Motor shaft locks only with power applied

[MrAl]

Hi,

I have a small 120vac house fan motor i was working on and all of a sudden it started acting very strange.

Without any power applied, the shaft turns freely by hand. When power is applied, the motor makes a noticeable hum and the shaft locks tight so it's very hard to spin by hand.

Nothing in the way of the armature or bearings, and it does turn by hand without power applied.

Anyone ever see this before?

 

[crutschow]

How were you "working" on it?

Is sounds like there may be a stray piece of loose magnetic material inside the motor.

 

[Mr RB]

If it's a shaded pole multispeed house fan then one of the windings is probably shorted. If you are lucky it might be something shorted in the multi speed switch (ie something you can fix).

 

[MrAl]

Hi,

Yes it is a multi speed motor, and it happened as if for no reason at all. I had replaced the thermal fuse, it worked although i had to push start it, then it didnt work at all because it just locked up once power was applied. I checked wires and they all look good.

The switch is just a multi position single pole switch. I might be able to check that by bypassing it. I'll try that today sometime. Thanks for the idea.
I checked the 'run' capacitor and it looks good at the required capacitance within close tolerance. Besides if that was it it would have had to have blown right as i was screwing the last screw in (that's just after the last time it ran).

 

[Mr RB]

They have very little startup torque. I have one here where the bearing is a bit stiff, and it turns "freely" enough by hand but wont start at all on the low speed setting.

However it starts fine on the max speed setting, so I start it like that, then when up to speed I can switch it back to the low setting and it keeps running. You could try that? Or even try to give it a bit of a "spin" to start it up (be careful!).

You also didn't say about what work you did on it, but if you used a heavy lube on the bearings like grease (or washed all the original lube out) that may be enough to cause a startup fail. I use a Lanox spray lubricant but light oil like sewing machine oil will work fine too.

 

[MrAl]

Hi Mr RB,


Well, i actually had that problem many times in the past with different fans. What has caused that kind of behavior on all of them was dirty sleeve bearings. What i did was took the whole motor apart by removing the armature, stuck a dowel inside the bearing and twisted it back and forth to get the dried gunk off the sleeve, then cleaned the shaft removing anything until it was shiny again. Then applied motor oil and it worked fine for another year. I did this with several motors and they all worked again no problem.

But this one is totally different. The shaft turns very nicely by hand. Then when the power is applied, it's like it 'grabbed' the shaft and wont let it turn anymore rather than make it turn as it should. It seems like a big magnet suddenly holds it so it wont rotate, and it's very very hard to turn by hand after that. Power removed, it turns by hand again. Strangest thing i've ever seen.

The ones that need a 'push' starts as you were saying, they all needed cleaning and oiling of the bearings, but this one is very different.
See what i mean now?

 

[gabeNC]

I have that same problem with one of my wood burning stove fans. I took it apart to take replace the sleeves with bearings but must have messed up something. It acts like an electro-magnet. DMM checks on the motor seem fine, not open, not shorted, the readings match the good motor.

Here's the link from my issue last year

https://www.electro-tech-online.com/threads/a-c-blower-motor-for-fireplace-dead.113729/

Interesting timing to your thread, just this afternoon I pulled the good fan and cleaned it all out with compressed air. I usually put some oil in there but it never really seemed to quieten the noisy sleeves. So this time I put brake caliber grease on the shaft and it is VERY quiet, almost sounds like a new fan.

 

[Mr RB]

...
But this one is totally different. The shaft turns very nicely by hand. Then when the power is applied, it's like it 'grabbed' the shaft ...
The ones that need a 'push' starts as you were saying, they all needed cleaning and oiling of the bearings, but this one is very different.
See what i mean now?

Yeah that's what I thought you meant but I threw some extra info in the pot just in case.

It sounds like a shorted winding or some electrical fault.

Does it do the same fault on all the different speed settings?

 

[MrAl]

Hi guys,

gabeNC:
Oh ok i'll have to review that thread. This is the strangest problem i've seen with a motor so far, and i've taken apart many of these things in the past and they always worked find when put back together after the cleaning/oiling. BTW i find that grease doesnt work as good as motor oil.

