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justplumducky 07-08-2012 10:11 AM

Condenser Fan Motor stopping after 10-15 minutes, but will restart
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). Can a locked bearing cool off that quickly?

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)

Thx much for any help.

beenthere 07-08-2012 02:03 PM

Winding shorting out to itself. Happens often.

beenthere 07-08-2012 02:03 PM

Winding shorting out to itself. Happens often.

justplumducky 07-08-2012 02:23 PM

Thank you beenthere!

I saw a youtube video where a tech was showing how to ohms test a motor like this. This shorted winding would show up on this test? Not doubting you of course - just wondering how to identify this condition in the future with a VOM if possible.

beenthere 07-08-2012 04:07 PM

Won't always show up on a cold motor. The varnish allows the winding to short to itself when it gets hot, and locks up the motor. When cold, it may read close to normal.

justplumducky 07-08-2012 04:44 PM

One more question sir, and I'll shut-up.

So it runs normally when the varnish is cold, but after 10-15 minutes (in this case), varnish heats up, winding shorts to itself, disrupting the magnetic fields (creates opposing fields?) and shuts it down and holds it there as long as power is on (in this case anyway).

beenthere 07-08-2012 04:50 PM

Yep. I was just at a place that was having the same basic problem. the motor ran fine for 20 minutes plus. but then would over heat and seize in place as long as power was on to it. Shut power off and the motor spun freely. I put on a nice new motor, and that had cooling again.

justplumducky 07-08-2012 05:07 PM

Thank you beenthere! ...think I love you! :notworthy:

justplumducky 07-08-2012 05:36 PM

Beenthere, I lied about shutting-up. :whistling2:

Would this condition be responsible for the burned terminal connection at the compressor. When I first checked this unit (for the same reported condition), I found a capacitor testing at half its rated uF values. I replaced it and unit was fine for a short while (now I know why - the winding shorting to itself). When I went back the second time, I found one of the terminal connections burned off the terminal post. The post wasn't burned - the female flag disconnect (spade connector) (just learned that " female flag disconnect" term last night) on the wire-end was burned. Replaced it and it ran for a while again, then shut down - that's when I discovered it (condenser fan) was locked up on with power on.

According to the homeowner, it had been doing this running/shutting down for a few times (they kept turning it off and back on later) before they ever called me for help. And on one of these occasions, it shut down and the power had not been turned off for a good while. When I arrived on the last visit, the power was still on - fan motor was overheating and I heard the compressor attempt to come back on, then switch off again (overheat cut-out I assume?)

Would all this draw too much amps (compressor) and burn that terminal connection (the spade connector only)? I believe it was the Run terminal.

JJboy 07-08-2012 06:08 PM

check the contactor...

justplumducky 07-08-2012 06:25 PM

Will do JJboy, thx.

beenthere 07-08-2012 06:30 PM

The compressor will run hot when the fan locks up like that, and can over heat the connectors. As above, check the contactor, as it can over heat from this also, and burn the contacts.

Now the bad news. The compressor may have been damaged from running like this. Depending how long they left it run like this.

justplumducky 07-08-2012 09:08 PM

Last time I looked at it (at which point I told them to not attempt to run it anymore), the suction line was cold with plenty of condensation on it, so I hope that's an indicator that it's still Ok (compressor).

That was a problem I didn't expect - the wife turned it on last time in attempt to use this package unit's blower to circulate air in the home - it shut down (cond. fan motor I mean) and was left in that state for quite a while (power still on). Your point about possible compressor damage is well taken.

We'll get a new motor (and appropriate capacitor) and check the contactor. Homeowner may want to change the motor himself, but I'll be checking it and starting it up when he's done (whenever he gets to it). Report back when we get it running again.

Thx so much for your help.

justplumducky 07-14-2012 10:18 AM

New motor (and matched capacitor) is running great, thank you beenthere and jjboy. Contactor is Ok and compressor is Ok and cooling good.

Big difference in the amount of air pushed out the top by that new motor. That should also improve cooling a bit?

Data plate on this package unit says Date of Manuf: '88. Will a motor usually last that long? The motor didn't have any vents on it for cooling - was closed on both ends. Apparently the cooling from the air circulation thru condenser, over the motor and out the top is enough.

Also had a small metal plug in a channel leading to the bearing (non-shaft side), for oiling apparently (a tech at Allexperts told me there wouldn't be any place to oil the motor). Maybe he didn't realize it was an old motor.

beenthere 07-14-2012 02:36 PM

Yep, it will help it to cool the house better. See a fair number of 20 plus year old condenser fan motors.

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