# What happens to a fan inside a system without air?

1. Jun 18, 2012

### KillerX

Hey guys!

I would like to know what would happen to a fan that is rotating inside a system without air. Imagine you take all the air off the system and after that you put the fan rotating. The only opposite force acting to make it stop would be the air resistance, but as no air, what would happen? Would the fan spin? Would it stop spinning, or would it spin forever?

Sorry if it is a dumb questing but physics is not my best.
Thanks :)

2. Jun 18, 2012

### Staff: Mentor

Without air, the fan would spin faster. Its speed would be limited by how fast its motor can drive it.

When stopped the fan would slow down as there is still friction about the axle.

3. Jun 18, 2012

### DaveC426913

Yup, jebodiah is right on both counts.

If the fan were powered, it would spin faster.
And once you stopped the motor, friction would bring it to a halt.

4. Jun 18, 2012

### Staff: Mentor

And depending on the fan motor design, it might fail if kept running for a while.

Quiz Question to KillerX -- can you think of a reason that it might fail if run in a vacuum?

5. Jun 18, 2012

### KillerX

I have no ideia as I'm considering the motor outside the vacuum, it would be just powering through cables to the vacuum. What could fail? I know that the energy should be PRESERVED, but the energy generated by the motion of the fan wouldn't be greater than the energy given by the engine? Just because it seems to me that it would be...

6. Jun 18, 2012

### DaveC426913

By cables, do you mean drive shaft or chain? Otherwise how does the motor deliver its rotation to the fan blades?

7. Jun 18, 2012

### Staff: Mentor

Hint: What are fans generally used for?

And are you seriously wanting to put a fan in a vacuum for some reason? Or is this just a thought experiment?

8. Jun 18, 2012

### DaveC426913

Presumably, the ... vacuuamity is a temporary, or intermittent state of the system...

9. Jun 19, 2012

### CWatters

What happens will depend entirely on the type of motor...

A fan in a vacuum would provide very little load on the motor. Might as well not be there. So what happens depends on how the motor responds to being operated without a load.

Some motors (for example most permanant magnet DC motors found in battery powered toys) are almost constant speed devices. They have a rated "no load rpm" that depends on the applied voltage. It's typically of the order of a 1000 rpm per volt. When switched on they will accelerate until they reach their no load rpm and then run quite happily at that rpm. At that point the back emf of the motor matches the applied voltage so the motor can no longer accelerate. The motor will draw enough power from the batter to match losses only.

Other types of motor (eg series DC) might behave differently. With series DC motors they don't use permanant magnets to provide the field they use an additional coil in series with the armature winding. If there is no load (eg the fan is in a vacuum) the motor won't draw much current from it's supply. Therefore the field generated by the field winding will be weak. That means the motor will need to spin very fast before the back emf matches the applied voltage. Perhaps so fast that it damages the motor or fan. For that reason a Series DC motor shouldn't normally be run without a load.

Other types of motor might just accelerate until something fails or losses in the motor exceed the capabilities of the power supply.

10. Jun 19, 2012

### CWatters

In theory if you were to spin up the fan and then dissconnect the motor from the fan the fan should continue to spin indefinitly. In practice there will be friction in the bearings that cause it to slow down and eventually stop.

The Pioneer 10 spacecraft has been spinning for 40 years (with only occasional help from thrusters to control the spin rate). According to info on the web the spin rate has been reducing very slowly over the years due to the heat emitted by the spacecraft and some gas leaks.

Last edited: Jun 19, 2012
11. Jun 19, 2012

### DaveC426913

Well yes, but the Pioneer 10 is an isolated system. It has no axle that it is spinning on. The fan by its very nature has a frictional component.

12. Jun 19, 2012

### CWatters

Agreed.

13. Jun 19, 2012

### Rap

I have run into this type of problem with well pumps. Sometimes the well runs dry. When this happens, the current drops down. There is an automatic sensor which senses the current drop and cuts off the pump before damage is done. I THINK The potential damage is mostly mechanical, the pump motor speeds up, but is not delicately balanced and at those high speeds, slight imbalance causes slight damage which causes more imbalance, which causes more damage, etc etc until it craps out.

14. Jun 19, 2012

### magicacid

I think I understand gas leaks, but why does the heat emission cause it to slow down?

15. Jun 19, 2012

### DaveC426913

The craft is a package of electronic components with a giant dish stuck to it. The dish points Earthward, meaning the heat-emanating electronic package points out of the solar system.

Any radiation from the electronics, which radiates outward in a sphere, will be free to flow outward from the craft toward interstellar space, but any of it that intersects the dish will act as a brake.

Essentially, because of its orientation the craft has a very, very low thrust propulsion system in the form of heat. It they turned it 180 degrees, the thrust would accelerate the craft positively rather than negatively.

Earth is up in this pic:
http://cache.gawkerassets.com/assets/images/8/2011/04/medium_pioneer_anomaly.jpg [Broken]

Last edited by a moderator: May 6, 2017
16. Jun 19, 2012

### CWatters

Photons have momentum so when emitted there is a recoil effect. Depending on the direction that the heat is emitted the recoil can have various effects... It can change the spin, push the spacecraft off course or speed it up or slow it down. If I remember correctly when I googled this earlier I found it slows the spin of pioneer 10 but speeds up the spin of pioneer 11.

Calculating the effect taking into account all the various sources solved one of the great mysteries..

http://www.popsci.com/science/article/2011-07/mystery-pioneer-anomaly-solved-last

17. Jun 19, 2012

### magicacid

O, yes I've read about this in "the trouble with physics" by Lee Smolins, as a possible proof that relativity is not as universal as we thought. But it seems it'S simpler than that

18. Jun 19, 2012

### DaveC426913

Yeah, that book is obsolete now.

19. Jun 19, 2012

### Staff: Mentor

Btw, another issue with a fan in a vacuum is it may overheat becacause of not enough air to cool it.

20. Jun 19, 2012

### DaveC426913

See post 5. OP is planning to obviate that problem by putting the motor outside the vacuum housing, connected to the blades by something he calls "cables". I presume the OP actually means shaft. (a belt or chain would not allow a vacuum-seal on the chamber).