MrRB:
Oh yeah that's cool too. Nice to gather all the information i can get about these things. I just feel that something very simple is wrong. I wonder if the armature or metal laminations could have become magnetized or something like that. I'll have to see if it acts like a big magnet.
Yes, same on all speed settings (3 settings: low, med and high).
I checked the capacitor and it reads 4.1uf when it is marked "4uf" on the case. Could still have internal short i guess, but what are the chances that would happen just when i finished putting the motor back together (tightened the last screw).
Maybe the cap shorts out when higher voltage is applied? I guess i could check it better. After all something did cause the thermal fuse to blow out originally, although that happened to this same fan several years ago and replacing it fixed it for quite a few years.

 

[MrAl]

UPDATE:


I got the motor to run but now it runs too slow, and after only about 30 seconds it will stop.
Strange huh?

I tried it with the cap disconnected and found that the shaft locks up when power is applied just like it was doing before. With the cap in place, it runs as above.
I guess i really have to try another cap next to be sure it isnt the cap.

BTW, this cap is a "motor run" cap, not a "motor start" cap. That means it is in use all the time providing a phase shift for one of the windings so it can run normally.

 

[Mr RB]

Cool, please keep us informed. It would be good to hear the final analysis.

 

[alec_t]

I tried it with the cap disconnected and found that the shaft locks up when power is applied just like it was doing before. With the cap in place, it runs as above.

My money's on it being a faulty cap. High ESR ? Or the cap terminal may have gone high resistance inside?

 

[MrAl]

Hi MrRB and alec,

Sure, i'll keep the thread updated as soon as i find out new information even if i find more on the web or experimentally.
I'm more interested in getting it going now just to be able to find out what is going on, more than i actually need the fan
My curiosity got the better of me here i guess, but if i get the fan running again that's great too.

alec, faulty cap? Yeah it could be and im hoping that's it because i ran out of ideas. Also, i didnt actually test the ESR, only the capacitance, and so far only at low voltage (like 3v). I should have ran it up with a resistor and 120vac to see if the cap acts like it should. I have the new cap on order now already though so i'll probably just wait for that to get here.

BTW, the only explanation i have for it running "a little" now is that maybe one of the internal connections was shorted to the metal motor case. That's the only thing i can think of because i didnt change anything else before it started to run a little and not simply lock up completely like it was doing. I added a little black tape to one of the connections and after that it started to run a little as i described before (where it slows down and stops after about 30 seconds).

 

[MrAl]

Hi again,


Little update...

I got the new cap, installed it, no help. Thumbs down. After looking at it a little more and seeing how it is sluggish to start at all, i have to come to the conclusion that the bearings are at fault here. Maybe one, maybe both. They are sleeve bearings.

BTW, i've also come to the conclusion that the shaft isnt really locking up. It's just trying to start, so it appears locked. Now that it's turning at least slowly (sluggish), i can see that the degree of sluggishness depends on how the four case screws are tightened to some extent. That tells me that the bearings are shot because the slightest change in angle between case and shaft and it changes the friction coefficient so the fan can go from totally not rotating to rotating slowly depending on the angle. My guess is that as the fan starts slowly, the bearings heat up and expand a little and so clamp down on the shaft little by little, which means the motor eventually stops dead again, and also wont start again at all (even slowly) until the unit is left to cool for a few minutes or so. If i can find my freeze spray i can try to cool them off one at a time and see if the fan takes off again once it slows down.

This is a good quality fan. Anyone ever replace the bearings in their fan?

 

[gabeNC]

I tried. It was just a sleeve, no bearings so I drilled out the rivets and took the whole assembly to the local bearing store. That's when I ran into my problem when putting it back together. It was an excuse to buy a rivet gun.

 

[Mr RB]

Usually they use bronze sleeve bearings, which are crimped into the pressed steel ends of the motor. Cheap and nasty.

I only use a light oil, something as heavy as engine oil might cause it to fail at startup. Try spraying some WD40 through the bearings to wash out any heavier oil. If you overtighten the screws it can warp the flimsy motor pressed metal ends, so you could try loosening off the screws some more even if you need to put a touch of locktite on them and leave them less than snug.

