If you put a noisy motor in a vacuum is it more efficient?

  • #1
[EDIT]
I am of the opinion that putting a noisy motor in a vacuum would make it more efficient, as it is not losing energy by sound anymore. Does this make sense? Or am I just confusing myself?
 
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  • #2
phinds
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I was wondering, if you put a noisy motor in a vacuum, does the motor become more efficient, as it is not losing energy by sound anymore?

I'd love to see what everyone thinks.
Well, ask your self whether the "sound" is a cause or an affect.

EDIT: and by the way, asking others opinions without first offering your own is considered bad form on PF. It's like asking to be spoon-fed an answer and that's against forum rules.
 
  • #3
NascentOxygen
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Some big generators are housed in low pressure enclosures to minimize windage losses. But I think that an ordinary motor placed in a vacuum will have problems with cooling, i.e., compromised cooling. The rotor relies on air to redistribute heat, and the fan blades on the rotor will have no air to circulate in a vacuum. Consequently, the motor capacity may need to be derated. This doesn't directly address your question, however.

My thoughts are that, without the damping effect of air, a mechanical apparatus may vibrate with a tiny bit more amplitude. This may or may not equate to higher losses, though whatever the case I'd expect the difference to be miniscule.

https://www.physicsforums.com/attachments/holly-1756-gif.110502/
 
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  • #4
Well, ask your self whether the "sound" is a cause or an affect.

EDIT: and by the way, asking others opinions without first offering your own is considered bad form on PF. It's like asking to be spoon-fed an answer and that's against forum rules.

Sorry. This is my first ever post and I was just trying to be kind. Thanks for the animosity.
 
  • #5
Some big generators are housed in low pressure enclosures to minimize windage losses. But I think that an ordinary motor placed in a vacuum will have problems with cooling, i.e., compromised cooling. The rotor relies on air to redistribute heat, and the fan blades on the rotor will have no air to circulate in a vacuum. Consequently, the motor capacity may need to be derated. This doesn't directly address your question, however.

My thoughts are that, without the damping effect of air, a mechanical apparatus may vibrate with a tiny bit more amplitude. This may or may not equate to higher losses, though whatever the case I'd expect the difference to be miniscule.

[PLAIN]https://www.physicsforums.com/attachments/holly-1756-gif.110502/[/QUOTE] [Broken]

in an ideal situation where we could cool the motor in a vacuum, wouldn't that mean that the motor would convert more energy kineticallly?
 
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  • #6
phinds
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Sorry. This is my first ever post and I was just trying to be kind. Thanks for the animosity.
No animosity intended and I'm sorry if it came across that way. I DID, I'm sorry to say, overlook the fact that it was your first post. I should have been more welcoming. This is a terrific forum but it's not one of those Q&A forums where you just ask whatever question you have and get an answer. We try to help people learn how to get answers on their own, rather than just give them the answer, so that was my intent.
 
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No animosity intended and I'm sorry if it came across that way. I DID, I'm sorry to say, overlook the fact that it was your first post. I should have been more welcoming. This is a terrific forum but it's not one of those Q&A forums where you just ask whatever question you have and get an answer. We try to help people learn how to get answers on their own, rather than just give them the answer, so that was my intent.

dont worry man, i understand that you dont just want random questions to be asked.
 
  • #8
berkeman
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So to get this thread back on track, in addition to the air resistance efficiency improvement mentioned, and the disadvantage of the loss of air cooling, there is one other important problem with trying to operate a traditional motor in a vacuum. @George Argyrou -- can you think what that problem might be? :smile:
 
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  • #9
So to get this thread back on track, in addition to the air resistance efficiency improvement mentioned, and the disadvantage of the loss of air cooling, there is one other important problem with trying to operate a traditional motor in a vacuum. @George Argyrou -- can you think what that problem might be? :smile:

if you are talking about the motor needing air for the fan to spin then yes. If not i have no idea
 
  • #10
berkeman
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if you are talking about the motor needing air for the fan to spin then yes. If not i have no idea
Hint -- think about the motor bearings and axle... Can you think about any issues that might come up in a vacuum?
 
  • #11
Hint -- think about the motor bearings and axle... Can you think about any issues that might come up in a vacuum?

only that they will heat up, but the questions states that cooling is not an issue. So i am still unsure :[ (P.S. i am not very good at this!!)
 
  • #12
berkeman
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only that they will heat up, but the questions states that cooling is not an issue. So i am still unsure :[ (P.S. i am not very good at this!!)
No worries. If we can guide you to think of the answer on your own, that is best. Lacking that, eventually we will try our best to help you out.

