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Why do LSM/LIM make a sound?

PeskyTrimBrake

PeskyTrimBrake

Hyper Poster
This might be a dumb question but why do LSM/LIM make a screeching sound when triggered? I know that LSM switch polarity in order to be a repelling force hence launch. But why does it make a sound? Aren't they magnets? Could it be the motor?

Here's what I'm talking about:
(go to 2:12)
[youtube]http://www.youtube.com/watch?v=25Z48FchWmI[/youtube]
 
Do the magnets and fins touch, or come very close? It could simply be the force of the two parts moving past each other either touching and screeching, or the air between them in such a tight space at high pressure?
 
furie said:
Do the magnets and fins touch, or come very close? It could simply be the force of the two parts moving past each other either touching and screeching, or the air between them in such a tight space at high pressure?
Click to expand...
They shouldn't touch. :P That's sort of the beauty of magnetic launches/brakes - no friction, so no worn out parts.

My guess is that the noise is produced by a combination of factors, but mostly probably the large alternating currents in the magnets. This would have two effects: the first being that any stray magnetic fields are going to cause parts of the track and equipment casings to vibrate; the second being "magnetostriction", which is essentially the materials physically changing shape due to the magnet fields (I only really know about this as one of those "things that happen", my knowledge of it is basically zero). Both of these sources of vibration make noise, and I would assume them to be the source of the launch 'squeal'. These two factors are, to my understanding, most of the reason why transformers hum (there is plenty of available material on that on Google) and they seem like they could also come into play here.

Other factors may include the rubber on the wheels squeaking, the train couplings/chassis vibrating under the rapid acceleration, and other electrical equipment nearby buzzing as the power increases.

My best guess at least.
 
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