Blew another damn transformer on my Trane XB80

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A continuation of the "Why does the 115V->24V transformer keep blowing on my Trane XB80?" discussion I started on 04/02/2011.
Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) here:
http://www.flickr.com/photos/bbqboyee/sets/72157626457562742 /
Trane's manual for the unit is here (for perhaps better viewing of the wiring diagrams that I also copied to my above flickr site as jpg images):
http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
As you can probably see in the pictures, there is visible charring of the 115V leads going into the transformer, and of course the 115V circuit is open (again). If you didn't see my first thread, this is the third transformer the unit has blown. In the previous discussion, it was discussed that perhaps the first one just blew because of old age (6 years), and the second blew because it wasn't a proper replacement (poor quality, made in China, etc.). This third unit is most certainly a proper replacement, and it's most certainly indicative of a real problem I have somewhere else in the unit. I didn't see any such charring on the previous two units, at least not like this.
I never got a real chance to test out the system after installing this third transformer. We had cool weather for several days, and I never tried to force the system to come on so I could monitor it; that was probably a mistake. Unfortunately, I was also absent from the premises during the extended times when the unit was most likely operational, so that didn't help either. However, my family tells me that it WAS working and cooling the house rather nicely, for at least a day, perhaps two. I'm getting 115V in all the right places, so it doesn't look like an over-voltage condition to me. Perhaps it's an overheating condition? It looks to me like the only real load on this circuit is the blower motor; could the motor be causing this? The blower spins freely when I turn it by hand. Start capacitor on the motor maybe? Relay on the control board perhaps?
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On 4/8/2011 5:52 PM Steve Turner spake thus:

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
How 'bout installing an in-line fuse next time you replace the xfmr? I'd put it on the secondary side. That way, if there is an overload, the fuse will blow instead.
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On 4/8/2011 7:58 PM, David Nebenzahl wrote:

Thinking maybe I should spring for a transformer with a manual reset, like this one:
http://www.pexsupply.com/White-Rodgers-90-T60C3-Transformer-60VA-50-60-Hz-120-208-240V-Primary-24V-Secondary-Foot-Mount-14937000-p
Would that protect me from having to replace the damn transformer every time? It looks like the circuit breaker is on the 24V output side; I'm a little fuzzy on how that would protect the input side of the transformer...?
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On 4/8/2011 8:22 PM Steve Turner spake thus:

http://www.pexsupply.com/White-Rodgers-90-T60C3-Transformer-60VA-50-60-Hz-120-208-240V-Primary-24V-Secondary-Foot-Mount-14937000-p
Why in the world would you think you need to protect the primary side?
Unless you've got a *really* weird problem--namely, severe overvoltage spikes--there's no way the damage is coming from that side. It's got to be because of overcurrent on the secondary side.
Unless there's something really messed up with your house's wiring ...
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On 4/8/2011 11:38 PM, David Nebenzahl wrote:

Well I'm no electrician so I don't *know*. What I really mean is that I want to protect *myself* from having to drop $20 or $30 a pop on a half-dozen transformers while I'm trying to figure out what's wrong. So again, would the transformer referenced above protect me from that? Many helpful things are being suggested, but unfortunately they all seem to require testing the unit while it's operational, and I can't do that unless I have a working transformer installed.

Ok, and how does that condition come into existence? Is there anything in the wiring diagram that jumps out at you as a potential candidate? I have two identical Trane AC/furnace units; one upstairs and one down. Should I swap control boards between units to see if the problem follows the control board?

The house is only six years old. Nothing's changed in terms of wiring and everything's been fine up until now.
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On 4/9/2011 1:37 AM, Smitty Two wrote:

That 3rd transformer looks like a dual voltage (120/240) input. If it is, it has two primary windings, and both need to be wire up to work. Like this:
http://www.eleinmec.com/figures/027_02.gif
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To OP this is a long shot but... I noticed on the Trane wiring diagram that you posted that the connections for the IGNITION circuit are shown to be near to the connections for the primary of the transformer.
If there is some insulation problem on the ignition wires and the VERY high voltage from the ignition circuit is sparking or jumping over to the transformer primary circuit, this could cause the symptoms you are seeing.
Make sure the wires that are part of the ignition circuit are not even close to the anything else. and inspect them (with the power off) for any sigh of cracks or other faults. These have very high voltage like in a car spark plug and can jump several inches if there is an insulation problem. And the problem would be intermittent. The transformer primary voltage would look perfect except when the high voltage spark jumps over to it and this could easily damage the insulation on the transformer which is not designed for very high voltages.
If you are an electrical novice, you may want to think about calling in for some help at this point.
Mark
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wrote:

Only one almost obvious flaw with that arguement - the ignition is NOT used when the system is running the "cool" cycle - which is what the OP says was running when the second transformer popped.
Also, the secondary resistance will be low enough, and the impedence of the ignitor (assuming it is spark ignited, and not hot surface) high enough, that the voltage across the primary would most likely be clamped to a reasonable level - and if not the electronics on the control board would be damaged, long before the voltage induced in the primary would get high enough to damage the windings or the insulation on them.
If, on the other hand, it is a hot surface ignitor, they run on line voltage (on any I've seen anyway) and a short as you envision would ALMOST DEFINITELY fry the control board.

