Author Topic: HVAC Temp Fix Help  (Read 477 times)

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Offline FlyingHacker

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HVAC Temp Fix Help
« on: June 18, 2018, 10:25:09 am »
.

Hi,

In 9 days we are getting a completely new HVAC and furnace system. So anything I do here only needs to work for 9 days...

My condenser fan motor keeps shutting off due to thermal overload. This of course happens in the heat of the day (6pm ISH).

UPDATE: THE SOLUTION IN THE VIDEO BELOW HAS WORKED! IT IS PRETTY CRAZY, BUT IT SEEMS TO BE GETTING US THROUGH. See the video for the experimental thermodynamic retrofit.

The motor is new as of last year, and we never had any problems last year. The first time it tripped I checked the run capacitor and it was 4.5 uF, rather than the 7.4uF called for by the motor. I replaced that run capacitor with one that reads spot on 7.5uF. It didn't trip again for a few weeks. These past few days it has been tripping every day. It has been hot (95F).

When this tripped today I cleaned the contacts on the contactor (they were somewhat dirty) with contact cleaner and paper strips. I also raised up the fan blades as high as they would go (went up about 1/2 to 3/4 inch higher than they were, not a huge change). They blades are still nowhere near the top of the condenser coils because the motor operates inverted and keeps them from going up any higher. The orignal motor and fan blades were similar in position.

It is possible that the fan blades are the wrong pitch for this speed motor. I think this motor is 1075RPM, and the original was 1075RPM. However, I had to replace the fan blades because we could not get the fan blades off the old motor (even borrowed a gear pullet from AutoZone... Tried a blowtorch... no luck).

So am trying to determine what my "fix" should be when this happens tomorrow. I could try to reduce the pitch of the fan blade to put less load on the motor. I am not sure how easy this would be to do well. If I don't get all three the exact same with the thing fly off and cause a problem?

Alternately I was thinking about buying about 8-16 pieces of copper pipe about the length of the motor's height. I would them use either zip ties (insulators, but small area) or metal hose clamps (with the screw) to affix these pipe around the outside of the motor in a vertical orientation (like hollow "roller bearing"). The idea would be that the pipes would act as a heat sink, and would take the heat out away from the motor, and also get the heat toward more where more air would flow over it.

I read someplace online that due to everyone else's AC motors and compressors running the power factor of the incoming power may be getting screwed up. Would increasing the run capacitor help? I have the old 4.5uF I could put it in parallel with the 7.5uF.

Any thoughts? The fix just needs to last nine days until the AC guys come to install the new system (they are booked up, like everybody else this season).

Thanks in advance. I know this isn't precisely electronics, but I might want to integrate an Arduino or Pic micro into the circuit to monitor things.. yeah!
« Last Edit: June 25, 2018, 02:43:29 am by FlyingHacker »
--73
 

Offline jheatac

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Re: HVAC Temp Fix Help
« Reply #1 on: June 18, 2018, 10:46:01 am »
Do you have an amp clamp?

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Online sokoloff

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Re: HVAC Temp Fix Help
« Reply #2 on: June 18, 2018, 10:52:41 am »
Spray water onto the condenser (not the fan, the condenser itself) during the heat of the day?
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #3 on: June 18, 2018, 11:03:28 am »
Do you have an amp clamp?


Thanks for the reply.

You mean a clamp on Amp meter? I have an old one semi-accurate. Or some sort of physical clap to use as a heat sink?
--73
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #4 on: June 18, 2018, 11:06:05 am »
Spray water onto the condenser (not the fan, the condenser itself) during the heat of the day?

Thanks for the reply.

Not a bad idea since I don't care too much about the long term effects (minerals in the water collect on the fins as the water evaporates and cause reduced cooling). This would likely let the unit cool better, but would it keep the condenser fan motor from overheating? Cooler is not necessarily better because the blower fan for the house (inside.. the fan that blows the forced air) is pretty poor and the airflow is terrible (yet another reason we are replacing).
--73
 

Offline jheatac

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Re: HVAC Temp Fix Help
« Reply #5 on: June 18, 2018, 11:08:07 am »
If you have a clamp-on amp meter, check the common wire leading to the contactor or defrost control board and compare it to the labeled rla amperage of the motor

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Offline jheatac

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Re: HVAC Temp Fix Help
« Reply #6 on: June 18, 2018, 11:09:02 am »
Let me clear that up by saying the common wire for the condensing fan motor

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Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #7 on: June 18, 2018, 12:03:10 pm »
Well, darn it, it just did it again even though it is about 4 degrees cooler than it was the first time.

The utility voltage is low (112V) due to everyone else’s AC. I assume the motor is drawing too much current. I could measure it, but I am not sure how much I trust this antique clamp on analog meter. Also, what is that going to tell me? That we need to reduce the load. I will also measure the voltage drop across the contractor, but last time is was nominal.

I may end up buying a new motor to use for 9 days. I guess ten bucks a day is probably worth it.
--73
 

Offline rhb

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Re: HVAC Temp Fix Help
« Reply #8 on: June 18, 2018, 12:26:55 pm »
Check the current in both windings and make sure they are about the same.  I had a compressor unit fail recently.  One winding was drawing 6A the other  4A so the internal thermal protection would shut it down.  If they are not the same I know of no cure except a new unit.

