Tony Miklos wrote:
On 4/11/2011 9:31 PM, John Robertson wrote:
Tony Miklos wrote:
On 4/9/2011 10:06 PM, John Robertson wrote:
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...oduct/22-1666-
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.
So the problem is the joint of the transformer wiring to the lugs. They
may have cold solder joints, or the lug may be slightly loose - enough
to crack the solder connection over time. Your transformer may still be
good!
When you are examining the transformer wire make sure it is stripped
enough to get good solder coverage. Burn off the insulation (enamel) -
do not sand, scrape, or try to erode it because if you scratch the wire
it will break at the scratch.
Re-solder the transformer connectors after first making sure the wire
has at least two complete wraps around the solder lug and the solder
makes a good meniscus joint to the wire and lug. Also use a good grade
of solder, not regular 60/40, find some 63/37 solder (Kester is best) -
this solder resists cold solder joints due to movement when cooling by
solidifying very fast.
John :-#)#
Hi John, fancy meeting you here! Wait, I didn't realize this is cross
posted to sci.electronics.repair Now I know why you are here. Glad you
liked the package.
Tony
Hi Tony,
Yes, we enjoyed the peanut brittle...
I do not think the original poster noticed my suggestion above, that
failure mode is really not going to be too much load or some weird AC
situation - it's either a short to the metal case the transformer is
mounted in or a problem with the wire to solder lug joint.
Would be nice to know if that idea was checked out...
John :-#)#
Yes, the thread got too long. If you saw the photo, and know the
primary went open on 3 transformers, it sure looks like a problem on the
primary side. If the load was too much, those wires on the secondary
side would most likely have been at the very least discolored from the
heat.
Tony
Yes, I looked at the pictures (rather poor resolution). The failure
looks like a connection issue because there is no evidence of windings
overheating. If the output was shorted the transformer would burn up -
assuming it could sink enough current, but it would be unlikely to flare
at one spot. This is a pretty standard 115VAC transformer, I've seen
many of them...
If someone tried to pump 5000 volts into the transformer then possibly
you could get an arc like that to the transformer frame, so that would
mean a serious miss-wiring of the furnace where the igniter somehow was
connected to the primary of the transformer and then turned on.
Hard to see that happening as the transformer powers the logic board
that would enable the igniter to fire up!
John :-#)#
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