A/C condenser fan runs continuously


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Old 04-24-17, 11:28 AM
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A/C condenser fan runs continuously

There were several older threads on this topic and it was noted in one of them that some newer condenser units have a dpst contactor which has a buss bar across one side/pole, thus making that side always on. Last year I had a service call on my Trane XL80 system because the system would not cool although the thermostat called for cooling. The technician found that ants had infested the control box outside and several had been crushed between the contacts of the contactor that controlled the compressor and the fan, thus making intermittent contact. He showed me the problem; now I have a stronger distaste for ants crawling into things I'd prefer they stay out of. Anyway, replacing the contactor and adding some freon got it back in fine shape.

Fast forward to late fall last year. I noticed that even though the weather had cooled off and we weren't using the A/C, the condenser fan was always running... It was cold out so I just pulled the disconnect putting off the investigation till next (now this) year. It's starting to warm up so I figured I should figure out why the fan never shut off before it gets hot out.

What I found was that my unit specifies a true dpst contactor; two independent circuits. The technician replaced the old contactor with a newer style one with a buss bar on one side, effectively turning the fan on all the time. Possibly he did that because that is all he had on the truck. If that was the case, I think he should have said he'd do a temporary repair and come back later with the correct part. No such luck.

So, the reason I posted this note is in case someone else has the unfortunate luck to have a similar problem. I didn't catch it after the repair because when I was outside, the compressor was running, so the condenser fan was supposed to be running too. Bad timing on my part.

When you have the outside compressor/condenser serviced and a contactor has been replaced, make sure the condenser fan shuts off when the thermostat is not calling for cooling, before the technician leaves.
 
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Old 04-24-17, 11:58 AM
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Nothing wrong with using single pole contactor.
 
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Old 04-24-17, 12:10 PM
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Welcome to the forums.

You have a bad condensor fan. Most likely a shorted winding.
The fan should not run with only one side powered.

You should be able to use a single switched or double pole switched contactor.

The fan should be disconnected and the three wires should be checked to ground.
There should be no continuity to ground.
 
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Old 04-25-17, 04:17 PM
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Thanks for the response.

The issue is that with the single pole contactor, both sides are always powered.

If a winding was shorted to ground, I'd expect it to run slower than normal since it's a 240V fan although it does have low & high speeds, both of which work when the outside thermostat switches).
 
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Old 04-25-17, 04:21 PM
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Re: Single pole contactor vs dpst contactor

Thanks, but since the system was designed to have the power switched to the lead connected to the contactor, now with a buss bar, there is no switching possible. Thus the motor continuously runs...
 
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Old 04-25-17, 05:32 PM
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A 240vac motor will not run with 120vac. So if you have a single switching two pole relay in place..... how is the motor running with one pole open ?
 
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Old 04-25-17, 08:53 PM
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A single pole contactor should provide 0 volts to the fan when 24 volts isn't present.

The motor is shorted or the contactor is incorrectly wired.

Post a picture of your contactor for better assistance.
 
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Old 04-26-17, 02:23 AM
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RE: single pole contactor when double is specified

The manufacturer calls for and the schematic shows a double pole contactor.

The compressor goes to one pole and the fan the other pole. The pole that goes to the fan is now shorted by the new contactor that has a buss bar on one side; the fan side, and the "real" pole for the compressor on the other side.

I repeat, the problem now is that the pole that should be open when no cooling is called for, i.e. coil is not energized, is a buss bar, so is never open, so the fan never stops.
 
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Old 04-26-17, 02:30 AM
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Let me explain one step further.

If I pull the lead going from the buss bar side to the fan, it stops. So if that pole was really open when not energized, instead of a buss bar, there would have never been a problem.

My original post was to hopefully save someone else from a similar problem. I know what the problem is and I have the fix. Just waiting on the new contactor to arrive since the local supply house had none in stock.
 
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Old 04-26-17, 09:59 PM
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It still sounds like you can move a wire to correct this issue.
You can have 2 wires to a lamp but turn off the light by interrupting a single wire.
The same will apply to a contactor.

Run or common could be moved be moved from L1 or L2 to T1 or T2.

Buying a 2 pole contactor is an inexpensive solution.
 
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Old 05-06-17, 05:10 PM
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shorted winding

Trane 240V motors will run on 120V. At least I know this is the case on Trane combustion fan motors with a grounded winding. Cant remember why. Have bumped into it several times though. Learned about it from the OEM Trane rep. Installed new motor and we were back in business...
 
 

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