Wednesday, August 1, 2012

12 V fans at 24 volts (overvolt 12V fan).

(dated 1st aug, 2012)
I'm talking about the average 12 V fans you'd find at $1; it appears their reliability is pretty good, and looking at their MTBF (4 years of continuous operation), it's desirable to reduce their life with expense of higher airflow.

So I over-volted 2 of these fans to 24 volts and another one (which I dumped a few days ago cause it's bearings were problematic) to 20 volts. Notice, you can't take these fans to 30V, they burn out. So below 30V seems to be safe.

The 20 v one is working continuously for 3 or 4 days, with no signs of wear and tear (despite the fact that the fan's bearings are at a bad shape), and the 24 V ones is used atleast 8 hours a day, and is working since 2 days.

All of these are working fine till now. Notice that people claimed, at these volts, these fans won't last more than a few hours, but their reliability seems good till now, many times their expectations.

I tend to think the first thing to come off are the bearings (being mechanical components), since I don't think the coil of the motor gets too hot to burn the insulation of the copper wires of the windings, although burning of the coil is the second possibility cause the insulating layer degrades over time.

Burning of the controller is ruled out completely cause they were ok even after long operations.

As of the performance, you should realize that the power delivered to the fans is not doubled, should increase 4 folds, cause with the volts, the amps also increases. This means 4 time the air flow, and 4 times the RPM (read fan laws). I measured the power consumption of one of these fans.

To experiment, I attached 2 fans facing (the output of one fan is fed into the output of another) each other and sealed together with insulating tape. When one fans runs, cause of it's airflow, the blades of the other fan will also run; the other fan will also generate an EMF. I'll measure that. It's a measure of RPM.
At 12V, the other fan generated 0.45V, and at 20V, the other fan was at 1.59V, that's more than 3.5 times. Expect 4x at 24V.

At these volts, these fans make a lot of noise cause of air motion, the motor itself doesn't make much any noise. The air flow is so strong that you can feel the suction of the fan from quiet a distance, like 2 or 3 inches. Come up close, and it sounds a bit like a vacuum cleaner.

As of the wiring in my setup I used an external 12V 500 mA adapter clubbed with the SMPS's 12 v source from the molex connectors for the purpose (2 fans).

Don't try to club 2 of the 12 V outlets of the different molex connectors. They come from the same source which will result in a short when positive of one molex meets wit the negative of the other.

Maybe this setup has more value for money than the deltas and 120 mm fans.

Update (Aug 5th 2012) --one of the fans (running at 20 V) broke. My dog bit it, shattering half of it's blades. It ran for ~9 days straight with practically 100% uptime. The same day I bought a new fan with the same specifications.

On night of 7th Aug, the new fan died. It was working continuously for 2 days. It didn't get burnt in any way, but there was a strange problem with it. After giving it power (12 or 24 V), the the fan will rotate for a few minutes, then it'll stop. On manually rotating the blades, it behaved like a spring, or like a very powerful motor with TREMENDOUS torque (and powerful permanent magnets) which's offline.

I've decided to replace the fan with a home-made one. The Q/A of the fan was questionable from the start. It was moving slowly.

Also note that rest of the 2 24 V fans still work

Update -- one of the 2 fans was replaced today by me (on oct 13th 2012). Problems came up with the bearings; it was making too much noise.
But it's to be noted that this problem started before I hiked up the voltage to 12 V; it was showing symptoms of failure.

Update -- Again the new fan that I replaced above gave the same bearing problem; I've replaced it with a new one on 27th feb 2013. However this time the fan was not at 24V all the time cause of cold climate.

Update -- I realized these sleeve bearing fans can be lubricated (search google). That'll solve the bearing problems. However if you ignore the noise for a long time, the fan may incur permanent damage. After this, the fan has practically unlimited life until it's winding burns.

Unlike what Google articles suggest, I'll ask you to fill the oil almost to the brim (but not so much such that when you close it with tape the oil will stick to that tape) and seal it off with a small portion of an insulating tape.

Cut out the tape such that it just covers the hole (keep a little bit of margin), and apply superglue around the border of the tape. Just put a small drop of suerglue at 1 corner of the tape and try to spread it all around the tape to form an airtight seal. If you apply less superglude, it'll be easier to open up the hole again for oiling.

The reason for the complications as above and the reason for using insulating tape specifically is that, these sleeve bearing fans tend to heat up at the bearings and the coil also heats up. Then the oil and air in the hole will heat up and will put pressure on the sealing of the hole. So if you use tape without superglue, the oil will leak out cause the adhesive of the tape is not strong enough to contain it.

If you use a rigid material to seal the hole (insulating tape expands under pressure), like straight and rigid plastic sheets, the pressure inside the hole will increase to a very large amount until the sealing between the shaft and cylinder of the sleeve bearing breaks and the oil leaks out from the other side (this will not damage the fan though). Insulating tape on the other hand expands to compensate for the high pressure, preventing such a thing from happening.

The fan heats much more when you're overvolting it.

The large amount of oil will act as a reservoir and compensates for the evaporating oil. The sleeve and shaft is an air-tight seal (like piston/cylinder) and wont leak oil unless something's wrong with the fan; you can figure that out if the oil disappears from the reservoir the moment you put oil in it (or after a few minutes).

Practically I've seen a suction being formed instead of high pressure in the oil reservoir, that depresses the tape over the hole. The low pressure will be there until all the oil is used up, and then high pressure will develop bulging the tape. After this, within a few days (or maybe months), the fan will start making noises again.

The problem with oil is that it evaporates quick; so grease will be good, but the problem with grease is that it doesn't flow -- so it wont go down the bearing from the reservoir. So what can you do is mix them both and you get the best of both.

Since grease has an oil base, machine oil forms a homogeneous mixture with it. Thin the grease this way till it form a thick mixture which flows easily, then fill it in the oil reservoir.

This mixture lasts much longer (4 to 5 times more as per my stress tests, but in real life, it should be much more durable) and the fan moves MUCH faster cause of reduced friction.

However there are downsides. Once the mixture is used up, there maybe carbon deposits on the shaft which will prevent new lube from going in and at the same thing slow then fan down to a crawl. In this case, you've to actually open the fan up to clean the shaft and sleeve. And before you put it back, you may like to grease the shaft.

See how to disassemble the fan --

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