Author Topic: Condenser fan running  (Read 1196 times)

Offline Porcupinepuffer

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Condenser fan running
« on: July 03, 2018, 05:57:53 PM »
Went to a service call. One of those old rectangular shaped units where you need to remove the top to access the panel where all the wiring is located.
Compressor was hot with no continuity on the common. Resistance of 4.5 ohms across start and run windings... The schematics don't show me having a thermal switch inside the compressor that could be opened up by the warmth of it... Not sure how long this a/c hasn't been working.
When the contactor pulls in, the fan goes on a high speed. Once the contactor lets go, it goes into a slower speed.
All wiring is correct per the schematic on the plate. Both capacitors were on spec... I just couldn't figure out why this fan keeps running at a slower speed. Only possible reasoning I could up with is that the direct sunlight on our sweltering day is making the temperature sensing fan switch come on since it's right on the side of the unit by the hot steel.

The contactor completely lets go. But it's not a dual contactor. Only one side gets disconnected.
I also don't really understand the logic behind this and why the fan can continue running. I've never seen it before and didn't have time to spend the rest of the day there when it seems the compressor has a problem.

Offline Sergroum

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Re: Condenser fan running
« Reply #1 on: July 03, 2018, 08:46:42 PM »
Mmm. A 'lot' of ACs dont show a thermal switch inside the compressor on schematics and yet those compressors 'do' have it. Just mentioning that.
I guess no model number for this?

Offline Porcupinepuffer

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Re: Condenser fan running
« Reply #2 on: July 04, 2018, 03:52:16 AM »
Here's a pic I have. I found nothing on it.

Offline Admin

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Re: Condenser fan running
« Reply #3 on: July 04, 2018, 06:47:25 AM »
It sounds like you have a winding issue in the compressor and it's back feeding power on one leg.  The condenser fan motor is getting that voltage and running at half speed as a result.

Offline Porcupinepuffer

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Re: Condenser fan running
« Reply #4 on: July 04, 2018, 05:45:22 PM »
That's the conclusion I came to when I thought back about it. The schematic and everything was plugged in where it's supposed to be. There really was no way for the power to give it 240 from the other side of the contactor with it disconnected. It didn't dawn on me at the time that it was almost certainly finishing the connection by feeding to ground... I should also point out there was at least 4 or 5 mice that were electrocuted on the contactor, and capacitors. I didn't see any obvious spots where they may have chewed wires, but that's likely a possibility.

Offline slo-115

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Re: Condenser fan running
« Reply #5 on: July 04, 2018, 09:09:28 PM »
Mice love those old green machines. The fiberglass insulation around the compressor is a great nesting material. We had a discussion the other day about the water shield that protects the condenser fan motor. I have yet to get one off in one piece.

Offline Trainerguy

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Re: Condenser fan running
« Reply #6 on: July 05, 2018, 12:13:59 PM »
The old Keeprite units used one of the compressor windings as a crankcase heater. there is a dual capacitor and it has a resistor across the comp. terminals. when the winding in the comp. opens the power goes thru the resistor to the fan and it will run on low speed. The fan only runs on high speed over 85 F.

Let the comp. cool and check the windings, still open winding, bad comp. showing resistance, low on charge, or bad capacitor.
 Good luck, hope this helps. I have worked on these units that are over 30 years old. Built to last forever.

Trainerguy

Offline Porcupinepuffer

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Re: Condenser fan running
« Reply #7 on: July 05, 2018, 04:02:41 PM »
The old Keeprite units used one of the compressor windings as a crankcase heater. there is a dual capacitor and it has a resistor across the comp. terminals. when the winding in the comp. opens the power goes thru the resistor to the fan and it will run on low speed. The fan only runs on high speed over 85 F.

Let the comp. cool and check the windings, still open winding, bad comp. showing resistance, low on charge, or bad capacitor.
 Good luck, hope this helps. I have worked on these units that are over 30 years old. Built to last forever.

