DIYbanter - View Single Post - Blew another damn transformer on my Trane XB80

Tony Hwang

John Robertson wrote:
Steve Turner wrote:
On 4/14/2011 2:54 PM, wrote:
On Apr 14, 2:37 pm, Steve
wrote:
On 4/14/2011 12:06 PM, Smitty Two wrote:

In ,
Steve wrote:

On 4/14/2011 12:41 AM, Smitty Two wrote:
In ,
Steve 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.