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My air purifier fans won't turn anymore, making humming noise

Discussion in 'General Electronics Discussion' started by computer-joe, Mar 1, 2020.

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  1. computer-joe


    Oct 6, 2016
    I have a small air purifier, this morning it stopped working. The fans won't turn on any settings, in the highest setting it turns a bit then stops.

    I took it apart and cleaned it but same problem. I noticed the black middle part is super hot. Probably a fire hazard. So i stopped using it.

    I wanted to fix it but not sure what's wrong. Anyone have any ideas? Here are some pictures



    and here is the black thing that gets super hot, almost burning(the black thing with an arrow)
  2. Alec_t


    Jul 7, 2015
    I'd guess a fan shaft bearing has seized. The black thing looks like a drive belt, so if that's not moving but the motor still is, then friction will make the belt hot.
  3. Hunter64


    Nov 20, 2018
    The 'black thing' looks like the motor.

    davenn likes this.
  4. Bluejets


    Oct 5, 2014
    As shown above, the shaded pole motor has oilite bushes which dry out and cause the shaft to seize.
    Remedy is to disassemble the motor taking care to introduce some light machine oil so as not to damage the bearing.
    Clean the shafts both ends with fine (400 grit) wet or dry sandpaper, oil up the bearings and the internal felt oil reservoirs with (for example) SAE 40 engine oil and reassemble.
    The bearings are self centreing as it reassembles but do not loose the thrust washers on the internal ends of the rotor.

    It will be no good to simply try to introduce oil without disassembly as it will simply seize again.
    The shafts must be cleaned of debris.

    Might pay to note that these motors do run hot, fry an egg hot if you need a comparison.
    Certainly does not help the heat situation if the fans refuses to run or runs slowly.
  5. dave9


    Mar 5, 2017
    I've not noticed much difference between just oiling them and further disassembly to clean before lubing. The main difference was that they get oiled before the bushings get that far chewed up so they don't have so much play in them that the shaft wobbles and pumps the lube out faster than (most) people would want to keep relubricating them.

    If you catch them earlier in their wear then you can often get much longer intervals between relubrication, but at this point I suspect your best bet is go ahead and lube it while it's taken apart but plan on getting a replacement air cleaner, then if it also has a shaded pole motor (as this type tends to have poor/cheap/shorter-lifespan bushings), remember to lube it before it gets dry enough to chew up the bearing.

    I suppose you could look around for a replacement motor, but something with the shaft extending out both ends to drive the twin squirrel cage fans is far less common. You might contact the manufacturer and see if they offer one but I doubt it.

    Frankly I hate to buy any consumer grade products with a shaded pole motor if meant for continuous duty. They don't just wear out before I'm done using the product, but also waste power the whole time. Maybe that's reasonable to arrive at a $15 USD price point table fan but not so much a > $100 air cleaner.

    Of course you can relube them, and I have, but i have more valuable things to do maintenance on, with what time I'm willing to devote to that.
    Last edited: Mar 2, 2020
  6. Bluejets


    Oct 5, 2014
    If the shaft is not cleaned the dry debris will pick up when the oil hits it and it'll lock up again.
    If you don't want oilite bearing type motor, replace with ball bearing type.
    I have never really found it necessary though.

    Waste power...??? it's a 15w motor ....geeess....!!
    Get real.
  7. dave9


    Mar 5, 2017
    ^ I've never had that happen to any I've lubed w/o cleaning. I kinda, woulda, noticed that.

    The issue is not just bearing type, rather that these cheap little shaded pole motors have such a small shaft diameter and bushings, with small oil reservoirs too. Further in an effort to be even cheaper in the device BOM, they often don't balance the load (fans in this case) so the wear on the bushing is even worse.

    A larger PSC motor, with a larger shaft, larger bushing, will tend to last multiple times longer. If the motor cost increase were a substantial % of the product price, then maybe it's not worth it, but when a whole $100+ product is trash because of it, not so much.

    You may feel the power is not much but let's do the math. A PSC motor can be twice as efficient (which isn't saying much, just that shaded pole are such low efficiency), so instead of 15W continuous, it's 7.5W. The warranty on that air cleaner was 3 years so let's just use this fairly short lifespan as the benchmark... that would be about 200KWH difference over 3 years. The US national average electricity cost is about $0.13/KWH in the US, so just under the warranty period, I could save about $26, more than the cost difference of a PSC fan motor, and still have a working widget because the motor still works, instead of having to buy a new one, so that many more years of energy savings too.

    There are many 1W, 3W, sometimes even 5W widgets I leave plugged in drawing power, but it does all add up. I'll gladly pay more for a product if it pays for itself in power savings over a mere 2-3 years span without any performance penalty.
    Harald Kapp likes this.
  8. duke37


    Jan 9, 2011
    I used to service tape players for the blind and saw a lot of these motors. If found early, they could be cleaned and relubricated but if the shaft was worn, they will soon fail again.
    I used various lubricants and settled on engine oil, I have often used a drop from the end of my dipstick to get the motor going without taking it home.
    I enquired as to what was the best lubricant was from the lubricant experts at work. They gave me a little jar of Chromatap but this did not improve things.
    The motor had a closed end bearing and an open bearing through which the shaft passed. It was always the open bearing which caused trouble.
    As said, these motors run hot.
  9. computer-joe


    Oct 6, 2016

    i was bored and decided to take a look at it again,
    i ended up putting oil where the rods sticks out! and boom! it started spinning again!

    its been working great all week now, and doesnt get hot anymore

    thanks everyone for the help
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