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On Sat, 09 Apr 2011 22:24:10 -0400, Congoleum Breckenridge

True - SOME dual voltage primaries are wired that way, and need both windings connected either in series or parallel to operate. Others have a center tapped winding - 110 on one half, 220 across the full winding - and still others have 2 separate windings that operate independently.
Real easy to find out - even in this case with the 110v primary open.
Connect 110 across the 220 (or 120 across the 240 - let's not get nit-picky over voltage ratings) or "high voltage" primary and check the secondary voltage. If he only gets 12 volts it is a totally separate and independent high voltage primary. If on the other hand he gets 24 volts, you hit the jackpot and HE screwed up..
This type of "dual primary" circuit is very "polarity" sensitive and the right colour wires need to be connected together, both in series and in parallel, or the "magic smoke" comes out very quickly.
Again , like I posted before - READ the instructions. FOLLOW the instructions, and VERIFY the result.
Part of verifying the result, or erifying an undocummented transformer, is to check the voltage on ALL winding, used or not, to fully understand their intended function.
If 110 across the black and white produces 110 across another pair, that other pair is is a "complimentary" primary and should, in all likelihood, be connected, phase for phase, across the black/white primary pair.
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On 4/9/2011 12:38 AM, David Nebenzahl wrote:

Look at the picture. The primary side has been overheated, the secondary side looks fine. Looks like an over voltage problem to me.
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On 4/9/2011 8:30 AM, Tony Miklos wrote:

I wonder if our intrepid OP has done the basics involved in troubleshooting instead of flitting around from place to place? In the case of a power problem, you always start at the source by measuring voltage and current. It could be a very simple problem like a bit of insulation skinned off a thermostat wire that only shorts out when everything is buttoned up and vibration from the running unit causes the short circuit. If you ever watched the TV program MASH, you would see the surgeons run the intestines looking for a nick in the wall of the organ. Wiring is the same way sometimes. I'll inspect the wiring and often find a small cut or tiny area where insulation is missing and sometimes the wire is bent over a sharp edge of the cabinet where it's fine until you close things up. I wonder if the supply voltage to the transformer is what it's supposed to be? Most domestic household air handler/furnaces are powered by 120vac but some may require 240vac if there are electric heat strips. I would check the voltage feeding the transformer first.
TDD
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On 4/9/2011 10:29 AM, The Daring Dufas wrote:

Yes, I've done all those things. The supply voltage is fine. I've inspected the wiring at great length and can find no evidence of problems; it all looks pristine. It's a gas furnace, and there are no 220 circuits running anywhere near that area of the house.
Thanks.
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On 4/9/2011 11:55 AM, Steve Turner wrote:

It doesn't take any 220 (240) circuits to cause that.
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On 4/9/2011 12:43 PM, Tony Miklos wrote:

I think someone already mentioned the possibility of a bad neutral connection allowing the supply voltage to shoot up.
TDD
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On 4/9/2011 5:19 PM, The Daring Dufas wrote:

You are correct, I mentioned that back a day or three ago.
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Steve Turner wrote:

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
Hi, Is the x-former gets hot when in use? Unless it is under rated for the application, it should npot blow so often. It is just 120-24V AC step down tranny. Buy one of hevier one(bigger size and really heavier in weight than stock) If it blows again, something on the load side is wrong.
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On 4/8/2011 8:44 PM, Tony Hwang wrote:

...
I'd venture something is already wrong...
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Put a voltmeter on the secondary side and measure the voltage when it is not running. THen turn on the AC and see what the secondary voltage does. IF it is supposed to operate a relay, and the current draw is appropriate for the size transformer you have, I would not expect the voltage to drop by more than 20%. Then leaving the AC on, for at least 1/2 hour, keep monitoring the transformer tempoerature. If it is a gradual heating, see how hot it gets. If there is a severe overload, the transform may blow out before it even gets hot to the touch. If it just gets warm over 10 - 20 minutes, then that is probably about normal. IF you can find out what the correct current drain is supposed to be, I would go for a transformer with double that amperage ratinbg just ot be ultra-safe. But you have to be there and monitor things, coming back 48 hours later is not going to work!!!
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I'd agree. Responding to his first post and before he put this latest transformer in I recommended measuring how many amps are being drawn on the low voltage side. This is electricity 101. So, he comes here asking for advice and instead of listening, he justs puts another transformer in and blows it too..... Go figure. Time to either get educated or hire a pro.
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On 4/9/2011 9:12 AM, snipped-for-privacy@optonline.net wrote:

I don't understand the lack of a temporary fuse or circuit breaker in troubleshooting this unit. The supply houses sell a little push to reset breaker that can be installed in series with the transformer. Heck, a box of fuses is less expensive than a transformer. :-)
TDD
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