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Air Conditioner Problem - Thermal Shut Down

06/24/2010 7:49 PM

Here is a problem that has me stumped. I am working on a 7.5 ton air conditioner condenser unit. The fan motor on the unit is getting hot and shutting down (thermal shut down.) When this happens the head pressure rises and the high side cut off then trips and kill's the entire unit. The unit is a 3 phase but the fan is running on 230 volt single phase. The fan pulls the air though the condenser coils fine and everything works fine until the fan thermals out. The fan blades are clean. I have replaced the fan motor and cap. With new ones, of the same make and model as the original one 1/2 hp. We have 2 other units on the same roof with no problems. I have also tried adjusting the fan blade up and down. The power to the motor is 230 volts which is good. Does any have any suggestions as to what to do or try next?

Thank you.

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#1

Re: Air conditioner Problem Thermal Shut Down Help

06/24/2010 9:20 PM

Have you checked the current draw on the motor?

Correct rotation?

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#2

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 7:25 AM

Since this fan is single phase and there is a single capacitor circuit, I suspect that the starter windings are expected to not work continuously, but they are now. This can be caused by the centripetal switch in the motor failing to open. Possibly the fan motor not getting up to speed due to bad bearings (not the motor bearings, you changed them with the motor replacement) or some other unexpected drag so the switch remains closed. There may even be a timed relay that is failing to cycle.

That's all that I can offer with what you've posted.

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#3
In reply to #2

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 12:04 PM

This is a direct drive motor no belts or pulleys and the cap used is a motor run cap not a start cap. The currant draw on the motor is 3.43 amps and the plate on the motor is 3.7 amps I can change the amp draw on the motor some by changing the height of the fan blade but the motor will still thermal out. Could I have a bad new motor? Any help is appreciated.

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#4
In reply to #3

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 12:24 PM

Take an IR thermometer reading of the motor when it shuts down and compare it to a reading from a known good motor on one of the other units.

If the readings compare, I would say you have a weak thermal.

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#5

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 2:42 PM

Just a guess, as your unit is bigger than mine and probably a little different. I had a similar problem last summer with my 4 ton unit. The fan was turning but it seemed like it was under powered and was getting hot. If I recall correctly it turned out to be the capacitor. On my unit it is a metal cylindrical thing that is about 7 inches long and has three wires that plug into the bottom. It was relatively inexpensive and corrected the problem.

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#6

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 11:01 PM

Is the condenser clean & the air flow path both in & out clear, sounds like the fan motor is overloaded - I suggest you clean the condenser first, as this will also push the head pressure up

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#20
In reply to #6

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 2:26 PM

A plugged condenser will actually LOWER fan power draw because it will move less air and the air that is moved will be slightly lower density (lower mass flow) because of the slightly increased suction pressure and higher thermal gain (less mass + same thermal = higher temperature)- similar to closing a damper on the inlet side of an air handler.

I think that we are back to motor issues. An independent post follows.

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#7

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 11:17 PM

All the above answers are really good. You say the ac is three phase, delta or wye? With delta is that high leg sometimes. Compare the wiring on the troubling unit with others, if there is a high leg, does it go to the fan or not?

Personally, I would measure each leg to ground, and each leg to the others, and go with the pair that gave a good clean 230 volt.

If you are getting low voltage, that makes for some real hot motors.

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#8

Re: Air Conditioner Problem - Thermal Shut Down

06/25/2010 11:38 PM

possibly your condenser is partialy plugged with dirt polen etc.clean wash out ???? You could try takeing one of the other fans off one of the other units that work and try on this one ,and putting the problem one on the other unit and see if it is the same problem ? Jerry

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#9

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 5:02 AM

If you are certain that the fan shuts down first,then it is for certain that 2 possible problems exist.1:Fan is actually getting too hot, or 2: Thermal trip is weak and causing premature shutdown.It appears to be #2 based on your description.The thermal OL's are located in the winding(stator) of the motor.If the spot where the OL is located is facing direct sunlight, there can be false trips in very hot weather.Try rotating the motor in the mount, to the bottom, or providing shade for the motor as a test.

If number 1, check voltage and wiring integrity for loose wires, etc.

