Transformer real output vs rated output

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Hi all!

I am always interested in making more and more electronic projects and keep searching for more stuffs all the time I get.
I always wanted to know what's the actual output that a power transformer can deliver to the load than what's rated by the manufacturer. I know that no machine or device can deliver their maximum full power continuously or not even go to 100℅ which is boasted by any company. So I wanted to know how much a transformer can deliver to the load at the most before overheating or even bursting out!
For example, how much current can a 12 Volts 3 Ampere transformer can deliver at maximum ?
Your question covers many very different types, bvrands and models of transformers, there´s no "one answer fits all" but each case will be different.

In principle, for any serious manufacturer, rated current is what you´ll get, as simple as that.

So a 12V 3A rated one will deliver 3A as long as you wish.
It will warm up, sionce losses arte inevitable, but within reason and specially not going above dangerous temperature, with a generous margin.

A very cheap under rated one, on the contrary, may give you 3A but become unbearably hot after , say, 1/2 hour operation.

But there is not an "universal" rule, except, maybe,"you get what you pay for".
JMFahey is correct, there is no one rule.

In general, a transformer is able to survive through-faults, which is a secondary fault that the transformer feeds. Assuming primary and secondary protection does not operate, of course, which is simply a time/current analysis. Most of the time you will find the protection operates fairly quickly, but in principle the transformer does not care about anything except heat generated in the windings. This takes time.

Mechanical forces are another concern, which is addressed in ANSI C57 standards for large transformers. They are required to survive for at least 2 seconds at what is called the ANSI point. Obviously, these units are out of scope for the little rectifier transformers you are asking about, but this is simply a general discussion of what a transformer can do.

The reason this is possible is that a transformer has leakage reactance (or impedance) that limits the possible fault current that can flow due to a secondary fault. So even if you apply a bolted fault to your transformer secondary, it can only allow a fixed percentage of FLA based on its impedance. It is a current limited passive device.

So the answer to your question is really based on a simple impedance calculation (1 / %imp = %FLA) followed by the significant quantity of time, which will vary greatly between manufacturers. It should come as no surprise that the damage curves for power transformers look logarithmic as a function of i^2*t. You could easily run a unit at 300% with no concern, the question is how long can you do it for? What is the preloading (which determines starting temp), how much cooling are you providing (oil, forced air, sealed), etc.
Don't confuse "maximum full power" with "rated full power". If Triad says their transformer will put out 6A at 24v, it will do so continuously. That does not mean you couldn;t drag more current from it over short periods, nor does it mean the current beyond which the thing would blow up. It only means what they certify it will safely do.


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All transformers will eventually die.

Good transformers will live so long you forget where you got them.

Some big-power transformers have been in service most of a Century and are not expected to fail anytime soon. (Historically they get replaced because power needs always grow, transformer design improves, it does not make sense to keep old/small/lossy lumps in a system many-many times bigger than it was in 1910.)

I've seen enough *cheap* transformers die so soon out of Warranty that I think they must have timers. (But really the makers know the game and can design to just-barely outlive warranty by pinching part-pennies.)

When It Matters, the engineer does math. Power company delivery transformers are expected to live 10 Years at FULL power. But typically carry 2%-10% of rating 364 days a year. So <1% will fail in the 30-50 years before the system gets changed enough to need new transformers. In a factory or mine where the power transformer works hard all the time, they will compare working a just-right transformer AT rating and replacing it every decade against an oversize lump to give 20-30 year expected service. (With present cost of investing for the future, it may not make sense to get the larger lump.)

Industrial Control Transformers, for incidental power in factories, will stand FULL rated power with life of decades. In the NA market an industry standard specs that they shall stand significant over-spec load (the electricians do not always do their sums) and not "burst out", just die in a few years instead of decades. (And the maker application guides urge very conservative derating for starting surges.)

ALL transformers will give SHORT-time output WAY over the rating. Your 25V 3A part may give 15 Amps for most of a minute (at about zero Volts) before it stinks and (couple minutes) fails. At 1.5A it may "live forever" (>50 years), or it may die young of sloppy construction. The 240V 100A transformer for my house would give 30,000 Amps to a dead short on its bolts (but I have proven that 2,000 Amps quickly burns a fuse above the transformer). As Zigzag says, on really big lumps the bolted-short current has to be controlled through design because electromechanical forces can rip the windings apart.
For example, how much current can a 12 Volts 3 Ampere transformer can deliver at maximum ?
On average (if the manufacturer got his statistics right), your 3A/12V xformer will deliver just that for 20,000 hours or whatever is specified.
These are average figures, and one sample might fail after 1,000 hours and another one could survive you and your grandchildren.

You need to remember that the Xformer rating is based on rms current (~resistive load): if you use a rectifier + filter afterwards, the power factor will come into play and limit the DC power available.

All of that is well-known, and you can find estimations, on-line calculators etc. to determine what actual power you can extract based on the basic ratings
And consider the example. Unlike a capacitor which can dry out or chemically break down, a transformer is just a hunk of iron with copper wire wrapped around it. SO unless you run it to the point the wire melts its insulation off, it has little reason to fail other than mechanical stuff.
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