One of my home fans here I have cut a hole in the plastic case, so I can squirt WD40 direct on the rear bearing without needing to pull the cover off. Actually I use Inox or Lanox, similar to WD but without that awful Kero smell being blown around by the fan.

 

[BobW]

When these motors are powered up, the rotors will want to centre themselves axially relative to the stator. If something has happened to displace the bearings, or thrust washer (or maybe a damaged thrust washer), the axial shift of the rotor could cause it to bind.

 

[MrAl]

Hi again,

MrRB:
I had read that WD40 isnt good for lubrication. You've found it to work well though?
I've used motor oil on my fans for quite a few years. I think it was 30 weight. I had a fan similar to this one years ago that needed taking apart and cleaning once per year, and re-oiling. After the re-oiling however it always worked very well.
I didnt do that with this fan, instead i used light machine oil.

Another little update:
I've determined that it cant be the bearings heating up (although they might still be bad). Here's the test i did...

1. With the motor off using a screwdriver i gave the blades a little push and they spin around maybe one time or so. Did the same with the screwdriver a few more times just to get a feel for how long the blades spin with a little push, trying to push the same each time.
2. Start up the motor, wait till it spins up, then wait till it slows down to a crawl. Turn off the motor and quickly spin the blades with the screwdriver again using the same amount of push force. The result is the same as before, the blades spin the same with the push. This says that bearings might not be heating, but one more test is required which follows.
3. Immediately start the motor again, and note that the spin does not spin up like it did the first time it was turned on.

The above shows that it's not the bearings heating because they were not truly affected by the energized spin. The motor shaft only slows down when it is powered by the incoming AC line voltage 120vac. Spinning by hand always makes the blades turn the same, regardless of how long the motor has been previously running.

This new test tells me it might be the windings heating up. When the winding heats up the resistance increases and thus the motor slows down.
Still, it could be that the bearings are the cause, making the windings heat up. So it is not the bearings "heating up", just possibly that they are causing too much drag. Maybe one of them warped or something.

I guess the other possibility is that one or more of the windings increased in resistance and stayed a little high. I dont have any other fan exactly like this one to test this idea with however.

 

[MrAl]

Hi again,


Ok it appears that the defect is more apparent now. There's either a partial short or a partial open in the winding.

Without the fan blade attached i was able to keep the motor running for several minutes. In the dark, i could look into the side of the motor and see the windings arcing in one spot. Taking it apart again, i could not see any burnt wiring or anything else for that matter, so it could be a partial short or partial open deeper inside the windings. Only way to repair that would be to take the windings off, and because of the complex shape of the stator core i would not want to get into doing that. Last motor i had a running problem with i was able to take apart and remove part of the windings and still get it to work for several years after that, but that was a very simply wound coil. The winding inside the motor i was looking at here is very complex, threading in and out of the core, and also it is kept in place with special cord that has been saturated with either varnish or some kind of glue, so it's a cord threading around the windings but it's hard like plastic now because of course it dried long ago. That would be so hard to remove (and there is a lot of it too) eventually i would cut one of the windings and have to repair that too. It's just not looking like this repair is worthwhile anymore, apart from changing out the whole motor or core or something like that, which i doubt i would go through the expense of doing either.
Thanks for all the ideas.

Looks like a new fan will be needed next year

 

[Mr RB]

...
I had read that WD40 isnt good for lubrication. You've found it to work well though?
I've used motor oil on my fans for quite a few years. I think it was 30 weight. I had a fan similar to this one years ago that needed taking apart and cleaning once per year, and re-oiling. After the re-oiling however it always worked very well.
I didnt do that with this fan, instead i used light machine oil.
...

As far as I know WD40 is just a light machine oil with a kerosene carrier. The kero dries out fairly quickly leaving some light oil in the bearing.

It's not a great bearing lubricant for sure, but the fan I use it on has a tight bearing and it needs the light oil regularly applied, and to wash out metal gunk. I used to strip and clean it, now I just spray. Recently I've been using the Lanox which has a light lanolin grease in it compared to just the light oil in the other product Inox.

Anyway congrats (I guess) on finding the fault in the windings. Once they have been arcing a while there will be carbon in there, so even trying to soak some varnish in is unlikely to fix it.