HTRN

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#10

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 8:55 AM

there are 2 considerations. there are different temperature ratings for motors, usually in centigrade. you can move to a higher temperature class. my choice would be to look at the amp rating on the motor which is the true rating of the work it can perform. if your half hp motor is rated for say 4 amp move to a higher hp motor with aan rla of 5.5 amps. once installed you will probably fing that it actually amps out lower. Your problem is very likely a power factor problem. when the temperature rises the draw on the local utility rises mostly with induction motors which is causing a lag between voltage and current. I have seen 208 drop to 170 under these conditions when read on a true rms meter. the book solution would be a capacitor bank but in the real world going to the higher rated rla is the only practical solution we have found. the only real test to prove i am correct about the problem is to actually read the voltage at the motor when the load starts to peak. when one unit drops it takes some of the back emf off the system and brings the sine waves closer to sinc. once you get to around 17' lag though you are going to start to lose motors. it's one of the things that makes this business so much fun.

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#11

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 9:23 AM

If the fan motor gets hot after a long run time, also suspect the bearning. The quickest low cost solution--replace the fan capacitor. If that does not work--replace the motor

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#12
In reply to #11

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 9:47 AM

Ahem, you didn't read the OP carefully. The capacitor and motor have already been replaced.

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#17
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Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 11:38 AM

Oops, and I got a highly coveted " good answer" I can't take that back, but, when I got my new capacitor I did find out that even though the new one looked like the old one, the plugs on the bottom were configured differently, (different manufacturers), so on my first attempt I was still experiencing problems, I had to seek out someone more knowledgeable than myself, they drew me a little diagram, and then everything worked fine. Just trying to help.

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#13

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 9:55 AM

Let me go back and answer some questions again. I replaced the fan motor and cap. It is a direct drive motor. The unit is clean and air flow is fine. The motor is rated at 3.7 amps the currant draw is at 3.4 amps. The supply voltage is 230 volts. I have also tried connecting to different legs on the line I have even bypassed all control circuits tying the fan direct to the line. It is the fan motor that is thermalling out. I have even tried lowering and highering the fan blade. I am wondering now if may the motor that I replaced may have been a replacement motor with a lower cutout temp. But the old one worked until it started doing this and it is a 1/2hp with the same rpm as the unit calls for.

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#14
In reply to #13

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 10:32 AM

Well when the normal electrical suspects are not the apparent fault, you have to look at the physics. To tackle this as a physics experiment, you will have to do some more measurements. Identify the case temperature before trip, at trip and after tripping. Compare this to the thermal protection rating. If the temperature before and after tripping never exceeds the thermal protection rating then likely the thermal trip protection is at fault. (I doubt this is the case but I'm trying to be thorough.) If the case temperature continues to rise after the internal temperature trip, then the dominant thermal load is the motor itself. I know that it is not likely that your new motor has the exact same failure as your old motor but sometimes this does happen. Now if the case temperature quickly comes close to the trip temperature on start up then I would suspect an external to the fan motor heating source has changed. While direct and reflected sunlight contributes thermal energy, I again doubt that this is the root of the problem. I suspect that the air cooling this motor is the air that has been drawn past the heat exchanger coils. Have you had the refrigerant levels checked?

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#23
In reply to #14

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 3:22 PM

I did not mention the refrid. pressure as I thought that would have been the second thing you would have done ,after you checked the air flow !!! Jerry

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#15
In reply to #13

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 10:39 AM

If the motor is mounted vertically, it may be going out of magnetic center.

Some fan motors are rated for vertical operation, some aren't.

Normally this would show up in the current draw, but this condition causes high rotor heat.

Maybe someone here can add to this.

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#22
In reply to #13

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 2:48 PM

Let me add this, when I bought my new fan motor, I was advised that it was made in China. Quality control is not a big issue there, these problems happen all of the time, whether it's car parts made in Mexico, clothes from Srilanka, it doesn't matter. The frustration of putting a new part in something and expecting it to work, apparently doesn't matter anymore. I'd rather spend $300 on something solid, than $75 and hours of labor on something that doesn't work. A tad off topic, but, I think this situation affords the United States an opportunity to once again reign supreme in the manufacturing sector, build stuff that works, stand behind it and price will become secondary.

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#16

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 11:37 AM

Sounds like the amperage is too high on the replacement motor. An amprobe would be just the thing to check the current. This could be caused by the blade set too higl or two low in the vortex. Check the other units to see where the blades set in the unit. You could also check the amps on the working units and adjust the blades to the same amperage.

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#18

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 12:01 PM

The original motor probably was wired to the electrical containment area. Sometimes the cap was wired so one load wire was fed through on one lug of the cap. The replacement motor might be cap fed by a brown/white and brown wire that does nnot connect to any of the load wires..........

Acheivement without flustration is nearly impossible.

