Blew another damn transformer on my Trane XB80

Page 7 of 12  

Phil Allison wrote:

Another question, does he keep buying same x-former over and over or something different little havier one? Along with fuse I'd put in series a low value proper Wattage resistor to lower the primmary voltage little bit. Crazy thinking but hope the x-former is not put in backward.
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snipped-for-privacy@optonline.net wrote:
<SNIP stuff previously said>

Measure both AC and DC current through the secondary of the transformer. If putting a multimeter into DC current mode gets a reading around or over 50 milliamps (,05 amp), then something is wrong with the load. If DC secondary current is near or over 200 milliamps, then, "Houston, we have a problem"! Probably with one of the recently-mentioned discrete diodes in a board's bridge rectifier.
Ideally, DC current through transformer windings should be zero. With exception to specialty cases, typically involving special transformers that use DC-handling means such as gapped cores.
If this tranny is running into problems related to DC, look for DC through the secondary. If that turns up substantially, then the already-mentioned 4-discrete-diode bridge rectifier on the board is likely to be the culprit. Look for solder joints there that need touching up, or else replace the bad diode if one is found to be bad (or all 4 of them), or the whole board.
Depending on ability and willingness to use a soldering iron and to troubleshoot and repair a board to component level, even with pointers to a suspect identified set of components on the board... It may be more practical to get a replacement board if the existing one causes substantial DC to flow through the secondary of the transformer that is prone to failing.
--
- Don Klipstein ( snipped-for-privacy@misty.com)


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"Don Klipstein is Full of Bull "

** Simply fitting that DAMN 1/4 amp s-b fuse will tell you immediately if any such problem exists.
Significant DC in the secondary will causes high current to flow in the primary ( due to core saturation) and BLOW the fuse !!!
If the 1/4 amp fuse holds and the tranny does not get stinking hot - all is OK.
.... Phil
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I have been (sort of) following this conversation. I assume this transformer also drives the contactor for the AC compressor. Maybe there is a short in that circuit and the transformer secondary gets shorted when the thermostat calls for cooling.
David
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It also could be powering a thermostat. Some of them will take power from the transformer circuit. For example, I have a Honeywell VisionPro that has an optional 24V connection to power the thermostat, with batteries then being the backup. Doing it that way allows the display to be backlit 24/7 too.
Also, it's not unusual to have humdifiers tied into the 24V circuit too.
Again, he needs to start doing some basic current measurements, starting at the transformer and find out how much current the whole thing is drawing and then if it's high, work to isolate it. If the current is normal, then I'd put in a 1.5 amp fuse in the secondary temporarily. Contrary to the suggestions to use a fast blow, I'd probably just use a regular one, as whatever is capable of destroying a transformer should be most capable of opening any fuse.

That's a good idea too.

I don't see how he could have a waveform that is so out of shape that it burns up this one specific transformer, yet there are no apparent complaints of any other problems in the house. Or how you'd get such a badly distorted waveform in the house in the first place.

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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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Everything I see on the diagrams says this is a HSI (Hot Surface Igniter) type of furnace. There is no high voltage in that type of igniter. David
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David, I think you are right...
well it sounded good anyway....
regards Mark
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"robb"

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
** From the damage to the tranny visible in the photos - the primary has developed an internal short ( due to insulation failure) and since there is no fuse link in series, the resulting current was high enough to make an exposed wire leading to the terminals explode.
An internal short could develop due to heat alone because of an overload on the tranny - but this requires a fault to exist on the secondary side which seems not to be the case.
High voltage spikes on the primary could also cause insulation failure leading to the damage seen in the pics - lightning does this sort of thing. So also could back emfs from the blower fan if the is a bad connection in the AC supply feed.
I suggest you provide the next replacement for that vulnerable tranny with some "protection" - firstly an in-line fuse of say 1/4 amp AND a capacitor wired across the primary of say 1uF rated for continuous use across the AC supply.
If there is an overload on the tranny, the fuse will blow.
The 1uF capacitor should suppress spike voltages enough to save the tranny from harm.
..... Phil
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"Phil Allison"

** Must be a "slo-blo" type fuse.
.... Phil
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On 4/9/2011 10:22 PM, Phil Allison wrote:

1 uF sounds a little high.
I would think .01 uF at 400V would be a better bet. Or an MOV (metal oxide varistor) or three (across the line and then from each side to ground).
I don't think this is a voltage spike problem though. Voltage spikes take out other components first.
Jeff

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"Jeff Thies" Phil Allison wrote:

** No it ain't.

** Think again.

** Bad idea.

** Not on the AC supply they don't.
..... Phil
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On 4/10/2011 7:47 PM, Phil Allison wrote:

I thought you had simply misspoke and that this was an honest error.
The reactance of a 1uF cap at 60Hz is: 2652 ohms (1/(2*pi*F*C) Online calculator: http://www.kusashi.com/reactance-c.php?f `&c=1&stage=results
V^2/R = W
Assuming primary, as why would you put it on the secondary:
120^2 / 2652 = 5.43 W
Does that not seem wrong to you?
If not then go buy a 200V non polarized 1uF cap. It is no easy chore.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat 1081
Jeff
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On 4/10/2011 9:47 PM, Jeff Thies wrote:

I have some big ass mylars, I think they are 4uF @ 200v.
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"Tony Miklos"
** They won't last long if connected across the AC supply.
.... Phil
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On 4/10/2011 10:48 PM, Phil Allison wrote:

I know, I was just about to reply to my post saying that the voltage rating is VDC.
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"Tony Miklos" Phil Allison wrote:

** Film caps rated at 600VDC will generally last a fair while wired across a 120 volt AC supply ( but not with a 240VAC supply ) - but is it far better to use a purpose designed and agency approved "class X1" or "classX2" capacitor.
... Phil
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"Jeff Thies" Phil Allison wrote:

** Completely.
Such a cap dissipates no energy at all.
What planet do you come from ??

** 1uF caps for use across the AC supply are cheap and plentiful.
Typical examples are metallised polypropylene " class X2 " types and sell for $1 or $2 each.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name99-5466-ND
What planet do you come from ??
.... Phil
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On 4/10/2011 10:46 PM, Phil Allison wrote:

What a jerk you are. You do realize that is 45mA running through that.
Jeff
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"Jeff Thies"

** What sort of know nothing JERK thinks that capacitors dissipate energy ??
Then proceeds to calculate the reactive impedance and treat it the same as resistance ??
Big bad.
Zero out of ten.
.... Phil
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