Bench-checking blower motor resistance

Fulton_Hogan

Fulton_Hogan

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Question for the sages:
Does removing the blower motor and checking its ohms via multi-meter give any indication to the motor's serviceability? If so, is there some numerical value that would indicate this item is bad, time to replace?

Background:
My HVAC control plate is becoming warm to the touch. I understand this may be a failing blower motor, maybe its resistor, maybe the controls and wiring, or some combination of any/all three. Otherwise, everything still currently works. I have no outward signs of failure; the fan works on all speeds, the doors actuate, the cold/hot switch makes air colder/hotter. The only symptom is a too warm HVAC face plate.

I'd like to start on the end of this chain (the fan) and work my way back (the controls) to see what may need to be replaced. I removed the fan and its spins via hand. Not surprised, it still works when installed. (However, this is such a subjective metric. I mean yes, it spins once released but not like multiple, full revolutions. Maybe that's enough to tell, I don't know. To me it's subjective.)

I tried checking the fan's circuit and ohms via its pins and I get a completed circuit and about 0.8-0.9 ohms. I've not found anything definitive saying those results are good or bad. Any thoughts or guidance here? I'm somewhat new to testing ohms, so maybe I'm missing something.

My next step will be a removal, visual inspection, and multi-meter test of the resistor. Same thing though, is there any series of numbers I would see that would say, this is bad, time to replace?

Thanks all!
 
Fan motor windings have a very low resistance like that. But it's common for the windings to start shorting together as they age which will cause the fan motor to start drawing excessive current which will start problems like melted insulation on the wiring, a blown resistor pack, or a burned fan speed switch. The windings are made from bare copper wire which is coated in shellac which serves as its insulation. The problem is shellac eventually starts breaking down/flaking off which causes the windings to start shorting together. It's really hard to tell the difference between normal motor windings and partially shorted motor windings with a simple ohmmeter. When in doubt it's usually best to just replace the fan motor.
 
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The resistor merely reduces the current for the lower speeds. High speed is no added resistance. If you have all speeds, it's good.

The only meaningful test of the motor would be to measure the current with it running. But then you would need to know the normal current draw of a new one to compare it to.

That's why I swapped out my original motor when I replaced the control panel. I didn't want to risk melting wiring and damaging the new control panel.
 
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Thanks all. Well, I can just visually check everything at this point.

I tried to replicate the panel heating up this morning on my way to work but I found myself more attentive to checking the panel's heat output than traffic.

I've already come to peace with ordering a motor.
 
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I popped the fan out before I left work to show it to a coworker for a second opinion.

I gave the fan a spin and he immediately, from across the room, said "Oh that's shot. Time for a new one".

Part is already ordered via Rock Auto.
 
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One more thought about the "does the fan spin freely" metric.

While I had them side by side I spun both fan cages to compare. They both spun about the same amount. Each made somewhere around three full rotations.

If bearings are bad, I think it's easier to recognize. Something would grind or the cage would hardly move at all. Neither of which I experienced.

Spinning a fan cage to test should result in movement, but those fan cages aren't ever going to spin like a fidget spinner. It's still a very subjective way to measure operation. That's why I don't like that metric.
 
Jerry Bransford said:
I don't see any relationship to how the HVAC fan motor spins to whether its windings are shorting out. Shorted out windings are common and the cause of the circuit getting too hot.
Click to expand...

I'm sorry, I wasn't really clear with my follow up. I do believe that my fan's windings were likely failing.

Regarding the spin test, I'm not meaning to imply that the shorted winding will cause the fan to stop spinning. It's just that the first thing every troubleshooting guide asks is "does it spin freely". I understand that's a specific thing that's being examined, the fan's capability to turn. It doesn't give any indication to the status of the windings.

I added the information about comparing my fans to each other because someone looked at mine and said (to them) it looked like the old fan motor was shot. But when I compared it to the new one, the spin rates were similar. Again, these fans spin, but they don't spin long like a bicycle wheel.
 
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