why on the primary side?

[mrssb]

As a student and always trying to learn I have a question for you guys.
I was working on a Trane furnace TUD080R, and upon arrival I found a bad XFMR with an open primary side. I inspected furnace and could not find any problems with the wiring so installed another transformer. Reset power called for A/C and the smoke came out of that transformer as well(I really hate when that happens, as I have never figured out how to get the smoke back in). So I started doing some digging and found out that the two wire feeding the condensing unit was shorted together. The weird part is that it never popped the 5amp fuse on the board, nor did it short out the secondary side. Can somebody please explain????

[hvacman14]

funny that you ask this, i was going to post the same question, i have a bryant heatpump airhandler doing the same thing, first visit, bad control baord, replaced two weeks later replace xfmr and now about every two weeks xfmr is bad, always on primary side fused every low voltage wire, hi to xfmr is fused, none of them will blow,have changed outdoor board, indoor board, all new low voltage wiring, relays, low volt coils, almost everything is new... will get a call unit down again, check it and sure enough no 24. secondary is closed, primary open, so if anyone has answers please help because im one more xfmr away from dynomiting this thing

gary

[Edited by hvacman14 on 03-25-2005 at 03:27 PM]

[cehs]

I'll take a stab at this,

Many furnace mfgr's use a 3A fuse.
Using ohm's law, P=EI which means that 3times24 is 72 watts.
Since most furnace transformers are 40VA (or 40 watts) they will run hot and burn out in short order when the 3 amp limit is close but not reached.
Most transformers will handle double the VA rating for short periods of time, but not for prolonged overages.

I would try using a smaller amp fuse in some of the 24V lines and see what happens...

[mrssb]

One thing I failed to mention the amp draw reading on the secondary side was @5.4amps, before I repaired the shorted wiring. Somebody correct me if I am wrong but the fuse on the circuit board is supposed to protect the board as well as the transformer. Why at this amperage would it not blow first? I have asked this question to Trane, but have not as of yet received an answer. Thanks for your help.

[fat eddy]

Put a bigger xfrmr in no the fuses protect the xfrmr not the board

[aircooled53]

Originally posted by mrssb
One thing I failed to mention the amp draw reading on the secondary side was @5.4amps, before I repaired the shorted wiring. Somebody correct me if I am wrong but the fuse on the circuit board is supposed to protect the board as well as the transformer. Why at this amperage would it not blow first? I have asked this question to Trane, but have not as of yet received an answer. Thanks for your help.

You have a short in the low voltage to be pulling 5.4 amps.
Try checking amps on each wire cooling,heating/fan circuit.
r=.4
y=.4
w=.4
g=.4

Never had a secondary side pull more than about 1.2 under load.

[bornriding]

The transformer primary winding is made of much smaller wire than the secondary, and on a 40VA control transformer, the primary current is only .67 amps. But when the secondary current starts to go up ( as from the shorted circuit you had ), the primary current will also go up, trying to maintain the same VA. The secondary current is rated at 2.6 amps. Hopefully you can see where the small wire rated for .67 amps ( primary ) will burn up faster than the larger secondary winding normally.
However, this is like the 'chain', it will break first at its weakest link. maybe the primary has a 'bad' spot on it - it will go first.
If the secondary winding has a 'bad' spot on it, it might open first.
Generally though, on a shorted secondary, the primary winding will open first.
And the fuse on the board has to be rated high due to 'inrush' current of the devices that is controlled by it.

[conrad1]

next time you have a xfmr out place a fuse in between and if you have a short it will pop the fuse rather than a perfectly good xfmr

[Wild Leg]

Originally posted by bornriding
The transformer primary winding is made of much smaller wire than the secondary, and on a 40VA control transformer, the primary current is only .67 amps. But when the secondary current starts to go up ( as from the shorted circuit you had ), the primary current will also go up, trying to maintain the same VA. The secondary current is rated at 2.6 amps. Hopefully you can see where the small wire rated for .67 amps ( primary ) will burn up faster than the larger secondary winding normally.
However, this is like the 'chain', it will break first at its weakest link. maybe the primary has a 'bad' spot on it - it will go first.
If the secondary winding has a 'bad' spot on it, it might open first.
Generally though, on a shorted secondary, the primary winding will open first.
And the fuse on the board has to be rated high due to 'inrush' current of the devices that is controlled by it.

That's not a bad description.

[docholiday]

Hmmm, I'll start with a question...

