I have a rheem central a/c unit (2 tons) that I have been knocking my head on. The fan keeps shutting down about 15 to 20 minutes after the unit has been running and trying to cool my house. I have checked the voltages on the leads to the fan and compressor and the leads to the relays from the breaker board to unit. At first I thought it was the fan motor sticking it's last leg, however, I noticed that when I would shut down the unit manually for a couple of minutes and then re-start it, the fan would come back on. I have checked all the wiring and all seems good. The unit is about 11 years old. I have cleaned it and checked for leaks. I did notice when taking voltage readings from the relay to the capacitor and to the motor and compressor, on one of the leads goind to the fan motor, it would drop from 271volts from start and then it will drop down to 157 and the fan will slow until it comes to a stop. This reading was from the capacitor to the fan motor. The other lead from the capacitor to the compressor remained the same.

Can you please help!


Thank you

Joseph :confused:

There is a thermostatic motor safety, once called a "clicks-on" embedded in the fan motor windings. Essentially, over current causes a bi-metallic disc to bend and open electrical contact to the motor. When the disc cools, contact is restored. This device may be doing it's job correctly... check the starting and running current of the motor and compare to the motor data plate. If it is high, motor bearings or bushings may be the problem. Does the fan manually spin freely, or slowly "drag" to a stop... good luck...

Thank your for your prompt response. I will check the information you asked. What has me puzzled is when I take the voltage readings from the capacitor to the motor they are pretty constant, however, after a few minutes I see the voltage starting to drop to the point where the fan shuts down, however, does not affect the compressor. Do you think I should be replacing the fan motor or just the capacitor or ?

Sorry! The fans does spin freely for about 2 - 3 complete spins and stops. I took the motor apart to check on the bearings. I cleaned the inside and lubricated the bushings.


Thank you

Joseph

Probably get ready to replace the motor,, always install new capacitor with every and all new motors.

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Originally Posted by wmproop

probably get ready to replace the motor,,,always install new capacitor with every and all new motors.

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Can you please elaborate on replacing the motor. Why would the voltage line from the capacitor to the motor drop after a couple of minutes of run time? The voltage to the capacitor remains unaffected.

If the issue is with the capacitor, how come it does not affect the compressor?

It was easier to check caps with analog multimeters. Discharge the cap. With the meter set to "Ohms" put the leads on the cap wires. The meter will show a dead short, which will increase in resistance as the dielectric field in the cap charges. If the resistance doesn't change in one or two seconds, there may be cap problems. The motor has 11 years on it,. just a thought...

BTW, the compressor is likely a hermetically sealed unit, with a separate motor encased in the housing... good luck...

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Originally Posted by rsain2004

It was easier to check caps with analog multimeters. Discharge the cap. With the meter set to "Ohms" put the leads on the cap wires. The meter will show a dead short, which will increase in resistance as the dielectric field in the cap charges. If the resistance doesn't change in one or two seconds, there may be cap problems. The motor has 11 years on it,..just a thought...

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If I check the cap per your instructions, how will it still be possible for the cap to maintain voltage to the compressor and not the fan? Is the cap divided independently from the compressor and fan motor? Now this cap has 3 terminals, 2 lines to the fan motor (line one has 271volts and line 2 has 121volts) and the other one to the compressor (271volts).

Do you think the cap is not maintaining its voltage feed to the fan motor and then the motor cutting off on low voltage protection?

You gave a lot of good info about the system. But to check the run capacitor you have to check for microfarad (Capacitance of the capacitor)not voltage. If the run capacitor is bad it will have the same symptoms of a bad motor-- over heating or not starting at all. If you have a bad capacitor it can be replaced and the motor should work fine. Take the run capacitor to a local dealer and let them test it for free if its bad buy one from them and give it a try.

Without being there, I don't know... In this application, the cap charges on one half a hertz, then discharges on the opposite half to overcome locked-rotor inertia during start. When I was growing up as "Mr. Wizard" and running around destroying things, Caps were made by hand. We used two pieces of foil and waxed paper... that means two wires... your three wire job is new to me... (I'm 60 years old, so I'm still learning... )

When you pulled the motor did you happen to notice what capacitor the motor needs,, they don`t cost that much,, try it you might like it,

Sorry! The fans does spin freely for about 2 - 3 complete spins and stops. I took the motor apart to check on the bearings. I cleaned the inside and lubricated the bushings. (quote)

What did you lubricate the bushings with? If you didn`t use the right grease you may have done more harm than good, this is why I said (probably get ready to replace the motor,, always install new capacitor with every and all new motors.) some stuff will flush the bushings out and wash away their life expectancy,they will get hot and seize up

(quote)sorry! The fans does spin freely for about 2 - 3 complete spins and stops. I took the motor apart to check on the bearings. I cleaned the inside and lubricated the bushings.

What did you grease the bushings with? This is why I said (probably get ready to replace the motor,, always install new capacitor with every and all new motors.) if you don`t use the correct grease you can do more harm than good. The wrong lubricate can wash out the bushings and when it runs a while the bearings will get hot and seize up,there fore taking away from the life of the motor.

Just my opinion, but I think the motor is overheating and the voltage drop is due to an increase in the amperage from the damaged winding.

Replace the motor and capacitor..

Note: the 3 terminal or dual capacitor uses a common common terminal ,one for the hermetic and one for the fan.

It is possible for the compressor to run but the fan to fail and visa versa

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Originally Posted by rsain2004

Without being there, I don't know...In this application, the cap charges on one half a hertz, then discharges on the opposite half to overcome locked-rotor inertia during start. When I was growing up as "Mr. Wizard" and running around destroying things, Caps were made by hand. We used two pieces of foil and waxed paper...that means two wires...your three wire job is new to me...(I'm 60 years old, so I'm still learning...)

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So far good news! After cleaning the motor and lubricating (high heat lubricant) the bearings sleeves, the motor is working. Knock on wood! I now know that eventually I am going to have to invest on a new fan motor. You guys are great with your support! Keep up the excellent job. I will keep you posted on any new findings or actions.

Spoke too soon! The motor seized again. This time after a couple of hours of operation. I did touch the motor and it was super hot!

Its time to move forward. Its not a fix what your trying. The motor may be over heating due to bad windings or it's a simple capacitor bad. Its all a guess at this point. If you buy a new motor get the capacitor its rated for. I would get the old run capacitor checked before I bought a new motor. $40 vs $140.

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Originally Posted by T-Top

Its time to move forward. Its not a fix what your trying. The motor may be over heating due to bad windings or its a simple capacitor bad. Its all a guess at this point. If you buy a new motor get the capacitor its rated for. I would get the old run capacitor checked before I bought a new motor. $40 vs $140.

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I will take your advise. Thank you very much. Keep you guys posted.

I finally replaced the fan motor along with the cap and this resolved my issue. Thank you to all that came to my rescue.