Trainerguy

So I found this schematic on one of our fellow sites http://hvac-talk.com/vbb/showthread.php?1965521-Older-Keeprite-Condensing-units

This one looks identical to the one I was looking at. I now notice a small black wire snuck around the line side of the NO contactor and going to the capacitor. I can now see how the power could get to the fan. I also see the fan is a two speed. I'm just wondering how it is that when the contactor is depressed, it goes into high speed; but once you let go, back to low speed... It was boiling hot in the sun where the unit was, and the only speed control of the fan I see is the actual fan control thermostat... I can't see how the contactor closing and opening should change the fan speed?

Offline NoDIY

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Re: Condenser fan running
« Reply #8 on: July 05, 2018, 06:30:20 PM »
Should have sold them a new unit by now :)

Offline Porcupinepuffer

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Re: Condenser fan running
« Reply #9 on: July 05, 2018, 08:18:18 PM »
Should have sold them a new unit by now :)

Yeah, a new one was installed yesterday as the customer couldn't stand his warm house. I'm just baffled as I had an issue I couldn't fully figure out.

Offline Trainerguy

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Re: Condenser fan running
« Reply #10 on: July 07, 2018, 07:32:38 AM »
What happens is when the compressor overload opens the power is diverted through the compressor windings back through the resistor across the capacitor and into the fan motor making it run at a very low speed.
Due to the high resistance of the compressor windings and the resistor, the fan receives much lower voltage resulting in the lower continuous speed even if the contactor is open.
When the contactor is closed the fan will run at the normal low speed as it is getting full voltage. Under normal conditions it will change to high speed over 85F.

If you follow the schematic through the closed pole of the contactor to the compressor overload, open it up and follow the path through the windings back to the pwer side of the open side of the contactor,  you will see it does make sense. You are one of many techs who were baffled by this happening.

The original problem was either low charge, bad compressor capacitor or tight compressor causing the compressor overload to open.

Trainerguy


Offline Porcupinepuffer

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Re: Condenser fan running
« Reply #11 on: July 08, 2018, 12:50:40 PM »
I sort off see what you're saying... My brain still turns a bit to mush when I try and follow it on the schematics. The C2 capacitor with the fuse and resistor throws me off with trying to understand what exactly it's doing. I'm pretty sure I have the path understood for how the fan is getting the power, but trying to figure out what difference takes place around that capacitor when the common winding has continuity that changes things that the fan won't run... I take it the fan wasn't actually running on it's slower speed, it just running on some sort of partial speed from the resistance caused by the windings/resitor? That would make sense since it was extremely hot and should have only ran on high speed in the way it did with the contactor in.

Offline Trainerguy

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Re: Condenser fan running
« Reply #12 on: July 08, 2018, 03:26:54 PM »
Correct, the windings and the resistor drop the voltage down to the point the fan will run slower than the regular  "slow speed". When the contactor is energized the full voltage goes to the fan allowing it to run properly.

Don't feel bad, many techs have changed the fan as well as a compressor when it was all due to a low charge.

Trainerguy

Offline Porcupinepuffer

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Re: Condenser fan running
« Reply #13 on: July 08, 2018, 04:10:50 PM »
I think I have it understood pretty good now. Under normal off operation the power is going from the L1 to the A terminal of the capacitor and leaving the C terminal on the capacitor to the Start windings. With a normal compressor motor (with the Common still connected) the power takes the easiest path back from the common to the T2 terminal. With the higher resistance of the run windings, there should be little to no electricity on the red wire going back to T1 (which would cause the fan to work)... I believe this setup is using the start windings as a crank case heater as already mentioned.

In the scenario with the open switch on the common of the compressor motor, the power goes in the same place, but now has no choice but to pass through the run windings and make its way back to the open T2 terminal with the red wires. Since this terminal is open on the contactor, and it still has no path back, it's now going through the fan windings that are connected back at T2 with the orange wire.

Maybe the resistor also plays a role in limiting the power consumption for using the windings as a heater. I still haven't quite got that part totally understood. But I'm a little more content anyways.

Offline Trainerguy

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Re: Condenser fan running
« Reply #14 on: July 09, 2018, 08:12:00 AM »
You have got the principal now. There are still a lot of the old Keeprite and ICG units out there yet so you might get a chance to put your new knowledge to work.

 Trainerguy