Rather annoying as the unit was slated for replacement in the fall after remodeling work was finished.  So the patch was a window unit.
 

Offline jheatac

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Re: HVAC Temp Fix Help
« Reply #9 on: June 18, 2018, 12:27:48 pm »
Most likely you have one of these problems.

1. Motor is overloaded and pulling more amperage than rated.

2. A high ambient rated motor was replaced with a standard temp motor.

3. Condenser coil is plugged with dirt causing a higher discharge air temperature thus overheating the motor.

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Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #10 on: June 18, 2018, 12:28:11 pm »
So I could not get old analog ameter to work..

However, I tried lowering the fan blade. This puts in farther down the shaft. Some website claimed this puts more load on it... is higher more load or less load?

Anyway, since it happened faster when I raised it I figured I would try lowering it.

I tried to bend the fan blades... that is not happening. Too strong, and too easy to break them.

So I guess if it fails again I will just buy another motor. The motor is crazy hot to the touch. So it is definitely being overdriven, but given the heat, it runs constantly.

Any other input is still appreciated. Thanks.
--73
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #11 on: June 18, 2018, 12:30:56 pm »
Most likely you have one of these problems.

1. Motor is overloaded and pulling more amperage than rated.

I think so.

2. A high ambient rated motor was replaced with a standard temp motor.

Not sure, this was sold by a reputable local parts dealer as a universal fan motor for HVAC units.

3. Condenser coil is plugged with dirt causing a higher discharge air temperature thus overheating the motor.

I forgot to mention I cleaned these with a vacuum a few days ago.

--73
 

Offline jheatac

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Re: HVAC Temp Fix Help
« Reply #12 on: June 18, 2018, 12:40:31 pm »
60C rated motors are the most common. Some older units would need a 70c or 85c motor due to smaller coils producing higher temperatures.

Pull the disconnect and pull the grill off the top. Using a jet stream blow water from the inside to the outside. This will flush more debris than a vacuum

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Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #13 on: June 18, 2018, 01:45:37 pm »
60C rated motors are the most common. Some older units would need a 70c or 85c motor due to smaller coils producing higher temperatures.

Pull the disconnect and pull the grill off the top. Using a jet stream blow water from the inside to the outside. This will flush more debris than a vacuum


Interesting on the temp rating. This is an old Carrier (General) unit. It is at least 20 years old if not older (hence the new one coming). So that could be an issue no doubt.

I could try cleaning the coils with a hose tomorrow.

I am wondering if lowering the fan blade isn’t providing better airflow over the motor, since the blades are farther from the motor I would think it might give the air more space to expand towards the center of the blade, and thus over the motor itself. With the blade really close to the motor there is not much opportunity for air to flow over the bottom half of the motor itself.

I should try it both ways and measure the temp with an infrared sensor. But at this point I just want it to work a few more days  :o
--73
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #14 on: June 19, 2018, 09:58:58 am »
So you want a good laugh? Check out this temp fix. We shall see how it works.



Don't try this at home. Sorry about the goofy voice. It seemed like the right voice for somebody doing this.
--73
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #15 on: June 20, 2018, 10:11:19 am »
Amazingly, this thermal experiment seems to be working, and hopefully will keep the HVAC running for the next week until we get the new unit.

Thermodynamics For the Win!
--73
 

Offline Gregg

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Re: HVAC Temp Fix Help
« Reply #16 on: June 20, 2018, 11:12:03 am »
If you have a 12 volt transformer rated for more than the full load amperage of the fan motor, connect it as a boost transformer.  The 12 volt secondary of the transformer connected in series with the power lead to the fan so that the 12 volts is added to the low 112 volt making it a happy 124 volts.  If connected the wrong way around, it will subtract the 12 volts making a buck arrangement.
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #17 on: June 21, 2018, 03:16:34 am »
Interesting idea, and likely a good one in the scheme of things.

I found this article about voltage effects on AC motors. Quite enlightening! It claims a 10% drop in input voltage will result in a 6-7 deg C increase of temperature, which is just enough to make the motor trip.

http://www.burfordphotos.com/EASA%20articles/effects%20of%20high%20or%20low%20voltage.pdf

I really need to double check the wiring, but last I checked I could not find any significant drop across the pull-out disconnect, or the contactor itself. Will have to check at the panel. I did try to tighten the screws where the wires go into the breaker. They seemed OK.

Anyway, a brand new system is coming in a week! (But I will tell them to check the wiring closely...)
--73
 

Offline FlyingHacker

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Re: HVAC Temp Fix Help
« Reply #18 on: June 24, 2018, 09:12:49 am »
I purchased a decent clamp meter. I was able to measure the condenser fan motor at 2.4 amps. The compressor is 17 amps.

Given that the FL amps rating is 1.8 amps we can see why it was overheating! The pitch the fan blades is too great for this unit.

Anyway, it has been working fine for the past week and the whole thing is about to be replaced this week.
--73
 


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