Blew another damn transformer on my Trane XB80

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On Sat, 09 Apr 2011 16:04:32 -0500, Steve Turner wrote:

An amp meter goes in series with the secondary (or primary) of the transformer. You can convert amps to volt-amps by multiplying volts times amps. 24 volts times 1 amp = 24 volt-amps. I don't remember if transformers are rated differently for the primary or secondary or which one is usually used. It might be in the technical data usually packed with the transformer or stamped on the transformer somewhere.
If the spec is for the primary then it would be 117 x amps. Of course the measurements should be taken while the unit is in heat mode.
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On 4/9/2011 2:30 PM A. Baum spake thus:

Using the primary side would be more conservative (= a bit safer), since it will include any losses between primary and secondary sides. But it really doesn't matter. And I'd much prefer to measure current on the secondary side, rather than deal with 120-volt juice.
He still has the problem of finding an AC ammeter, since a DMM won't handle AC current.
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On Sat, 09 Apr 2011 14:42:39 -0700, David Nebenzahl

As noted previously, this is NOT the case. An AUTOMOTIVE DMM might not have an AC current scale, but virtually all general purpose meters do. They generally have a "third terminal" used with the common for the high range, which will be what he wants to use since the low range that uses the standard (vom) connections generally tops out at 300ma, more or less.
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On 4/9/2011 6:11 PM snipped-for-privacy@snyder.on.ca spake thus:

Well, all I can say is that *none* of my 3 general-purpose multimeters (2 analog and 1 DMM) have AC current settings. But apparently this is a non-issue in this case, as the OP's meter definitely has AC current capability.
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wrote:

The rating is "output" which means the secondary. Primary power will always be slightly higher due to efficiency issues.
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On 4/8/2011 8:52 PM, Steve Turner wrote:

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
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.
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On 4/8/2011 10:16 PM, Tony Miklos wrote:

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?
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On 4/9/2011 12:19 AM, Steve Turner wrote:

It happens all the time.
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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.
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wrote:

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
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On Sat, 09 Apr 2011 18:33:34 -0700, David Nebenzahl

I read them. I understood them. I was just replying to the notion that he had connected the transformer wrong, based on wire colours.
NEVER ASSume anything - check the instructions, follow the instructions, and verify the results. Particularly with today's supply chain issues you NEVER ASSume something is what it says it is. Quality control is not anything close to what it was in, say, the seventies, and evenTHEN, it was not unheard of to have wrong documentation, or improperly assembled product (but not nearly as common)
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On 4/10/2011 7:30 PM snipped-for-privacy@snyder.on.ca spake thus:

True that.
One thing nobody's yet explained about this whole puzzle is why there's an extra set of leads from the transformer (as shown in the O.P.'s pictures) that aren't being used. What's that all about?
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On 4/10/2011 9:43 PM, David Nebenzahl wrote:

The orange and red leads are to be used when you have 208 or 240vac. On the Honeywell transformers the black is the common and black to white0vac, black to red 8vac and black to orange$0vac supply. The transformer was manufactured to be used on single or three phase systems and you are more likely to wind up with 208vac on a three phase system. The 32volt difference between 208 and 240 could adversely affect your secondary control voltage if you use the wrong tap. Oh yea, I for got to mention that on an electric furnace your supply voltage could be 240vac instead of the 120vac for a gas furnace.
TDD
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<stuff snipped>

Got a propane nozzle for next winter's icy steps from HF that came with instructions for assembling a pick axe hammer. Got an inspection camera, too, that was short the mini-plug to RCA plug cable to allow the video to be sent to a large TV or a recorder. Seeing that it was a video only tool, I assumed it would take a two conductor cable. Tried a video only cable, no joy. Tried a video/1 channel audio cable, still no joy. Tried a video and *stereo* cable, STILL no joy. Called up and ordered the correct cable, although by this time I was suspicious that the cable was deliberately excluded because the function didn't work. Then, sitting at my desk, looking at the picture on the manual, the picture was clear enough to reveal it was a video/stereo audio cable. Then I remembered that they've always been somewhat less than completely interchangeable, so I had several sets lying around. The one for my Sony Handicam didn't work but the cable from a portable LCD cable did! My assumption was a video only device would use a video only cable. I still think it was a good one, but obviously not in this case.
As for quality control, I believe it was Micro$oft who began the outrageous procedure of charging people 30 bucks to get a preview (aka doing the final bug hunt in the real world) of Win98 when it first arrived. IIRC, you didn't even get a discount on the release version for being a beta tester. That's when it seems that QC finally fell off the face of the earth.
-- Bobby G.
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On 4/9/2011 12:19 AM, Steve Turner wrote:

Yes, it's called an open neutral. Can be quite dangerous.
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On Sat, 9 Apr 2011 06:53:25 -0700, "Bill"

MWBC is not legal for a furnace and never has been (at least in Canada) and a main panel neutral problem should have manifested iteself with light bulbs flaring/dimming or other indications of "wonky" voltage elsewhere in the house.

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On 4/9/2011 2:42 PM snipped-for-privacy@snyder.on.ca spake thus:

I agree. Suggesting that the OP's problem is due to a buggy Edison circuit (and why don't they just call them that, since that's what everyone else calls 'em?) is just grasping at straws. Possible, sure, but highly unlikely.
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On 4/8/2011 8:52 PM, Steve Turner wrote:

It would seem to me that you are going to have to get yet another transformer. Put a fuse on the primary, 1/2 A I would think. Slow blow, perhaps less.
Then I'd check a couple things.
Note where it says that the integrated controller is polarity sensitive. Check to make sure it is correct.
Then I would look to see if anything looks overheated on the integrated controller.
I doubt it is any of the motors as those probably all run on line voltage.
Jeff

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
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On 4/8/2011 8:52 PM, Steve Turner wrote:

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf

Steve,
If you are not comfortable with taking current measurements, or do not have an ammeter, or do not wish to sit there waiting for some unusual condition which is drawing too much current, you could temporarily install an in-line fuse holder and automotive cartridge fuse rated at or above the secondary side amperage which the transformer is rated. With this fuse installed on the secondary side, in series with the load, you could then determine if the load is indeed drawing too much current from the secondary and burning out the transformer, versus primary side excess voltage being the problem.
A rough guess would be that the contactor coil should maybe be drawing about a quarter to a half an amp of current at 24V. The transformer secondary should not need to source a lot more than that amount of current to provide adequate power to the coil of the contactor.
Fuses are a lot cheaper than transformers......
There may be an intermittent short in the wiring to the coil, a short in the coil itself, a breakdown of the coil insulation allowing a short to ground when the coil heats up or cools or vibrates, etc.
Smarty
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On 4/9/2011 3:43 PM, Smarty wrote:

Yes Smarty, I would very much like to try this approach because I don't want to zap any more transformers. Lots of people have suggested it, but there appear to be more types of fuses than there are insects, and I have no idea what *exact* type to look for. That's why I asked earlier in this thread if this particular transformer:
http://www.pexsupply.com/White-Rodgers-90-T60C3-Transformer-60VA-50-60-Hz-120-208-240V-Primary-24V-Secondary-Foot-Mount-14937000-p
with a built-in manual reset would be a much cleaner solution that serves the same purpose, but I got no responses.
The specs on the original transformer were: Class 2, 115V primary (60hz), 24V (35VA) secondary. Any chance you could point me to some _exact_ fuse solutions for this application? There's a virtual six-pack of beer in it for you. :-)
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