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#19

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 1:19 PM

From you description it seems to me your fan though original is undersized, you admit having replaced it before was it for the same problem? What is the capacity of the other units that are ok?

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#21

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 2:36 PM

Since you have other units running with no problems, and you have adequate voltage, macro-system problems are not the issue.

This appears to be a motor problem.

Suggest that you try an ohm-meter reading on both windings. It is possible that either the insulation was damaged during manufacture OR some form of transient voltage hit that motor hard enough to damage insulation. A low-grade short circuit would trigger the higher amperage and the higher motor temperatures.

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#24

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 4:20 PM

Have you bench tested this motor? Amps,rotation and temp? Did you check the fan to see if the Fan pitch matched the other units? You will also want to check the air flow on this unit, that motor is a air cooled motor, lack of air flow will cause it to over heat as will high amps.

Check your voltage with the other units running so you might see a voltage drop on one leg of your power.

You say that you have two other units, have you checked there motors and cap. to the one you are having trouble with? You just might have two different production runs !

Even thought the units were made by the same company sometimes they are not the same, check the data plate, the factory has put the wrong things in the right unit before. If you still have the paper work that came with the unit you might find the build sheet,this is what tells the workers what goes in a unit as it goes down the assembly line.

Don't rule out that you have a bad batch of motors with that overload that were sent to that Maunf.,they might have bought 500 units and changed over from one shipment to the next.

This is just some things that I have ran across in my service work.

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#26
In reply to #24

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 9:32 PM

50 or 60?

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#25

Re: Air Conditioner Problem - Thermal Shut Down

06/26/2010 6:59 PM

from all the discussion it would appear to be a weak thermal overload..

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#27

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 1:06 AM

No one has considered the 3.4A the 1/2 HP 230V is drawing.

At 230V a 1/2 HP motor should be drawing 1.13 ish Amps.

Is this motor energized through a contactor/relay device? I could see a high resistance from a bad contact set causing a 'single phase' operation.

Just a thought.

Skies.

Jay.

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#28
In reply to #27

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 1:18 AM

The FLA for a 230V Single Phase motor is 4.9 Amps according to the standard FLA tables.

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#33
In reply to #28

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 6:08 PM

Something has been bugging me about your FLA rating numbers and I think you led me to the root of this problem. You are correct that the Full Load Amperage of a single phase motor is rated at a higher number (6.5A @ 240VAC) than this particular motor is rated at. But I then checked another FLA table related specifically to HVAC use. In this table they have a completely different set of currents (3.5A @ 220VAC) that relate to 1/2hp single phase. Now in the second table I noticed that there's also a separate column for two phase motors. By constantly using a capacitor delayed secondary winding, isn't this actually a two phase motor and not a single phase motor? Also why would motors with the same power rating have such dramatically different FLA current ratings for similar voltages?

This makes me think that the original failing motor was not really replaced with an identical motor, but with a motor that had the same rating values but the values were calculated/measured under different conditions.

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#34
In reply to #33

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 7:56 PM

I am inclined to think redfred has captured the root of the problem. Performance was what I would have expected of a 50 hertz getting 60.

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#29

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 10:30 AM

Thanks for all the input, I don't have a temp probe to test the motor temp but that is a very good idea. The other 2 units on the roof are not the same as the one giving the problem. They appear to be about 5 ton units and only have a 1/4 hp fan motors on them. The idea of the new motor being bad is where I am going with my thinking. The motor cap can't be connected wrong since it connects to the wire as they come out of the motor. The line voltage is not dropping when the unit shuts down. I didn't install the old fan motor which was in it so I know if it was original or not it has been in it for a long time so I don't know about it. The coils are clean If I ever get it corrected I'll post the cure on here.

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#38
In reply to #29

Re: Air Conditioner Problem - Thermal Shut Down

07/01/2010 4:33 PM

Whyme, check the motor nameplate for the "Service Factor" or SF; The orignal motor may have had a SF: 1.15 or 1.25 this is the amperage the motor can work above the rated amperage of the motor. Also make sure the fan blade is right and posistioned right.

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#39
In reply to #29

Re: Air Conditioner Problem - Thermal Shut Down

07/06/2010 9:44 AM

Thanks for all the comments and suggestion. I believe we have found the problem. The new fan motor that we put in was also bad. After installing a different brand motor in the unit it has preformed perfectly. What a headache getting a new bad part can cause.

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#40
In reply to #39

Re: Air Conditioner Problem - Thermal Shut Down

07/06/2010 10:07 AM

Glad the problem is solved. At least you had lots of help getting there.

