I guess in many cases it isn't considered a key design parameter.

Title: Why do electric motors make sounds?
Post by: wolfekeeper on 18/06/2010 23:30:39

The main source of noise is going to vary from motor to motor.

In a lot of motors though a big one is 'cogging' where the strength of the magnetic field that is seen by the armature varies as it spins. This causes it to cyclically twist on the shaft and this makes some of the noise.

Also, bearings are likely to be another major source.

Title: Why do electric motors make sounds?
Post by: traveler on 20/06/2010 01:15:56

Those are true statements, but I would say that the cooling fan at the opposite end of the output shaft makes the most noise.

Title: Why do electric motors make sounds?
Post by: tommya300 on 20/06/2010 02:39:05

.
Depending on the motor there are many different designs.
The DC motor commutator to the brushes make different pitch sound relative to the RPM

The  AC makes a humming sound due to the Lamentations of field coils sometimes.
The magnetic flux between the armature and the field coils oscillate as it cycles, such as a speaker operates, the housing of the motor accentuates this oscillation. Airflow around the spinning parts also.

This also can apply to a solid magnet field of a DC motor.
Sometimes inferior manufacturing can also contribute, such as; inferior bearings, bad balancing of the armature, poor bonding of the laminations, etc...
Any time there is an oscillation in any sound conducting environment or medium, there is always noise.
.

Title: Why do electric motors make sounds?
Post by: Piterson on 21/06/2010 07:48:22

Hi Guys
The motor was purchased new a decade ago and run by VFD since day one for a total of only about 400hrs.  I have disassembled the motor and found that the sealed bearings felt good.  No catching or roughness and a little drag, not dry endless spinning. I did not remove the bearings from the shafts. The drive end bearing is clamped into its hole with a little plate with two screws. The NDE just has a spring wavy washer dropped into the end bell hole it slides into.

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