Why a computer fan can be used as a generator ?

Hello :)

I have a question about DC fan, such as the ones inside a computer.

I was discussing with a friend and he told me than when you clean such fan with a vacuum cleaner, you should block the fan so it doesn't turn (which I never did), because it could damage the motherboard (which never happened to me). He couldn't explain to me because he isn't into electronics.

I replied that it didn't matter because there was most likely some protections inside the fan (such as a diode?), which disallowed any power to be sent back into the motherboard.

After testing with my multimeter connected to the wires of a disconnected fan, and using the vacuum cleaner to make it turn, it appears that there is a small amount of power being generated and sent into the wires. I tried with 3 different fans, same thing. I knew fans could be used as generators, but I didn't expect computer fans to be like that.

Why is that? And what happens when computer is shut down and the fan is still turning for a while?

Sorry for noob question!

Years ago I had a professor for an "Electric Machines" course who was fond of saying that he would give zero credit on test and homework problems where one made a sign error because the direction of power flow is broadly the difference between a motor and a generator.

I would expect that any power circuit intended to drive an electro-mechanical device to be tolerant of back EMF, because it's almost certainly going to happen whether or not you vacuum your computer.

I think the losses inherent in the wiring to/from the fan are enough to deal with the relatively slow speed of a fan being driven by a household vacuum compared to normal use. That's not even considering all the circuit protection that is probably on the board anyhow.

If you turn any motor, it will generate electricity. The relevant physics works both ways. Of course, a motor is optimized for turning when you power it, not generating electricity, so it doesn't work as well as a real generator might.

The tiny amount power generated by spinning the fans with a vacuum isn't anything to worry about. With any sort of load on the fan (like the rest of the computer - or if you repeated your test with a small resistor across it), the amount of power generated would look a lot smaller.

I've seen some reports of problems resulting from using canned air aggressively on fans and causing them to spin faster than they're supposed to, though I'm skeptical that these cases don't also involve a fan that was already near death - I've seen a lot of dying fans that were in working equipment and spun just fine there, but when you take them out, pull the dust off the blades and stuff, they don't work anymore - a dying fan can be pushed over the edge by small changes to the fan or it's surroundings that cause the shaft to sit differently in the terribly worn bearing. Those fans are really cheaply made most of the time.

guix: because there was most likely some protections inside the fan (such as a diode?), which disallowed any power to be sent back into the motherboard.

Nope, but your motherboard probably has such protection, that is if it's from a reputed manufacturer.

btw, have you seen this?

https://www.youtube.com/watch?v=2wQikiHF7WA

DrAzzy: If you turn any motor, it will generate electricity. The relevant physics works both ways. Of course, a motor is optimized for turning when you power it, not generating electricity, so it doesn't work as well as a real generator might.

Not any motor, a squirrel cage induction motor won't do it. It requires a permanent magnet.

Thanks all :)

Yes Noobian my PC is made from quality parts (including fans). I just wondered why the protection isn't inside the fan itself. Cost saving maybe? Anyway I rarely use a vacuum cleaner for this job. As I'm a smoker, I prefer to clean my fans with alcohol :)

guix: I was discussing with a friend and he told me than when you clean such fan with a vacuum cleaner, you should block the fan so it doesn't turn (which I never did), because it could damage the motherboard (which never happened to me). He couldn't explain to me because he isn't into electronics.

I think the usual cleaning method should be using a compressed air machine to blow air into heat sink fins - to get rid of any dust clogged up in the fins. A screwdriver shaft can be placed between fan blades to prevent the fan movement. I like to prevent the blades from spinning.....especially for laptop fans, to avoid over-speeding and messing up the fan bearings.

As for fan motors working as a generator..... you would expect a voltage to be generated if you spin the fan manually. But usually, the speed won't be fast enough or done for such a relatively long duration as to overheat components and burn things out. For a vacuum cleaner, the fan could spin extremely quickly, which could be an issue.

KeithRB: Not any motor, a squirrel cage induction motor won't do it. It requires a permanent magnet.

No, induction generators are a thing. They are basically the same as induction motors, just with a prime mover.

Link

Yes, but they require voltage to get started, which is not the case here.

guix: I just wondered why the protection isn't inside the fan itself. Cost saving maybe?

I don't think any fan has such a protection. Maybe they figured people might want to use their motor as a generator. :D

Prop-Forge: I think the losses inherent in the wiring to/from the fan are enough to deal with the relatively slow speed of a fan being driven by a household vacuum compared to normal use.

You clearly never vacuumed a computer fan :D They will turn faster then under normal operation easily! But yeah, I'm more concerned with the bearings then with the power generated :p

MrMark: Years ago I had a professor for an "Electric Machines" course who was fond of saying that he would give zero credit on test and homework problems where one made a sign error because the direction of power flow is broadly the difference between a motor and a generator.

Hehe, recognize that a bit. But if you notice the wrong sign it was fine to just say, "crap, that sign shouldn't be there because power flow/etc, just ignore it" and get full point :p Because it's so easy to mix up, maybe you used your left hand, maybe you forgot a electron is negative etc. Just flip it in the right direction :p

Hehe, recognize that a bit. But if you notice the wrong sign it was fine to just say, "crap, that sign shouldn't be there because power flow/etc, just ignore it" and get full point :p Because it's so easy to mix up, maybe you used your left hand, maybe you forgot a electron is negative etc. Just flip it in the right direction :p

Seems like a sloppy way to do things. I'd rather go back and find my mistake. Maybe it isn't a mistake?

If it's not a mistake then your explanation is wrong, A good/correct explanation why the sign should be wrong is a sign of insight. Same for order of magnitude. That's all that matters with hand calculations these days anyway :) No need to spend time debugging and rewriting everything because of a sign error. "Ain't nobody got time for that!" :D

guix: Hello :)

I have a question about DC fan, such as the ones inside a computer.

I was discussing with a friend and he told me than when you clean such fan with a vacuum cleaner, you should block the fan so it doesn't turn (which I never did), because it could damage the motherboard (which never happened to me). He couldn't explain to me because he isn't into electronics.

I replied that it didn't matter because there was most likely some protections inside the fan (such as a diode?), which disallowed any power to be sent back into the motherboard.

After testing with my multimeter connected to the wires of a disconnected fan, and using the vacuum cleaner to make it turn, it appears that there is a small amount of power being generated and sent into the wires. I tried with 3 different fans, same thing. I knew fans could be used as generators, but I didn't expect computer fans to be like that.

Why is that? And what happens when computer is shut down and the fan is still turning for a while?

Sorry for noob question!

With the fan unpowered the windings voltage is appearing on the drains of various switching MOSFETs which are off, so all will be OK until the fan is turning so fast the voltage rating of the MOSFETs is exceeeded and you blow the whole board... Some circuit topologies will mean the DC supply to the fan will rise when the fan is turned, some won't. You may also cause over-current damage to free-wheel diodes potentially.

Basically don't do it, it may destroy the fan (and possibly even the motherboard). You could also damage the motor mechanically, a vacuum cleaner on a high efficiency fan can cause enormous speeds, way beyond anything the fan was designed for.

If you do spin a fan up like that and put your finger in the way of the blades, expect to lose your fingertip too....