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Old 09-11-2020, 04:05 PM   #1
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Kenmore AC 240v/24v transformer keeps blowing up


Hi Guys

I have kenmore AC that I decided to change the thermostat from analog to digtal. Seemed easy enough.

I turned the power off, took photos and replaced with digital, connecting the correct wires (red, yellow, white, green etc). Turned the power back on and the AC worked cooling the house.

Couple of days later the AC stopped working. I diagnosed the problem to the AC transformer. There was 267v going in but nothing coming out. I was expecting 24v.

I replaced the AC transformer and it started working again. Then couple of days later the AC transformer died again.

Question. What could be causing the transformer to blow?
What more diagnosis can I do?

Thanks for your help in advance.
Andy
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Old 09-11-2020, 04:24 PM   #2
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Re: Kenmore AC 240v/24v transformer keeps blowing up


You really should not have more than 240 volts going in. Usually it is between 230-240 volts.

Try contact your elec company and have them check the voltage to your house. That may be blowing it.

You should also check all the thermostat wiring going to the outdoor unit for damage from rodents chewing it or lawn mowers and weed wackers. If it old and bare and shorting then replace it.

You can also add a McGyver 3 amp GM auto spade type fuse in the Red wire ( hot from the transformer ). I use spade connectors and stick them on a 3 amp GM fuse and then wrap it with elec tape and make sure it is not shorting to ground.
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Old 09-11-2020, 05:07 PM   #3
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Yuri,

I have called the power company to check the power coming into the house and make sure it is between 230-240v.

Great catch.
I will keep you posted on what happens.

Thanks
Andy
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Old 09-11-2020, 05:11 PM   #4
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Yuri,

Some video's suggest that the relay switch could have shorted and that could be blowing the transformer.

Could that be possible?

Andy
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Old 09-11-2020, 05:35 PM   #5
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Usually there is no "relay" but a 24 volt contactor in the outdoor unit. They don't usually short out. 99% of the time it is wire damage/bare spots in it shorting to ground.
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Old 09-11-2020, 05:36 PM   #6
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Yuri,

The power company came and checked and said everything is normal.

What's next to check? Why the transformer is dying? Two in a row.

Thanks
Andy
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Old 09-11-2020, 05:40 PM   #7
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Re: Kenmore AC 240v/24v transformer keeps blowing up


The outdoor contractor is not receiving 24v.
It is all coming from the inside transformer. When it dies, then there is no 24v anywhere. Not in the thermostat and not in the outside contractor.

The question is what is killing the transformer?

Thanks in advance.
Andy
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Old 09-11-2020, 05:53 PM   #8
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Did you install a transformer with current protection? Or separate current protection?
If not, and there’s a short, it’ll kill the transformer.
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Old 09-11-2020, 06:05 PM   #9
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Roughneck,

I didn't install a transformer with current protection or separate current protection. I just replaced the original transformer.
The original one worked for 6 years.

Anyway, how could I find the short? Also how could I install transformer with current protection?
Any procedures? Video's

Thanks
Andy
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Old 09-11-2020, 06:17 PM   #10
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Re: Kenmore AC 240v/24v transformer keeps blowing up


I would check your power again to see if it is over 240 volts periodically. Do you have a high quality meter or some el cheapo possibly inaccurate one?

I had autobody shop once blowing transformers on a new furnace every day. I had to get the power company to put a 24 hour metering and monitoring clock on the power supply and lo and behold there were spikes during the day.

They moaned and groaned but eventually put it on and monitored the voltage for a day.

Next day they replaced the pole transformer and all is well.

However if you have a cheap unreliable meter you will look awful silly when the voltage is really OK.

HDepot has some reasonable priced Klein meters.
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Old 09-11-2020, 06:36 PM   #11
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Re: Kenmore AC 240v/24v transformer keeps blowing up


You can put an inline mini-breaker instead of a fuse so you don't go through a lot of fuses.
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Old 09-11-2020, 06:39 PM   #12
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Quote:
Originally Posted by isaggu View Post
Roughneck,

I didn't install a transformer with current protection or separate current protection. I just replaced the original transformer.
The original one worked for 6 years.

Anyway, how could I find the short? Also how could I install transformer with current protection?
Any procedures? Video's

Thanks
Andy
Just get a transformer with an integrated breaker.
What is the output voltage and current of the transformer while the unit is running?
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Old 09-12-2020, 09:44 AM   #13
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Re: Kenmore AC 240v/24v transformer keeps blowing up


Hi Guys,

Right now I don't have access to the AC unit. But I have the transformer I took out.
I am also using a new 600V CAT III multimeter to test the transformer.

I test the primary and secondary connections for resistance. The secondary is showing 1.5 omes and primary is OL

Please explain.

Thanks in advance.
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Old 09-12-2020, 10:18 AM   #14
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Re: Kenmore AC 240v/24v transformer keeps blowing up


OL=Open Line which means it is a open circuit and shot.

I have a bad suspicion that your power company is sending you too much voltage. I would insist they put a recorder clock for 24 hours which measures their voltage and records it on your line. Talk to a supervisor and go over their head if necessary.

Do you have overhead power lines and pole transformers or underground wiring with no overhead wires?
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Old 09-12-2020, 11:13 AM   #15
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Re: Kenmore AC 240v/24v transformer keeps blowing up


It is going to be tough to get the power company to admit their mistake.

I will double check the high voltage on the transformer using the new multimeter.
I will also take out the 2nd transformer and see if it's primary is also showing open line.

I heard sometimes the relay shorts can cause the primary and/or secondary to open in the transformer. What relay can I test to make sure that it is not causing the transformer primary to fail?

The power company already have come once to check high voltage. I want to test all I can before calling them again. How could I eliminate all other failure possibilities?

Thanks in advance.
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