sci.electronics.repair@googlegroups.com

Today's topics:

* Blew another damn transformer on my Trane XB80 - 7 messages, 4 authors
http://groups.google.com/group/sci.electronics.repair/t/f883cd09a3a0b791?hl=en

==============================================================================
TOPIC: Blew another damn transformer on my Trane XB80
http://groups.google.com/group/sci.electronics.repair/t/f883cd09a3a0b791?hl=en
==============================================================================

== 1 of 7 ==
Date: Thurs, Apr 14 2011 7:20 pm
From: clare@snyder.on.ca

On Thu, 14 Apr 2011 19:52:52 -0600, Tony Hwang <dragon40@shaw.ca>
wrote:

>
>
>John Robertson wrote:
>> Steve Turner wrote:
>>> On 4/14/2011 2:54 PM, trader4@optonline.net wrote:
>>>> On Apr 14, 2:37 pm, Steve Turner<bbqbo...@swtacobell.net.invalid>
>>>> wrote:
>>>>> On 4/14/2011 12:06 PM, Smitty Two wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> In article<io785f$t8...@dont-email.me>,
>>>>>> Steve Turner<bbqbo...@swtacobell.net.invalid> wrote:
>>>>>
>>>>>>> On 4/14/2011 12:41 AM, Smitty Two wrote:
>>>>>>>> In article<io5tea$b6...@dont-email.me>,
>>>>>>>> Steve Turner<bbqbo...@swtacobell.net.invalid> wrote:
>>>>>
>>>>>>>>> Just mentioned elsewhere that I have a new transformer on order
>>>>>>>>> and I
>>>>>>>>> can't
>>>>>>>>> really do anything until that arrives.
>>>>>
>>>>>>>> Sure you could. You could open up those other two transformers
>>>>>>>> and check
>>>>>>>> for an open thermal fuse. As I recall, you said neither of them
>>>>>>>> failed
>>>>>>>> in the same manner as the third. I still say you could have 3
>>>>>>>> distinct,
>>>>>>>> unrelated failures.
>>>>>
>>>>>>> The design of the first transformer makes it virtually impossible
>>>>>>> to open it
>>>>>>> up
>>>>>>> (remove the tape) without destroying the windings, (which I did)
>>>>>>> so there is
>>>>>>> no
>>>>>>> possibility of "repairing" it by installing a thermal fuse (as some
>>>>>>> suggested).
>>>>>>> There is evidence of burning at the input contactors just like
>>>>>>> there is on
>>>>>>> the third transformer (the one I took pictures of), but not quite
>>>>>>> as bad. I
>>>>>>> wouldn't call it an "explosion". There is no evidence of a thermal
>>>>>>> fuse.
>>>>>
>>>>>>> On the second transformer (a completely different brand and style)
>>>>>>> there was
>>>>>>> no
>>>>>>> evidence of burning. Again, it too was virtually impossible to
>>>>>>> open up with
>>>>>>> destroying it, so I didn't bother. However, because everyone has
>>>>>>> been so
>>>>>>> curious, I just did open it up (destroying it in the process) and
>>>>>>> there IS a
>>>>>>> thermal fuse on the input side; it is open, which explains why
>>>>>>> there is no
>>>>>>> burning.
>>>>>
>>>>>>> The third transformer is similar to the first in design (except
>>>>>>> for the extra
>>>>>>> 208 and 240 legs, which, per the instructions, I taped off and
>>>>>>> ignored). It
>>>>>>> is
>>>>>>> also virtually impossible to remove the tape to gain access to the
>>>>>>> primary
>>>>>>> windings without breaking through the wires, but I did, and again
>>>>>>> there is no
>>>>>>> evidence of a thermal fuse.
>>>>>
>>>>>>> I see no reason to conclude there are "3 distinct, unrelated
>>>>>>> failures".
>>>>>
>>>>>> Just because you can't fix something, it doesn't mean that failure
>>>>>> analysis is moot. I'd be doing serious exploratory surgery on all 3 of
>>>>>> those transformers to try to understand what the hell happened.
>>>>>
>>>>>> I'm not "concluding" that there are 3 distinct unrelated failures, I'm
>>>>>> suggesting it as a possibility.
>>>>>
>>>>>> The first, OEM transformer lasted 6 years. The second transformer
>>>>>> lasted
>>>>>> a few days, and by your own admission was a cheap knockoff that may
>>>>>> not
>>>>>> have been suited for your application at all.
>>>>>
>>>>> Others sorta concluded that for me. It was "Made in China" and has a
>>>>> bit of a
>>>>> different form factor, but that's the only evidence I have so say it
>>>>> might not
>>>>> have been "suitable". The transformer was similarly rated, and
>>>>> internally it
>>>>> doesn't look much different from the others, so I still have no real
>>>>> reason to
>>>>> believe it shouldn't have worked.
>>>>>
>>>>>> And the 3rd transformer may well have failed due to improper
>>>>>> wiring, or
