Blew another damn transformer on my Trane XB80

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:

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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):

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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?

Reply to
Steve Turner
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On 4/8/2011 5:52 PM Steve Turner spake thus:

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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.

Reply to
David Nebenzahl

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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.

Reply to
Tony Hwang

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I'd venture something is already wrong...

Reply to
dpb

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!!!

Reply to
hrhofmann

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Three times it fried the primary? I'd guess it's getting 240vac at times. It's very unusual for the primary to go bad from a short on the secondary side, and the picture shows that the secondary wasn't overheated like the primary was. Where's all those electricians talking about an open neutrals when you need them? If it measures 120vac tie a light bulb into the primary side and leave it where it can be seen. Have the family let you know if it ever gets brighter than normal. Or better yet, call a pro.

Reply to
Tony Miklos

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Thinking maybe I should spring for a transformer with a manual reset, like this one:

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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...?

Reply to
Steve Turner

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That very thought crossed my mind, but I'm not sure how that could happen. The unit is on a dedicated 115V circuit tied to a single-pole breaker. Where would the possibility exist for the neutral to become hot? In the breaker box?

Reply to
Steve Turner

On 4/8/2011 8:22 PM Steve Turner spake thus:

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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 ...

Reply to
David Nebenzahl

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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.

Reply to
Steve Turner

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Look at the picture. The primary side has been overheated, the secondary side looks fine. Looks like an over voltage problem to me.

Reply to
Tony Miklos

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It happens all the time.

Reply to
Tony Miklos

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.

Reply to
trader4

The problem with that theory is that the 120V circuit that supplies power to the transformer also powers the rest of the furnace, ie the inducer blower, main blower, etc. If it were getting 240V, one would think by now one of those other components would have failed. Also, with

240V it would be delivering 48V to the logic board, which one would think would have opened a fuse on it or destroyed some electronics by now.

I can't say I can make much sense of what is going on either. But until someone with some basic electric skills capable of taking some measurements looks at it, I doubt we're gonna fix it by remote analysis.

Reply to
trader4

To OP, I'm not going to wade through all the previous posts...

you pictures show a transformer with 6 leads. the wiring diagrams show a transformer with 4 leads

how are you connecting the transformer..

Do you have a voltmeter... when you connected the transformer does it blow up immediately or after a time?

Did you check the pri and sec voltages..

A failure like shown in the pic with big black scorch marks would indicate a MAJOR overload like you have connected the transformer completely wrong ...or there is some metal box or something that shorts to the terminals. Does the main breaker blow too?

Find a ham radio guy or someone that knows a bit about electricity to help you.

Mark

Reply to
Mark

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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

Reply to
The Daring Dufas

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

Reply to
The Daring Dufas

In case you hadn't noticed, I've been getting LOTS of (much appreciated) advice from many people, but it's also been very contradictory with no single suggested approach. It's been an interesting and informative discussion, and I've been happy that nobody's been a jerk about it... until *now*. And I DID listen; plenty of people suggested that the original transformer was likely to be under-rated and to replace it with something more heavy-duty. Also, it seems to me that having a working transformer is a pre-requisite to following your advice of measuring the amp draw on the low voltage side, no? Unfortunately, time constraints prevented me from being there to perform those measurements when the unit was running, and unfortunately again the transformer blew in my absence. But of course, I'm repeating myself...

Reply to
Steve Turner

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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.

Reply to
Steve Turner

Steve I really hope you can figure out the problem with the furnace and I can understand the bit about time constraints. Installing a fuse in series with with either side of the transformer is cheap insurance to keep from losing another transformer. The fuse holders are inexpensive and it's a lot less of a hassle to replace a blown fuse.

TDD

Reply to
The Daring Dufas

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