Trane XE-90 blower makes noise and barely nudges


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Old 01-21-12, 09:50 AM
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Trane XE-90 blower makes noise and barely nudges

I live in New England and I have Trane XE90 forced hot air by gas furnace which is approximately 10 years old. As of yesterday, the blower is not working.

Interestingly, the noise is exactly what they all make for fraction of a second before the blower starts and picks up the speed. At least that is how it was on mine since new. A youtube video explaining and cleaning the blower exhibit similar noise, once again only for a fraction of second on startup. (Oiling the furnace blower motor - Oiling the furnace blower motor - YouTube @5:40)

As of yesterday, my blower only makes that big noise all the time and the cage fan barely attempts to spin. I already had a technician but he did not have a replacement motor. While he showed up, the blower ran for few cycles for him but not anymore. The fan cage has no obstruction. I can spin it by hand very easily. However, as soon as controller commands it to spin, it behaves as if a giant magnet is preventing it from spinning. I tried to "assist" in starting but as I said, it "chokes" itself as long as there is power going to it. I had technician swap the starting capacitor but the behavior did not change.

I have measured voltage at the controller with the blower disconnected (YEL wire) and it shows 110V when controller commands the blower. The two pins on the capacitor shows 140V (?) when the blower is hooked up back to the controller. I would still like to rule out any controller problems by directly starting the blower. What is the best and safe way to do that? My idea of sticking a wire between wall and blower power lead scares me.

What bothers me most is that doing research on web has not come across this particular type of failure i.e. blower fan spins freely by hand but jams itself when the power is applied. I do not see any lateral movement of the fan. Fan starts "nudging" in the correct direction. Given that and also trying the new capacitor and trying to push it by wooden stick should rule out the bad capacitor.

Are internal rotor winding burned up? So instead of creating opposing emf, the core of the rotor is being attracted to the stator and preventing it from spinning. Does that explanation make sense? Do the rotor even have internal windings?

I already have the motor on the order and the technician is going to install it on Monday. I want to make sure it is the motor which is bad and not the controller. The house is getting colder and I would like to see if there is anything that I can do to get the heat starting at least temporarily. Any way to stick a leaf blower somewhere in the system? :-)

Thanks!
 
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Old 01-21-12, 11:16 AM
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110 VAC is a bit low. Usually 120 VAC would be expected. You might want to measure the AC voltage that you are getting at the house and see what it is.


It's unlikely but possible that you have a fan motor relay that is making a poor connection and reducing the voltage available. Were that the case, when the motor is trying to start and needs the maximum amperage, the voltage might be reduced further and be the cause of the problem.

Cutting an end off an extension cord and wiring the motor directly into the house power supply as a test might be worth trying.

I've seen stranger things.
 
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Old 01-21-12, 11:53 AM
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I measured it again and it is actually 120VAC at the relay terminal (motor disconnected) and never goes below 118VAC when the motor is making the noises. I also measured the resistance (to the ground) of the motor connections. They are in the 4/5/6 ohm range depending upon which connector (high/med_high/med_low/low) (BLACK, YELLOW, BLUE, RED) is selected. That sounds about right given that circ. blower output is listed as 14.4 FLA and 26.0 LRA on the inside of the furnace cover.

Given the minimal drop in voltage, I am inclined to believe that relay or controller is NOT the culprit here. If there is too much resistance in the relay, I would see reduced voltage at the relay terminals. I do not have clamp-on ammeter and my ammeter can not handle more than 10A.

The failure mode I am seeing on this motor is definitely not common.

Thanks for your reply
 
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Old 01-21-12, 12:18 PM
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Replacing the motor is the thing to do.
 
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Old 01-21-12, 12:48 PM
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I just tried leaf blower; of course that failed! There is no way to get the air moving without the blower fan rotating; Duh :-)

One thing bothers me though. The furnace is on 15A breaker. If that is the case, blower motor should be drawing quite less than 15A current. Why is the rating plate showing it otherwise? The utube link was showing significantly less current than the 15A (1.5-3.5A) for his blower. The resistance of my motor (4 ~ 6 ohm) sounds low or should I only be worried about the inductance of the motor? How does a 15A breaker handle the listed 14.4 FLA and 26.0 LRA? As a matter of fact, my hot water is on the same breaker as is a freeze (for last umpteen years!).
 
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Old 01-21-12, 01:47 PM
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Motors have a large spike in amperage when they start, and circuit breakers usually have a slow blow characteristic to accommodate such things.

Inductance greatly limits amperage flow through a motor.

(Inductance is a quality of AC voltage that acts to restrict current flow, similar to resistance but not consuming energy the way resistance does.)


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Old 01-21-12, 02:35 PM
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Have you seen this type of failure? I am not seeing fault in stator or stator windings or in mechanicals. That means the fault has to be with the rotor but rotors are so simple. What can go wrong with a rotor??
 
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Old 01-23-12, 05:17 PM
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It was the motor! It is pretty hefty 3/4 hp. Someday I am going to open the old one to find out what went wrong with it.
 
 

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