How to measure the rated current of a step down transformer?

[jonslab]

How to measure the maximum current a transformer can handle?

[inse]

If it gets hot, you‘re overdoing it…
There is no general rule, transformers can carry a lot of short term overload.
E.g. (simple) welding transformers have a duty cycle rating and microwave oven transformers wouldn‘t do it without the cooling fan.
Or my soldering gun which packs 100W in a tiny transformer says 15s on / 45s off

[Picuino]

You should not measure it, the manufacturer should give you that information. But if you want to do that job, you should measure the inside temperature of the winding and know the temperature supported by the winding varnish.

Note that the supported current is RMS current, not average current. Therefore, at the output of a transformer with rectification and capacitor filtering there are pulsating current peaks (to charge the capacitor) that have a higher RMS current than the average output current.

[CaptDon]

A simple 'one primary winding / one secondary winding' type of transformer can be estimated by the core size. I tend to look at transformers as fitting into some common core sizes. There are little ones up to around '10VA' which are the doorbell type transformers. There are the very common 25 to 50 VA size, common ones are small filament transformers 6vac@2A or 25.2VAC CT @ 2A Then you get to some common bigger stuff in the 50 and 100VA sizes, these are often seen as 'control' transformers with typical dual primaries / dual secondaries like 120/120 primary and 12/12 secondary. Often these transformers don't even show their current rating, only the VA rating which provides enough information to do the calculation. You can find online information as to calculating the approximate VA rating based on the physical core size. Sadly, with transformers of multiple secondaries such as a filament winding 63vac, a plate winding 700VCT and maybe a bias winding 70vac you can only guess knowing the total VA of the secondaries added together will be approximate to the core size.

[Konkedout]

How to measure the maximum current a transformer can handle?

The simple answer is "based on temperature rise."  So as others have replied, you really need to depend on the manufacturer's rating.

Even the size of the transformer is not an accurate/easy guide.  Mains frequency transformers can be constructed with split bobbin for easy safe construction, or using tape for highest performance/size.
There is also the question of the grade of steel laminations in the core.  Higher grade steel permits higher core flux density which reduces the required number of winding turns, so increases performance/size.

It is complicated....

[MathWizard]

So magnetic metals become demagnetized with high enough temperature, and only the coils inductance keeps the current to sane levels. I guess the non-magnetic wires would loose their insulation long before some Iron core would get hot enough to loose it's coherent magnetic field. But yeah what happen's to a copper inductor's mag field ? I'm just thinking out loud. I'm planning to do a bunch of magnetic's this winter.

When testing transformers, I would find the load line by measuring the voltages for different loads. But yeah, I never tried short circuit tests, not with my old junk transformers from TV and stereo's.

[Ian.M]

The Curie point of all irons and steels is *way* above the max. insulation temperature.  e.g. NiFe is approx.  400 °C,  soft iron is 770  °C,  and typical electrical steels are in-between.   The highest enamelled wire temperature rating is 260 °C, for modified Polyimide coating,  but transformers typically have 200 °C or lower rated wire.

[Picuino]

The temperature does not increase linearly, so it is easier to determine a limit and make fewer errors.
For example, with 4 amps on the secondary, the Joule heating will be proportional to 16 and a proportional temperature increase will be noted (say +48°C, which is acceptable). However with 5 amps on the secondary, the Joule heating will be proportional to 25 and a proportional temperature increase will be noted (say +75ºC, which is no longer acceptable in class A enameled wire).
In this example case, a small increase of 25% of current produces an increase of 56% of temperature. This is without taking into account the losses in the iron.


https://www.linkedin.com/pulse/13-understanding-insulation-class-enameled-8etge

[edpalmer42]

How to measure the maximum current a transformer can handle?

There are no hard answers to this question for mere peasants like us, but there are some rules of thumb.

As has been stated, measure the size of the core and compare it to catalog listings & drawings.  This will give you an estimate of the transformer's total power capability.

But if the transformer has multiple secondary windings, you still have to figure out the output power for each winding.  One reference that I saw said to measure the open circuit voltage and then load the winding until the voltage drops by 10%.  Use that as the load rating.  I would then total the numbers for all the secondary windings and compare it to the rating based on the core size.  If the winding total exceeds the core size value, start derating the windings until everything lines up.

This only applies to mains frequency transformers.  If you're dealing with a transformer for a switching power supply, all bets are off. 

Note that if you're trying to measure temperature rise, it could take an hour or more for the temperature to reach steady-state and, even then, the temperature in the center of the transformer probably can't be measured and will be much higher than the surface.  Also, it's normal for a fully-loaded transformer to be too hot to hold onto.  But, cooler is better for longevity of both the transformer and the other components near it.

[LinuxHata]

If there's single secondary core, than it is very easy.

You'll need wattmeter at input and volts and ampermeter on the output (or another wattmeter) and some high power adjustable resistor.

Start loading transformer and note input wattage and output wattage.
Start to increase load slowly, compare wattages, their ratio should be somewhat fixed. At certain moment, you will notice that transformer starts to draw far more power than it delivers  - this is the peak power. Say, at 100W load transformer inputs 120W power, but if at 200W output, it takes 300W instead of 240W as it should - than you're overloading it.

Winding heating measurement is nice method, but it can take very long period of time on larger or potted transfomers. Wattmeter method is much robust and fast, but it has drawback that it works reliably only on single 2ndary winding transformers.