Blew another damn transformer on my Trane XB80

That is a damn EXPLOSION !!

It happened very suddenly and made a loud bang too.

I said:

" High voltage spikes on the primary could also cause insulation failure leading to the damage seen in the pics - lightning does this sort of thing. So also could back emfs from the blower fan if the is a bad connection in the AC supply feed."

If the insulation on the enamel wire of the primary is punctured by a HIGH VOLTAGE SPIKE, effectively shorting out most of the primary - then the

Cos they just became the fuses.

It just so happens that many small transformers made in China, India & Sri Lanka etc are very prone to this sort of failure - due to bad manufacturing practices.

.... Phil

"Stormin Mormon"

... Phil

I have a guess, maybe the board has to be in sync with another power source. Would reversed polarity make it 180º out of sync?

It doesn't have to dissipate energy to explode. That is a lot of joules.

So I left out the PF, so what?

My first encounter with you, but no doubt others have had the same reaction. Telling an inexperienced OP to put in a 1mF cap without any specifics, and that for the dubious goal of "suppressing" the back EMF of a motor running off the line has a disconnect from reality. Transformers are generally much hardier devices than the solid state components they feed.

Say what you want. You've already made your mark and I really don't care what you add. I've taken away what I need to know about yourself, and the OP is off somewhere else.

Happy now?

Jeff

On 4/11/2011 6:27 AM Tony Miklos spake thus:

Now you're *really* grasping at straws ...

The sad part is Phil usually is right with things involving pro-audio gear. I would take his word hands down on guitar and power amps. But you cannot know everything about any kind of electronic device like he would like others to believe he does. And why he traps people with one decent reply then comes back scathing is beyond me.

"Jeff Thies" Phil Allison

You are one colossally narcissistic fool and a public menace.

God knows what it is you DO know something about - but certainly electricity and electronics are not among them.

Piss off and stop TROLLING .

.... Phil

"Smitty S***ty Two"

I only call f*****ts f*****ts.

Shame is, the whole of usenet has been taken over by them.

.... Phil

"Jeff Thies is a Moron "

I suggested a capacitor of " 1uF rated for continuous use across the AC supply"

Standard practice with inductive loads for decades.

F*CK OFF - DAMN TROLL !!

AC does make things worse, by rapidly repeated severe voltage swings combined with some development of heat.

The capacitor needs to have an actual AC rating, and one that includes your AC voltage with a comfortable safety margin. If it has a DC rating, chances are that will be around 400 volts DC. And not every cap rated

Preferably, it should be "UL recognized" or the like, for some assurance that it is reasonably reliable against failure, or at least an unsafe failure.

Back in the early 1980's, in an experimental sodium lamp ballast, I have blown an 800VDC cap and two 600 VDC ones with 240-260 VAC 60 Hz with less than 10 operating hours of this combined among the three of them. One of those capacitor blowups was a spectacular one that left a major oil stain on the ceiling above. I learned the hard way that actual AC ratings are required here.

===================================

One more thing: If there is a switch upstream of a capacitor across a power line, then the switch may be in for severe contact pitting.

Then again, I doubt a voltage spike lasting long enough to burn out a transformer will be absorbed by a 1 uF cap across a 120V AC line.

I would lean to looking for intermittent overload by the transformer's load, the transformer's load intermittently drawing a large amount of DC (such as by failing-open one diode in a bridge rectifier).

Or, the transformer being connected incorrectly.

(Such as a 120V/240V one with 2 primary winding sections needing both primary sections to be connected in parallel with each other for full power handling at 120V, but only 1 of them is being used.)

"Don Klipstein"

A voltage spike ( or a series of them) can easily cause insulation failure in the enamel winding wire of the primary - then the energy to explode the lead in and lead out wires comes from the 120 volt AC supply.

.... Phil

With AC, volts times amps is not necessarily watts.

If determination of volts and amps are both "true RMS", (non-"true RMS" meters are usually OK for line voltage and for current through a cap across an AC line),

then volts times amps is "volt-amps". Ratio of power consumption (in watts) to VA is power factor. Capacitors have very low, ideally zero, power factor.

Digi-Key has 3 easy enough to find 1uF ones with X1 or X2 rating, all $3-plus plus shipping with minimum order requirement to avoid a surcharge.

