I have a 12V portable box fan that is acting funny this spring. It will run fine for a few seconds, then slow to almost a stop for a few seconds, then pick up speed again. It does this in both high/low speed with the slide switch, and both battery/wall transformer power. I have found than when the motor slows the voltage to it drops and the two resistors in the circuit become hot. The wall transformer also becomes hotter than normal. My only guess is that there is some sort of short in the motor even though it isn't getting hot. It also turns freely by hand so it isnt seizing. All connections seem good. Any ideas?

 

I'm not entirely sure but have you tried using a different power supply?

 

Maybe some dirty bearings getting hot?

 

Maybe some dirty bearings getting hot?

Two votes for this.

"Shorts" generally stay shorted. Hot resistors say the supply is still supplying power but the fan is drawing extra current.

Does the fan also make a funny noise. Doesn't always do this when the bearing are a problem, but worn or un-lubricated bearings can "chatter" when the shaft wobbles around the bearing.

It is probably easiest to just try to get some lubricant on the bearings. Use a drop of something like motor oil on the end of a toothpick or something, don't just spray it with WD40. Generally, one bearing is easy to get to, behind a label on the back side of the fan. One of Murphy's Laws says that the bad bearing will usually be the one that is almost impossible to get to. Depending on the fan, you can try to take the blade off to get to it, but at that point, you should probably plan on having to buy a new fan.

 

The motor does not make any noise or give indication of bad bearings and every power supply gives the same results. It appears that it is just simply dieing. What is confusing is why the speed is varying randomly. I would think that its resistance would increase and it would run really slow or not at all. Anyhow, thanks for your help.

 

Cannot be excess of oil.

 

If your going to oil the bearings don't use motor oil. Use light machine oil like 3 in 1.

 

If your going to oil the bearings don't use motor oil. Use light machine oil like 3 in 1.

If it were a new bearing, I'd agree. But it's not new, and there is now a large clearance between the bearing and shaft. If it wasn't worn, you probably wouldn't need to be lubricating.

When your old truck bearings start rattling, do you put in 20 Weight oil? No... you fill it with 50 Weight to get a few more miles! The fan doesn't even have a pressurized lube system.

Light oil will still allow the shaft to rattle, there will just be a single point contact, and it will all run out in a few minutes and you'll have to do it again. A heavier oil will fill the gap and the bearing (actually probably a bushing) will work the way it is supposed to for awhile longer.

If it were my fan (like it has often been), I might even use a grease depending on the size of the bushing, but you might have to disassemble the fan to get grease to where it is needed. Even a heavy oil will flow into the gap even while still cold.

I'll still go with... use a heavier weight oil on worn bearings/bushings, motor oil over one of the light oils. Just my opinion.

 

I would also recommend against 3-in-1 oil. The vegetable component tends to turn to varnish and gum everything up. Any kind of mineral based oil - gun, machine, or motor - will do better.

 

3 in 1 is not vegetable oil. It's light machine oil.

 

Check its brushes.