(hvac blower) Motor trouble suspects

nutnspecial

nutnspecial

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I have installed a few small forced air-furnaces and am fairly aware of the common problems/maintanence, but my current issue is totally new.

5yo furnace, and it kinda vibrates from the blower motor /squirrel cage. I pulled the assembly out and it's clean and fairly straight/balanced.

I guess it's an ecm motor- it looks like there's one neutral and a few (4) hot (+) wires for various speeds or whatever. Then there's two for the capacitor.

When I then energized it, a shudder or miss/skip became very evident as it winds up and seems to be the issue of the shudder at full rpm, transmitting thru the furnace.

I don't see any brushes, but it almost seems like if it's only 'running on one cylinder', like if one brush was screwed up etc. I didn't think it would be the capacitor but maybe it is?

I did google some stuff most 'vibrations' etc seem to be caused by dirt or messed up / imbalanced squirrel cages, which this def doesn't seem to be.

Just wondering if anyone knows anything about this type of motor that could help. Thanks for reading!
 
The two types of motors I've seen used in HVAC are

--induction motors, with capacitor start/run, and caps go bad over time. If you have another cap you can just swap it in and test it. (like out of another HVAC unit, maybe even just an old refrigerator, freezer, window AC unit, etc off the compressor).

--BLDC motors, with integrated controllers. Some of these have quite a few wires coming from them for control/power, like the one in my present AC system--they've had to replace it once already, as the controller section failed (motor was fine); I forget the symptoms. If it hadn't been under warranty and thru the landlord otherwise, I'd probably have tried using an ebike controller on it. :)
 
what color wire for what speed. Try a slower speed?
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I'm not sure what this would rule out or highlight- (perhaps the 'EC' -electronic control?). Totally worth a shot.


--induction motors, with capacitor start/run
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I guess that's what this is. I also am guessing induction is 'brushless' because of the different operating principle or whatever. Anyway, I will totally also swap over a known good cap from an identical (but slightly older) furnace. Or maybe buy a new one for 15$?
I should probably also read up on 'induction' type motors if that's what alot of our ac motors are.




It 'fires' up and runs wot just like an old car that has dropped a cylinder, except it's more like a 50% miss than 1 in 4 or 1 in 6. Thanks guys I'll post the outcome!
 
An update- I swapped the motor out for another I had. It's slightly more hp and wound a little faster according to the ohmeter but works properly.

The original problem is still perplexing/unsolved. I ruled out the start/run cap, and all the speeds do the exact same thing. being it's a single phase 120v motor, I don't know what could be wrong with it electrically given the clues. I pulled the covers off and one bearing is just a bit worn and loose, so although i wouldn't expect these symptoms for a bearing, maybe that's the culprit.

Even though the furnace is only 5yo, I had caught the prev tenant running the fan 24/7 - and she was clueless it was even on or how to turn it off :| I'm guessing a slight cage imbalance led to bearing failure after more like 10yrs of normal duty cycles.

The extra weird part I don't understand is that when running the old setup out of the furnace, if you held your hand over the exaust side of the plenum the slight added resistance would somehow trigger the motor to up it's rpm's and airflow. Take hand away and it would drop back down. I'll at least get some bearing replacements so the ~80-150$ motor has any chance to live again.
 
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