Condenser Fan Motor stopping after 10-15 minutes

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Condenser Fan Motor stopping after 10-15 minutes

Postby justplumducky » Sun Jul 08, 2012 10:20 am

Nordyne Model No: MSC36E- C3
Serial No: MK0688-02860

This is a Nordyne, self-contained A/C system, for a mobile home 14' x 70', manufactured in 1988 (the A/c unit I mean, not the home), but I have no idea of the condenser fan motor's age. Not sure when it was oiled last either - the fan blade didn't turn as free as a newer one would (as in giving it a push and watching it continue to turn for a few seconds), but it turns ok (obviously so, because it restarts without delay). I did oil it (there was a single port with a cap in it to do so, at the bearing end of the motor). It improved its free rotation, but still is far from the continued rotation of a much newer one. I'm wonder if, over time, an older bearing like this (not maintained with oil on a regular basis) would eventually stress the motor windings into a condition like this (maybe my gray matter is just working overtime, though?)

When I first started working on it, I found a dual run capacitor at about 1/2 the rated microfarads - replaced it, but the condenser fan motor continues to stop after about 10-15 minutes. The compressor continues to run (noisy), but then goes off on "overload" (switch) I assume. Fan motor will restart no problem (but then shut down again after another 10 to 15. If left in this condition (fan motor stopped with compressor initially running, then going off on overload), the fan motor will heat up the metal in the center of the grill work that holds the fan motor.

It's apparently not a locked bearing...
After the fan motor shut down, I opened the side panel on this package unit, grabbed the fan blade while the motor was still humming (rather loudly) and it didn't want to move. I could force it if I pushed hard on it, but the "power still on" was holding it "locked". It's apparently not a locked bearing, because right after it stopped, I turned the power off and the fan blade was "free" again (and it will restart no problem).

An obvious case of needing to replace this motor?

What causes the power-on to hold this motor in a "locked" position? (after it initially stops running) When it does stop running (witnessed it twice), you can hear it begin to slow down gradually, then come to a stopped (and "locked" position), which takes it only about 15 seconds after you first hear the fan motor start to get louder (which is at the point when the fan first starts to slow down).

After it first stops (fan motor), there is normal voltage (240) at the "outlet" side of the contactor. The motor pulls normal amperage while running (about 1.2 A) and about 2.0 A initially after stopping. Compressor pulls normal (according to the data plate on the package unit) amps also while running (about 14 A).

What causes the power-on to hold this motor in a "locked" position? (after it initially stops running)

A very experienced tech from the DIYChatroom just told me this condition is a motor winding shorted out to itself and it happens often. Said ohms checking the motor when cold may reflect numbers fairly normal. He said the motor will run for however long and when the windings' coating of varnish gets hot enough, it will short to itself, stop the motor and keep it stopped/locked up as long as the power is still on. Then soon as power is turned off, fan blade will rotate freely again. Then unit can be restarted and will repeat the problem: run for a while, then fan will stop and lock up again until power is turned off. If it is left unattended for any length of time in this power-on, locked up condition, he said compressor damage may result.
Luckily, I think this one is ok, so far at least. Will change the motor (w/appropriate capacitor), check the contactor & see what happens.
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Re: Condenser Fan Motor stopping after 10-15 minutes

Postby justplumducky » Sat Jul 14, 2012 2:31 pm

New motor with matched capacitor - all is well again.
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Re: Condenser Fan Motor stopping after 10-15 minutes

Postby cnyguy » Mon Jul 16, 2012 7:44 pm

Hello:
Glad to hear you "won one" in the HVAC battles. I have a similiar system as yours and I made the same fix you did. What I have heard is critical for your outdoor condensing unit is for you to tarp it over for the winter. I seem to have had problems with capacitors, not fan motors. I guess excess moisture in there takes them out. I replaced the original "clamshell" GE motor three years ago, and have just replaced the capacitor since then. I also installed a "kick start" kit for the compressor which is across the line from the dual capacitor compressor wiring. The compressor (copeland hermetic) contains an internal overload tstat to take your compressor offline if it gets to hot, no condensor fan. When this happens, its usually too late, system is toast. I thought about installing a high head pressure switch (which comes standard with the new refrigerant R410A units),but I think when this switch opens, again it's too late.
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Re: Condenser Fan Motor stopping after 10-15 minutes

Postby cnyguy » Mon Jul 16, 2012 8:21 pm

oops!
didn't realize you were talking about a packaged unit , not a split till now. If your unit is like a Miller, which I think is the same as as an Intertherm or Nordyne one, these units are tough as nails. We installed a bunch of these on school bldg rooftops for computer labs, and I'm totally impressed. They're at least 20 years old, very little problems. If I ever got rid of my split system, I would definitely get one of these.
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Re: Condenser Fan Motor stopping after 10-15 minutes

Postby 1987Commodore » Thu Jul 19, 2012 10:28 pm

I have had one of those Miller units for 23 years, and have done nothing to it other than an annual cleaning. :D
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