Electric Motor Failure Mode?

[Todd, KA1KAQ]

Once upon a time I knew the answer to this, but damned if I can recall now.

Thinking of your typical armature/brush-type electric motor as you'd find in a fan, or blower on a big transmitter: motor gets overloaded for whatever reason, heats up, stops working. Cooling it down results in the motor coming back to life, but after it runs for a while, it starts to slow down and eventually dies. Like something is saturating or weakened.

Somehow I thought this was related to overheating the windings of the armature, but can't recall the mechanism at work. Or is it a case of brushes burning off too much carbon and filling up the commutator?

[KA2DZT]

Most likely the bearings,  try some light oil or WD-40

[W3NE]

Hi Todd, Does the motor in question actually have brushes and an armature with a comutator?

Most fans and other constant-load devices powered by a-c not requiring high starting torque are driven by induction motors, which do not have a commutator and brushes. When higher starting torque is required on a-c, repulsion-induction motors are employed with a starting mechanism that permits high current during rotor run-up to near synchronous speed with high torque; after that, the motor runs as a plain induction motor with load current depending on "slip" from synchronous speed. A variation of that are split-phase motors employing a capacitor in starting and running. D-c motors can use brushes with an armature, or can be of the more modern "brushless" type.

If you are describing a fan motor that has simply stopped, it could be a bearing problem as suggested above, failure of its capacitor if it is a split-phase motor, or failure of the motor itself -- an open stator, for instance. More details of your situation would help analyze your particular problem.

Bob - NE
 

[kb3ouk]

What you describe with it stopping after running a while and then starting to work again after a cool down sounds like bearings to me.

[Todd, KA1KAQ]

I was thinking if it was the bearings, wouldn't the armature be seized or otherwise stuck? It's certainly possible that the lube was cooked out of them when the motor overheated.

Bob/NE: definitely has an armature, commutator, brushes, etc. Not a terribly high quality motor. Re-purposed from its original use and riveted together, though the brushes are accessible for changing.

Basically it will start running fine but within a minute or so it begins to slow down until it stops, like it goes weak. Let it cool off and it repeats.

When I get a chance to drill out the rivets and pop the covers, I'll give the bearings a shot of lube and try again.

TNX all.

[KA2DZT]

Todd

Just spray a half can of WD-40 in any opening on the motor in the morning and the other half can in the evening.  The motor should run top speed.

Just another helpful hint from a Forum expert.

[K7LYF]

Sounds like typical cheap electric motor with bronze bushings......grease is what is needed, not WD40. But then again the armature could be rubbing the field coils  because the bushings are worn beyond repair.

[KA2DZT]

Sounds like typical cheap electric motor with bronze bushings......grease is what is needed, not WD40. But then again the armature could be rubbing the field coils  because the bushings are worn beyond repair.

Usually light oil instead of grease.  I just fixed a sander that had worn bronze bushings.  Motor would run OK for a short time then slow down to a stop.  Replaced the bushings with ball bearings, motor runs perfect.

Fred

[Opcom]

But then again the armature could be rubbing the field coils  because the bushings are worn beyond repair.

the true death of many household induction motors.

[WA2SQQ]

Some years ago at a trade show I spoke with a WD-40 rep. He told me that WD-40 is technically not a lubricant, in fact it dissolves oil that has thickened with age. When applied, it will give you the illusion that it's lubricating, but after a short time the device will start acting as if no lubricant is present. Some time later I took my car into the dealer complaining that the door lock was very stiff. The mechanic came out and told me, "you used WD-40 on your locks, right?" I did! He also explained that it washed away the lubrication. A shot of some graphite fixed the lock.

WD-40 has many uses, but not as a lubricant.

[WD8BIL]

I agree.......... clean and relube the bearing/bushings.

[W3RSW]

Yes, WD 40 is "water dispersant 40." IFIRC concocted in a Navy lab.
   Not a lube.  This has been discussed several times on AM fone over the years.

I've taken the label off Rotron Fans and the like to find hidden screws underneath allowing complete re-oiling of the bearing felts. You can buy sewing machine oil sized tins labeled as "3 in 1" light machine motor oil, 20 wt. SAE sold primarily for furnace blower motors and the like, " specially blended for 1/4 HP motors and larger." This blend won't evaporate under heat loads as quickly as sewing machine oil.

1/10 horsepower motors and up, AC synchronous or DC brush jobs also may have thermal breakers imbedded near the core. These motors also start up again after cooling off from overload.

[flintstone mop]

I think the noisy cooling fan in the T368 was a brush driven motor and I replaced that with a nice Dayton motor.

[Todd, KA1KAQ]

This is a cheapy motor with bushings, not bearings. You've convinced me that it's really not worth the effort to drill out the rivets and try or hope to repair. Easier and cheaper to just replace it.

WD-40 was originally developed by a company out on the left coast (or thereabouts) to combat rust and corrosion on Atlas missiles sitting in their silos. Water Displacement, 40th formula attempt which finally worked. Something like that. I use it a lot for cleaning gears or surfaces needing to be re-lubed. It's great for things like a grubby R-390A gear train to get the crud off and get things moving again. It will offer some lubrication by virtue of its presence (something vs nothing or crud), but is not a long term solution. I buy it in the big can size and use it for hosing things off when working on engines, etc.

[flintstone mop]

Smells nice too
Good for cleaning, de greasing... A quick lube job. Not permanent.

[WB6NVH]

Apologies for getting a little off topic but instead of 3 in 1 oil, I have recently switched to synthetic automatic transmission fluid, which is quite a bit cheaper and I am hoping will never turn into sludge or attract dirt.  Seems fine so far in motors that were otherwise OK and on R-390A bushings.

I have had pretty bad luck dealing with lubing motors that are simply worn out at the bushings.  They seem to run fine again after lubrication, for awhile, but then I am back at square one before too long.