>>>>>> a loose connection leading to excessive current leading to insulation
>>>>>> breakdown and a hard short, or as has been suggested repeatedly, a
>>>>>> fault
>>>>>> somewhere on the secondary side.
>>>>>
>>>>>> You've also agreed that there are wire color discrepancies and several
>>>>>> respondents have opined that both halves of the primary should,
>>>>>> perhaps,
>>>>>> have been wired in parallel to accommodate the current requirements.
>>>>>
>>>>> I brought the old transformer to a local HVAC supply house, and the
>>>>> sales
>>>>> person (whom I perceived to be at least somewhat of an expert), gave
>>>>> me the
>>>>> third transformer as a suitable replacment. I chose to connect the
>>>>> transformer
>>>>> according to the instructions, and not according to any speculation
>>>>> "opined" in
>>>>> this group.
>>>>>
>>>>>> So, I too am curious, have you acquired some appropriate fuses,
>>>>>> figured
>>>>>> out how to monitor the current, and been able to schedule some time
>>>>>> off
>>>>>> from your other responsibilities so that you can actually work through
>>>>>> this puzzle once the new xformer arrives? Are you equipped to bench
>>>>>> test
>>>>>> the new transformer to verify wiring color scheme before you
>>>>>> install it?
>>>>>> Do you know how to test that bridge rectifier? Seems like you have a
>>>>>> *lot* you could be doing while waiting for the UPS guy.
>>>>>
>>>>> I'm sorry you feel that I'm not "doing" anything or that I'm leaving
>>>>> you
>>>>> hanging, but I'll be making the repairs on my own schedule, as time
>>>>> permits.
>>>>> My own place of employ has me working 12 and 16 hour days, so I
>>>>> can't be
>>>>> everywhere at once. Plus, the weather here is pleasant and there's
>>>>> been no
>>>>> real need to run the air conditioner, and the house has two A/C
>>>>> units anyway so
>>>>> I'm in no hurry. I don't have the fuses or fuse holders yet, but the
>>>>> transformer isn't going in without them, so it will be taken care
>>>>> of. My
>>>>> common sense tells me that I really shouldn't *have* to test and
>>>>> verify the
>>>>> living daylights out of a new transformer, but I will most certainly
>>>>> get
>>>>> bitched out here if I don't, so yes, that will be done. I don't
>>>>> really recall
>>>>> the discussion about the bridge rectifier, but I'll look for it. I
>>>>> will be
>>>>> reviewing the entire discussion and make a checklist of all the
>>>>> steps I need to
>>>>> perform.
>>>>>
>>>>> Anything else?- Hide quoted text -
>>>>>
>>>>> - Show quoted text -
>>>>
>>>> After you put it in, measure the current and voltages in both
>>>> the primary and secondary. I'd measure them both with the
>>>> AC running and with it not running.
>>>>
>>>> Also, not sure if anyone ever covered this, but when the transformers
>>>> have failed have they all been in cooling mode, heating, or both?
>>>
>>> Yep, I'll do that. It's only in cooling mode that the transformers
>>> have failed, but I'm in Texas and we're long past the point where
>>> we'll be needing the heater for a while, so it's hard to say whether
>>> or not running the heat would also trigger the problem.
>>>
>>
>> Steve, As said before the best thing to do is to fuse both the primary
>> and secondary windings.
>>
>> If you have the recommended (original) transformer ratings handy that
>> should make it easy to figure out. For example, if the secondary (24VAC
>> as I recall) is rated at 1A, then the primary fuse is 1/5 of that
>> (24:120 - plus a fudge factor) suggesting that a 0.25A slo-blo should
>> last fairly well, and a 1A slo-blo on the secondary winding. If the
>> secondary is rated at 2A, then I'd go with a 0.5A slo-blo on the
>> primary, along with a 2A slo-blo on the secondary.
>>
>> Two fuse holders should be fine - one for the 120VAC side and one for
>> the 24VAC side.
>>
>> Hope this helps - at least it will reduce the cost of replacing
>> transformers!
>>
>> John :-#)#
>>
>Hi,
>Giving up for real solution? Just band aid fix? VChanging blown x-former
>to blown fuse? If I am having problem like that I'd fix it for sure
>dragging out all the tools I need. O'scope, clamp ammeter, DVM, etc.
>Maybe it is asked already but is happening all of sudden or been like
>that since the system got installed? Every symptom has cause.
The fuse is not being put forward as a solution - just to avoid
blowing the transformer while finding the real problem.

