Grant wrote in
:

On Sat, 09 Apr 2011 20:55:37 -0500, Steve Turner
wrote:

On 4/9/2011 7:29 PM, Jim Yanik wrote:
wrote in
m:

"Steve Turner" wrote in message ...

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) he

http://www.flickr.com/photos/bbqboye...7626457562742/

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):



http://www.trane.com/webcache/un/fur.../product/22-16
66- 07_04012009.pdf

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?

a bad start cap would not affect the 24v control transformer,that
just powers the controller board and relays.
either something is loading the transformer or a problem with input
voltage.


Just a thought,

When you put the new transformer in does the "Diagnostic Light"
LED turn on ?
Does the LED blink or flash in one of the patterns indicated in the
"diagnostic codes" section that you posted a link to ?

The "diagnostic codes" imply that the control board can detect
several of the problems mentioned by others.
Maybe a good starting point as there is not enough time to test
with test equipment.

robb



a line voltage monitor may be needed to see if there's some
short-term overvoltage applied to the transformer,and a oscilloscope
may determine if the input waveform is sinusoidal or
otherwise,because 60hz iron core tranformers don't like extreme
distortion on their input,it gets converted to heat,not output
voltage. a DMM will not show line distortion or short term
overvoltages. are the wires burned close to the transformer,or over
their entire length?

I peeled the insulation back from the hot and neutral wires and the
burning only occurred right at the terminals, no more than 1/4" into
the wiring.

Is that a dual primary 115V (230V) xformer? Did you wire both
primaries in parallel for 115V operation?

Grant.


another poster on the alt.home.repair NG made that suggestion/comment.
A very good one,that I had overlooked. good catch!

--
Jim Yanik
jyanik
at
localnet
dot com