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#41
In reply to #39

Re: Air Conditioner Problem - Thermal Shut Down

07/20/2010 3:57 PM

hi yall:))

i totallly agree about new parts, causing problems. i've worked on rtus before. did you say fan motor normally runs on single phase 230 volts?

all ones i repaired had three phase. dont remember brands.

gerald b.

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#30

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 1:27 PM

Do an amp draw test and change the fan blade, then retest. You have to check the charge on the unit. Newer refrigerants require precise charges and run much higher head pressures, so they may run well up to the point that they are severely loaded. My thinking is that your running high head and to much heat is causing poor cooling at the fan and the weakest link is the fan thermo heaters.

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#31

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 5:01 PM

is the air flow clear no blocked filters on blockage outside

have you changed the thermal cut out if separate from motor

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#32

Re: Air Conditioner Problem - Thermal Shut Down

06/27/2010 5:31 PM

Things to consider, is condenser drawing in much hotter air, is the pitch on the blades right, is the fan cavitation (1/2 way on the shroud), are the RPM's right, and h.p. of the motor, voltage right (not "brown power"). hope this helps.

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#35

Re: Air Conditioner Problem - Thermal Shut Down

06/28/2010 8:54 AM

As another post said change the fan blade
Fan blades wear out, the props get bent almost every time someone changes a motor
during the act of changing it.
Blades also go out of balance but usually the props get bent by the mechanic or birds
or any animals or a garden hoses that get caught in the moving prop
Check the revits on the blade to search for stress cracks in the blade (out of round)
Place a straight edge on the shroud touching the tip of one prop then turn the blade
by hand to see if all blades also touch the straight edge at the same height
Sometimes you can salvage the blade by bending a low prop back to height
but it's always best to install a new blade and always check a new blade too

Go to manufacture's blade charts to match the old one and you might be surprised
as the mfg chart may tell you that blade now requires a 3/4 hp motor not 1/2 hp
My theory is the a/c mfgs use the smallest hp motor possible to get high EER ratings
in the lab but in the field the blade gets damaged, goes out of balance, blocked
condensors causing high ambient on the motor, low voltage whatever causes motor
overheating and shuts down on thermal

What's better a new blade and 3/4 hp running cool and unloaded
or a fully loaded 1/2 hp shutting down on thermal and losing compressors?

I have also found by increasing fan motor hp the condensors seem to stay cleaner
longer with the increased flow through the fins here in the USA with cottonwood
tree fuz blockage nightmares

Good luck
John

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#36

Re: Air Conditioner Problem - Thermal Shut Down

06/28/2010 9:25 PM

Yes I know the OP said the cap was replaced. I just repaired my 37 year old central air conditioner for exactly the same symptoms. Fan motor getting real hot, then slowing down to a stop and of course the compressor starts making ugly noises before it shuts down due to the high-pressure cut-out switch.

Check the run capacitor. Again.

Disconnect the leads from the capacitor and use an ohm meter and watch the resistance start near zero and approach infinity. If it goes to infinity instantly, then the cap is open. If it doesn't go to infinity then it may be shorted. (You can always insert a resistance in series with your ohm meter to "slow" it down.) My capacitor was a nice 70 ohms. I thought the motor was bad and paid $84 for a new motor (Since I replaced the blower motor ten years ago, I thought my bearings were seizing, Fasco Model D908 1/3 hp with the sleeve bearings.) (The new motor was a Fasco Model D7908 with sealed ball bearings.) Same problem but it took longer to slow down. I checked the run cap and found the low-grade short. For another $2.49 I fixed the problem. It runs like a champ now.

I have 220VAC 1-p system (not surprising for home) and the run caps are rated at 370 VAC. Interestingly enough, if you put a voltmeter across the cap while it is operating you may find like I did, that the voltage is even higher due to the resonant effect of the capacitor and the motor windings. Mine is running at about 385 VAC but these oil filled film caps are rated conservatively so no need to panic.

On my single phase system, my blower motor run cap and compressor motor run caps share a common point. On my system, the brown/white wire was also tied to the compressor run caps. Not sure if it makes a difference. You might want to try switching the brown and the brown/white wires. The cap doesn't care, but the motor might if your setup has any common points.

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#37

Re: Air Conditioner Problem - Thermal Shut Down

06/29/2010 8:00 AM

Hey Whyme, May be your power factor has gone from good to really bad. Check it out, just in case. A bad power factor will heat up some motors.

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