I have a Vornado table fan, about 14" across, in my bedroom that I run at night. It has three speeds, and I might use any one of them depending on conditions, and I might change from one to another during the night. It is probably around 8 years old, and it has always worked fine, up to now. I could afford to buy a new fan, but that’s not the point. I just want to know why it is behaving this way, and since I am ignorant of the workings of electric motors, I thought I would consult the experts on SDMB.
In the past couple of weeks, when I hit the On switch, it sort of hums for about three minutes (timed, but not with a stopwatch) before the blades start rotating. After that, it works fine for the rest of the night. If I turn it off after it has started up, and then turn it back on, it starts up right away. The “hum” time hasn’t changed since it first happened, as far as I can tell.
I’ve seen this when the bearings are getting dried-out. The motor barely has enough torque to spin the fan blades, but eventually the bearing warms up and becomes looser, and it can spin it pretty fast. Also, some motors have more torque the faster they spin (up to some maximum), so once it starts spinning faster, it tends to accelerate.
Probably the motor shaft has become dirty with dust and grime. Eventually the motor warms up and softens the grime allowing the shaft to begin spinning. In some fans there is a start up capacitor which may be failing, but generally the fan won’t start rotating at all if that is a problem. If you turn off the fan and it starts up again a few seconds later without a problem then you are looking at dirty shaft. Sometimes it can be cleaned with compressed air or vacuumed out, or disassembled and cleaned. Sometimes it’s not worth bothering with.
I think the dirty motor shaft is probably it, because (and I purposely withheld this bit of information) a few days before this started to happen, my husband took the covers off the fan to clean off several years’ worth of dust that had collected on it. Quite possibly enough of that dust made its way onto the motor shaft to make this happen.
So it may not be able to clean the shaft, but at least I know where to start trying. Thank to TriPolar, and beowulff in case it turns out to be the bearings (which I’m pretty sure would mean a new fan).
This. Cheap/small induction motors (like those used on portable fans) are made with Oilite bushings on the shaft and usually have small felt-pad reservoirs soaked with oil at the factory. These reservoirs keep the OIlite bushings properly lubricated for a long time, but not forever; eventually they dry out.
Running the fan for a long time with underlubrication will wear the bushings, making them noisier over time. In extreme cases, the slop in the bearing can be so bad that the shaft rolls around inside the bushing instead of sliding, a loud vibratory condition called whirling.
You can try saving this fan by taking those felt-pad reservoirs off of the ends of the motor and putting oil in them. Common automotive motor oil will work just fine. You can soak the pads good for long life, but not so much that the oil oozes out onto the floor later.
It’s dumping heat into the motor housing this entire time. If it gets hot enough, this could be a fire hazard. Some motors have a thermal fuse that cuts power if things get too hot, but it’s not something you want to deliberately count on. For your safety, you should fix or replace this fan ASAP.
Yeah, don’t do this. Seriously. Replace the fan. NOW.
Speaking as an electrical engineer, never let a fan (or most other motors for that matter) just sit there and hum like that for more than a few seconds. We call that a “locked rotor” condition (because the moving part, aka the rotor, is locked in place and can’t move), which also happens to be the condition where the most current is going through the motor windings. Some motors are designed to handle the locked rotor condition, but many motors will overheat and fail.
Note that fail in this case can involve the fan motor bursting into flames.
Is there any reason I shouldn’t try to fix the fan before throwing it out, if in fact the problem is either a dirty rotor shaft or dried out bearings? If I can remove the cause of the locked rotor condition, doesn’t that alleviate the danger of overheating?
I’m leaning towards dirty rotor shaft, by the way, because of the timing.
Sure, you can try to fix it.
Lubing the bearings can ameliorate the problem for awhile (years, sometimes), but it will eventually come back. And, BTW - a “dirty shaft” is a bearing issue - the dirt on the shaft itself (the part you can see) is of no consequence. It’s the dirt inside the bearing that is causing the issue.
Yes, try to fix it. Beowulff and I were referring to the same problem, the shaft spins in a bearing, Dirt and dust can clog that up and ruin the bearing. But as suggested above also, a little bit of lubricant may make it work again. You have to keep an eye on it after that, but sometimes a bit of hair wraps around the shaft and if you can pull that out or it disintegrates it may run like new again for years, but in my experience once clogged it’s got a limited life span.
However, I don’t think fans like that cost very much, a replacement is the simplest solution. You wouldn’t want to leave that fan running unattended for very long even if you do fix it. The chance of it starting a fire is extremely low but not at all non-existent.
Also you pointed out that cleaning may have caused the problem, sadly true, although maybe only because it was pretty dirty to start with which prompted the cleaning, but maybe about to have a problem anyway.
Not that I’m encouraging people to let jammed motors sit and hum indefinitely, but current draw on a shaded-pole induction motor (the kind used in almost every small fan) is not much more than normal running current, so there’s little if any risk of catastrophic failure.
Universal and split-phase motors, on the other hand, will happily release their magic smoke if left in a locked-rotor state.
