I am having problem with my central A/C system. It keeps blowing transformers. I have been battling this for the last month and I live in AZ, not having my A/C is getting old. Here is what I have done so far.

A month ago the fan on my A/C quit working. I could continue to hear the compressor running but the fan motor would not kick on. I went on the roof and on my initial inspection I found the 30 sec timer delay melted at the black wire which goes to the 208V terminal on transformer. I thought, simple fix, new timer. I installed the new 30 sec timer and turned the A/C on. It ran for about 2 min, mean while I could smell electrical burning. Then the transformer popped and threw the circuit breaker on the house.

I figured maybe the transformer had been bad from the beginning and the cause of the failed 30 sec delay relay. I replaced the transformer, attempted to turn the A/C on and once again the compressor kicked on, I could hear the 30 sec relay click on but the fan would not run once again. After 30 sec I could hear the 30 sec relay click on, but the fan didn't come on. After further diagnosis I found the fan run capacitor bad.

I replaced the bad capacitor, turned the A/C on again and was pleased to see that it came on. I left for the night and with the way the A/C had been acting I elected to leave it off while I was gone. I came home the next day, went to turn on the A/C and nothing, not even a click from the thermostat. Back up on the roof I went, and now I was not getting any power out of the 24V side of the transformer. Once again the transformer smelled burnt. Now I have to admit I ordered a used transformer since I was on my second transformer and I still wasn't sure I had the problem fixed.

I thought maybe that's what I get for buy a used electrical part. So I ordered another new transformer. I am now on the 3rd one if your keeping count, I installed it tonight. Fired up the A/C, compressor came on, 30 seconds later the fan kicks on and everything seems great. No burning electrical smell, sounds normal, put all the panels back on and just as I go to step off the roof, poof the A/C shuts off.

After I got it back apart, I could smell the burnt electrical, the transformer was hot, and once again no voltage out of the 24V side of the transformer or the 208V side of the transformer that goes to the 30 delay relay. I did some research online and someone mentioned the compressor contactor being shorter. The blue wire that comes off the 24V terminal of transformer to the contactor does have continuity to ground. I unbolted the contactor so that it was not longer mounted to the metal frame, incase it was grounding through that, and my volt meter still says I have continuity to ground. I am lost at this point, I know that something is over loading the transformer, but I don't know what. Any body have any ideas? Its an older A/C unit, could a fan motor that is wearing out draw to much power to cause this? Do you think its the contactor?

3 transformers, a 30 sec relay, and a capacitor. I am out of ideas..........

 

Welcome to the forums.

You didn't mention what type or brand system you have so I'm going to make a few assumptions.
This is an A/C only system. It sounds like a package unit since the transformer is in with the compressor.

When they said the contactor was shorted..... they meant the coil.... not to ground.
Some systems connect one side of the transformer to ground. That would mean that if any control wire in the unit or going to the thermostat touched ground you would get a short. So you may have a problem in the thermostat wiring.

You need to look at the schematic/wiring diagram and see if the transformer has one leg connected to ground. Normally an A/C only system doesn't ground one of the leads to ground so that if you are measuring either wire going to the transformer showing a short to ground.....trace it out and see where the ground connection is coming from.

A make and model would be of some help.

 

The Unit is a Inter-City Products forced air furnace with cooling unit. Its an older unit manufactured in 1994.

As of right now, with no power to the unit I am showing continuity to ground through the transformer leads and contactor leads. My transformer has 5 leads, 240V, 208V, COM, and the two 24V leads. When the transformer blows I am losing power to the 208V and the two 24V leads. One of the 24 Volt (light blue) leads go to the contactor and then to the thermostat. The read wire off the other 24V lead and the 208V lead go to the 30 sec delay timer. Hopefully this provides more useful information.

It seems like I am only blowing transformers once the fan runs for a min or so...

I have attached a few photos of the contactor/coil and the transformer. I don't know if that helps, not the best pictures. They were taken last night.

 

Since the transformer blowing issue only started after you replaced the delay timer, I'm wondering if there is a possibility that the timer is mis-wired. Did you either take a picture of the old timer (installed) before you removed it, or make a sketch of how it was connected? Also, is the new timer identical to the old one, or is it a different make/model (possibly requiring different wiring)?

 

Stop using the 208 volt lead. Insulate that lead and use ONLY the common and the 240 volt lead.

 

A model number would still be helpful.

 

The timers look identical and the specs match. I took pictures of how it was all wired prior to removing it. You can see in the first picture, what the original one looked like when the A/C first failed. Again at this point in time, if I switched on the thermostat to A/C the compressor fan would fire up and the compressor ran. It was the blower fan that failed to come on, once replacing this 30 timer the blower fan ran for 1 min or so and the transformer blew. The part you see melted in the first photo is the terminal where the 208V lead from the transformer connects to.

The second picture is the new timer

 

Model number reads: PGMD36G0951

 

@Furd, The 208V lead off the transformer runs the 30 sec delay relay to run the blower fan. Without the 208V lead going to that relay the fan won't kick on. Are you saying there is a way to bypass that?

 

In the wiring diagram for your unit I don't see a 30 second delay relay so maybe it was a part installed because it was needed in your application or a later design change.

The following link is your service manual and the wiring is on page 11 for a three ton unit.
ICP/GAS PACK/PGMD 18-60 1PH.pdf

However....I believe I've spotted your problem. In the picture I believe I have a yellow arrow pointing to the 208v tap. There should be NO wires on that tap. The wire that you currently have there needs to go to the 240v connection.

You are drawing the blower power thru the transformer winding and melting it. Power is coming in to transformer on the 240v terminal and right now the blower is trying to run on the 208v tap.

 

That makes sense, when I replaced the delay relay that wire that is currently going to the 208V terminal had fallen off before I got any photos of the original transformer. I saw the bare tap, and it had looked clean, I assumed it had go to that. I didn't have a wiring diagram for the unit. Live and learn I guess, I will have to give that a shot and see how it goes. Thank you for the help, I really appreciate it.

 

It's our pleasure