On 4/8/2011 8:52 PM, Steve Turner wrote:
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...7_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?

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