First transformer apparently lasted several years - 6 I think from
what I remember of the post. - so no, it was not a problem from the
first install.

== 2 of 7 ==
Date: Thurs, Apr 14 2011 8:04 pm
From: "Phil Allison"

<clare@snyder.on.ca>

> The fuse is not being put forward as a solution - just to avoid
> blowing the transformer while finding the real problem.

** The fuse will actually help you diagnose the problem.

If the fuse opens soon as AC power is applied - the tranny is being
overloaded.

If the fuse opens after some time because insulation in the primary side has
failed, replacement fuses will open immediately despite the secondary being
disconnected.

...Phil

== 3 of 7 ==
Date: Thurs, Apr 14 2011 8:18 pm
From: Tony Hwang

Phil Allison wrote:
> <clare@snyder.on.ca>
>
>> The fuse is not being put forward as a solution - just to avoid
>> blowing the transformer while finding the real problem.
>
>
> ** The fuse will actually help you diagnose the problem.
>
> If the fuse opens soon as AC power is applied - the tranny is being
> overloaded.
>
> If the fuse opens after some time because insulation in the primary side has
> failed, replacement fuses will open immediately despite the secondary being
> disconnected.
>
>
>
> ...Phil
>
>
> Hmm,
This isunthinkab;e crazy idea but is the x-former being put in backward?
Rgwew i a such thing as current limiting x-formers. One thig I'd try
then I'd put in proper Wattage low value resister to lower the primary
voltage.

== 4 of 7 ==
Date: Thurs, Apr 14 2011 8:23 pm
From: Tony Hwang

Phil Allison wrote:
> <clare@snyder.on.ca>
>
>> The fuse is not being put forward as a solution - just to avoid
>> blowing the transformer while finding the real problem.
>
>
> ** The fuse will actually help you diagnose the problem.
>
> If the fuse opens soon as AC power is applied - the tranny is being
> overloaded.
>
> If the fuse opens after some time because insulation in the primary side has
> failed, replacement fuses will open immediately despite the secondary being
> disconnected.
>
>
>
> ...Phil
>
>
>
Hi,
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.

== 5 of 7 ==
Date: Thurs, Apr 14 2011 9:04 pm
From: "Phil Allison"

"Tony Hwang"

> This isunthinkab;e crazy idea but is the x-former being put in backward?

** You must be totally schizo.

== 6 of 7 ==
Date: Thurs, Apr 14 2011 9:26 pm
From: don@manx.misty.com (Don Klipstein)

In <dc989a93-2114-4712-b192-aa6441ae2ce8@r33g2000prh.googlegroups.com>,
trader4@optonline.net wrote:

<SNIP stuff previously said>

>After you put it in, measure the current and voltages in both
> the primary and secondary. I'd measure them both with the
>AC running and with it not running.
>
>Also, not sure if anyone ever covered this, but when the transformers
>have failed have they all been in cooling mode, heating, or both?

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 (don@misty.com)

== 7 of 7 ==
Date: Thurs, Apr 14 2011 9:58 pm
From: "Phil Allison"

"Don Klipstein is Full of Bull "

>
> 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"!

** 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

==============================================================================

You received this message because you are subscribed to the Google Groups "sci.electronics.repair"
group.

To post to this group, visit http://groups.google.com/group/sci.electronics.repair?hl=en

To unsubscribe from this group, send email to sci.electronics.repair+unsubscribe@googlegroups.com

To change the way you get mail from this group, visit:
http://groups.google.com/group/sci.electronics.repair/subscribe?hl=en

To report abuse, send email explaining the problem to abuse@googlegroups.com

==============================================================================
Google Groups: http://groups.google.com/?hl=en