If you put some water in a vacuum environment, do you know what happens to it? And remember that motor bearings usually have some lubricant on them to keep friction low and provide long bearing life. How may these things apply to your question? :smile:
 
  • #13
sophiecentaur
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Motor noise is a major issue in some places. Many different things have been tried but one thing that will always help is a chunky case and some really good bearings. Expensive and not standard practice in domestic equipment.
 
  • #14
I was wondering what the motor noise is assumed to be caused by in the problem statement. Worn bearings? The additional friction and vibration from that would not be helped by a vacuum environment.

If the noise is the air as it's blown over the case (TEFC: "total enclosed fan cooled" type) then that has been addressed as to possible cooling deficiencies introduced by a vacuum.

What other noises could there be? Electrical hum? Again, vacuum independent as far as any efficiency difference.

OK hold on. I think I'm beginning to understand the question as intended. If any noise, no matter the cause, is assumed to be a symptom of efficiency losses is eliminated by a vacuum, are the efficiency loss(es) eliminated? Noise is a bi-product. It's elimination by removal of air will not fix mechanical deficiencies.
 
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  • #15
sophiecentaur
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Rattling laminations ("Batman") can be offensive, too.
 
  • #16
sophiecentaur
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You need to remember the sound energy is 'trivial', even when it's deafening. Our ears are about as sensitive as any other system known to man. We have a wood mouse that kicks up a 'terrible racket' in one of the internal voids of our house. It is probably producing just a few mW of vibrational noise whilst chewing on the timbers but it interferes with our sleep. You have to get these things in proportion when looking to improve efficiency. Best value is to sort out the thermal losses first. I2R really can count.
 
  • #17
i think what i was really trying to get at was can we make a motor that is near 100% efficient
 
  • #18
sophiecentaur
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i think what i was really trying to get at was can we make a motor that is near 100% efficient
Getting to grips with the 5 to 10% that's due to thermal loss is more important than the 0.5% (or whatever) from the sound energy.
 
  • #19
Baluncore
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This is an electrical engineering forum, can we assume it is an electric motor, or maybe it could be a diesel motor driving an alternator ?
What type of electric motor ?
Why is it noisy ?
Hydrogen is used for cooling big electric motors and alternators because it is less viscous than air and transfers heat better. Hydrogen cooling will probably make less noise than air cooling.
 
  • #20
tech99
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[EDIT]
I am of the opinion that putting a noisy motor in a vacuum would make it more efficient, as it is not losing energy by sound anymore. Does this make sense? Or am I just confusing myself?
Of course, the energy lost by sound radiated into the air is very small, quite negligible. But from a theoretical point of view, when the motor starts, energy will be supplied to all parts which vibrate or rotate. Due to inertia (mass) and springiness in the construction, the energy will be stored until the motor is switched off and will then help it turn a bit more. But all the parts which rotate or vibrate will also radiate some energy as sound, and this energy will be lost continuously. So it will appear as very small resistance in series with the motor. But if the motor is placed in a vacuum, the parts will still vibrate and store energy but but the continuous loss of energy as sound radiated into the air will be removed. The vibrating parts will now vibrate more and store more energy.
This operation is an exact parallel to the working of a radio transmitting antenna.
 
  • #21
CWatters
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i think what i was really trying to get at was can we make a motor that is near 100% efficient

You need to look at the sources of the losses. Some permanent magnet DC brushless motors are 93-95% efficient. The remaining 7% isn't attributable to just one cause. There are papers on the web that describe some of the causes but audible noise isn't usually significant enough to feature....

http://www.novatorque.com/aboutus/white-papers/060_John_Petro_final_paper.pdf
"Achieving High Electric Motor Efficiency"
 
  • #22
CWatters
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Of course, the energy lost by sound radiated into the air is very small, quite negligible. But from a theoretical point of view, when the motor starts, energy will be supplied to all parts which vibrate or rotate. Due to inertia (mass) and springiness in the construction, the energy will be stored until the motor is switched off and will then help it turn a bit more.

I doubt that. The vibrating parts are not ideal springs that convert KE to PE and back again without loss. Most likely the vibration will end up heating the motor so that energy is lost that way.
 
  • #23
tech99
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I doubt that. The vibrating parts are not ideal springs that convert KE to PE and back again without loss. Most likely the vibration will end up heating the motor so that energy is lost that way.
Thank you. Yes, agree, but the principle involved is interesting, the division of "reactive" and "radiated" energy.
 

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