Isnt the entire secondary side (lov voltage circuits) nothing more than an iductive load for the high voltage side?

A 240v-24 volt transformer is easily explained since the secondary is 1/10th of the primary.

The primary winding is only respoinsible for 1/10th the current the secondary is. For example if you are pulling 1 amp on the secondary side, you should only read about .1 amp on the primary side. As mentioned the wire gauge is much smaller for several reasons. First, there are 10 times as many turns around the iron core on the primary than the secondary in the 240-24v transformer, a 115-24v transformer will have roughly 5 turns on the primary for every turn on the secondary.

A 115v-24v transformer with a 40VA rating has a secondary capacity of 1.67amps. The 230v-24v 40VA has obviously twice the current capacity at just over 3.2amps.

If you short the secondary, which is the primary's load the primary side will fail as it is the weakest link. A 3 amp fuse on the secondary would be the weakest link if it were present. a .3 amp fuse on the primary would also be weaker than the primary winding. Lesson, learned? A fuse is alot cheaper than the transformer.

Many lower cost units do not include fuses because it is an added feature, not a requirement. The only requirement is to have a load within the boundries of the transformer used. Picture the fuse as a high pressure switch, an optional control used to prevent larger damage should a mishap occur.

Kinda like expecting a yugo to have the same safety features a volvo might have.

[cem-bsee]

a bad xformer primary can also be hazardous! I ALWAYS specified having a transformer fused in the primary -- even larger than 2000 kVA @15kv/ 480V 3ph.

Also, I do not think that most electronics designers understand anything over 6 volts, especially since solid state devices came into general usage.

[Jultzya]

Originally posted by docholiday
A 115v-24v transformer with a 40VA rating has a secondary capacity of 1.67amps. The 230v-24v 40VA has obviously twice the current capacity at just over 3.2amps.

40 VA = 40/24 = 1.67

How do you figure that the 240 V transformer has twice the capacity on the secondary? If it did, it'd be rated a 80 VA transformer.

[bornriding]

I apologize - misquoted - sec amps should be 1.67 not 2.67
I should be flogged
I was thinking too fast

[dmoneyc22]

i work on and install alot of trane and carrier systems and i to have had this same problem however i have been able to locate the problem. i usually find that there is a rubbout in the condensing unit that causes this either the low pressuce control or the rv def sensor all controls check it out

[docholiday]

Jultza,

Me too, you are right. I stand corrected, thanks. VA is volt amps. I think I was heading to make a point about primary and secondary fuses and got sidetracked.

So a 240v-24v transformer with a .32 amp primary would give you a 3.2 amp secondary or about 80VA. and a 115v-24v transformer with the same primary capacity of .32 amps would give you about a 1.67 amp secondary or 40 VA.

[Edited by docholiday on 03-26-2005 at 11:38 AM]

[bodhisatva]

"If you short the secondary, which is the primary's load the primary side will fail as it is the weakest link. A 3 amp fuse on the secondary would be the weakest link if it were present. a .3 amp fuse on the primary would also be weaker than the primary winding. Lesson, learned? A fuse is alot cheaper than the transformer."

Yeah but where can I get a .3A fuse? Something they sell @ Radio Shack? I have enough trouble getting 3A fuses.

[dmoneyc22]

you can go to a gas station and puchase a 5 amp auto fuse and do the same thing
push come to shove you do what ya gotta do lol

[docholiday]

I didnt say it was easy but yes Radio Shack would be one place. My point was that either location would save the transformer. The last post suggested a 5amp fuse. In this case it will be the last thing to fail and pretty much a waste of time, especially if you are dealing with a 40va 115-24v transformer which is only rated at 1.67amps.

[wb]

Those newer style automotive fuses work good with regular slip-on type crimp connectors. Put one of these fuses in series with the secondary.

[majormickey]

I ran into similar problem with a furnace board and a hot surface ignitor burning out for no obvious reasons.I was originally sent to the job site to do a fall service/cleaning.A couple of days passed and then the call-back came,{no heat}.After troubleshooting and finding nothing I checked the off/on switch at the ceiling servicing the furnace.The problem was that they were switching the neautral and not the hot.On my first visit I turned the switch to off thereby causing the 110 volt to back feed through the entire system.{Everything ran fine as long as the off/on switch wasn't used}.The fuse on the board never did get damaged but it burnt out the board and the H.S.I.Anyway after rewiring the off/on,and replacing the parts,everything was OK.