Among those 3, my favorite is FC233820105, which is by Vishay/BC Components. Its datasheet refers to 2 UL standards and 2 CSA ones, and has a link to an application note making a claim that the referenced UL ones are sufficient for achieving the "UL Recognized" ("backwards UR") mark.

That one needed me to delve into its datasheet to see that it claims to meet the two relevant UL standards and that it has X2 rating. Its AC voltage rating is 275 volts. 91 cents each plus shipping, with a minimum order requirement to avoid a surcharge.

Any of the 4 capacitors above (2 mentioned specifically) look good, in the unlikely event what is needed is a capacitor across the line.

PF of a capacitor suitable to connect across an AC line is usually less than .01.

Phil Allison is well known in sci.electronics.design to be quick to get brash, even calling people names and sometimes a little worse. If only he would avoid that, a lot more people would think a lot more highly of him than they do now, since he does fairly well know what he is talking about when it's electrical.

I think Rod Speed is much worse. Meanwhile, it appears to me in my experience in s.e.d. that P.A. is, like R.S., an Aussie.

This one is a grasp at a straw. Was a magnet stuck on the transformer at the time that it blew?

I realize that not too many people run transformers with magnets stuck to them. But doing that does make their cores more prone to saturation.

Is the O.P. having other things in his house blowing from voltage spikes severe enough to blow transformer primary winding insulation?

I have seen lots of things blow from line voltage surges, but no transformers indoors blowing when line voltage spikes blow other things.

My experience is that usual 120V-primary step-down transformers can produce pulses of 2 kilovolts when used on pulses in reverse. In fact, I have done that with about 8 different transformers dozens of times each, and none of them lost their ability to do that. (I am aware of line voltage spikes being noted to get even higher.)

There is also the issue of line voltage surges not easily being loaded down by capacitors for whatever reason. I have experience with them blowing things that had capacitors, including a CFL that had probably a 22 or 47 uF capacitor across the output of its internal bridge rectifier as they usually do. No transformer failures in the same house from the same event, though there were electronics failures.

The usual solutions to absorb line voltage spikes are MOVs and other devices that absorb voltage surges by becoming conductive in response to excessive voltage.

The furnace unit and the transformer in question are all we are discussing.

You have clearly not bothered to read my first or my other posts in this thread.

Eg:

" High voltage spikes on the primary could also cause insulation failure leading to the damage seen in the pics - lightning does this sort of thing. So also could back emfs from the blower fan if the is a bad connection in the AC supply feed."

Whenever AC power to that furnace is disconnected, the blower fan will deliver a back emf spike - meanwhile that poor, little tranny is wired in parallel with it. Other devices in the house are NOT involved.

Most AC supply transformers can tolerate repeated 2kV spikes on the primary till the cows come home - but a badly wound one cannot. This is a specific and fairly recent problem with small transformers made in China and elsewhere in Asia where the makers are not fully aware of the issue of insulation failure in the enamel windings.

Once there is a layer to layer insulation failure ( between adjacent wires) in the primary of that tranny - the AC current draw will jump up to many amps and may cause the feed in and out wires to explode - as seen in the pics.

If the spike voltage is suppressed, the tranny will likely survive.

A 1uF cap provides a low impedance path for such a back emf spike, virtually shorting it out.

OTOH a varistor provides no conduction path until its breakdown voltage is exceeded, but is also a means of suppressing the spike voltage to a safe value.

.... Phil

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 :-#)#

So you are agreeing with what I said before - most likely problem is cheap crap component.

We can be relatively sure it wasn't lightning 3 times. If the back emf from the motor is causing the problem, the bad connection to the motor should have made itself VERY evident by now. Not saying it is NOT part of the problem - like you, I wasn't there watching it fail - and neither you nor I heard the noise you speak of.

And IF the problem is what you say it is (and I'm not saying it is not), then probing the old primary and getting to the winding beyond where the solder joint "exploded" you should be able to measure a significantly lower than normal primary resistance.

That measurement has not, as far as I know, been made and reported.

If it was mine, or if I had the transformer at hand, I'd have it apart and analysed in no time. If the primary is shorted, I'd know, within an hour or two of the failure.. And with that second primary, it would not be hard to determine if the

At this point no-one has actually posted FOR SURE what the primary configuration is. Is it a mult-tapped primary, set for 115, 208, and

I don't know this, and you don't know either unless you are clairvoyant, because it has not, to this point, been established and reported.

So we are all guilty of the same thing - making ASSumptions.