On 4/9/2011 5:21 PM, Steve Turner wrote:
On 4/9/2011 3:43 PM, Smarty wrote:
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
Yes Smarty, I would very much like to try this approach because I don't
want to zap any more transformers. Lots of people have suggested it, but
there appear to be more types of fuses than there are insects, and I
have no idea what *exact* type to look for. That's why I asked earlier
in this thread if this particular transformer:
http://www.pexsupply.com/White-Rodge...unt-14937000-p
with a built-in manual reset would be a much cleaner solution that
serves the same purpose, but I got no responses.
The specs on the original transformer we Class 2, 115V primary
(60hz), 24V (35VA) secondary. Any chance you could point me to some
_exact_ fuse solutions for this application? There's a virtual six-pack
of beer in it for you. :-)
It's not that hard, or that critical.
Put this on the primary:
http://www.radioshack.com/product/in...ductId=2103752
..25A
This on the secondary:
http://www.radioshack.com/product/in...rodsPerPage=60
1.5A
You can put them in this:
http://www.radioshack.com/product/in...rodsPerPage=60
But first do as I suggested in another thread,look carefully for
anything burnt and check the polarity of the line. Make sure the neutral
is neutral. You can measure from the ground prong. See the note in the
schematic. I'm not so sure this didn't arc to the transformer shell,
from looking at the pic. You may wish to measure voltage (AC) from the
transformer shell to a guaranteed neutral.
It is OK for the transformer to run a little warm, it shouldn't be hot.
There are two kinds of failures, one is a long term thermal failure
(which seemed likely when you said years had gone by), the other is a
transient short, or maybe not so transient. If that is the case then
something else will be bad. It may look burnt, or if it is a diode or
transistor it may just be shorted. You can Google how to test those.
Don't be afraid to put a quick finger on a transistor to see if it is
hot, or on the transformer.
As far as calculating what size fuse, you should know Ohms law. Current
* voltage is watts, or in this case V(olt)A(mps). 120V * 1/4A = 30 VA,
on the out side, 24V * 1.5 = 36 VA.
Note that there is a small surge when you turn on a device. So it may
peak higher at the instant of turning on. A fuse usually can absorb
that. The white ceramics are faster blowing and the slow blow ones
usually have a little coil inside the fuse to show it down and are
marked slow blow. When/if you fix this, leave in the fuses and we can
give you some guidance on more properly sizing them. But some fuse in
the ballpark is definitely better than nothing.
Jeff