How to determine the power capacity of a transformer

Question

I want to use a transformer as a step-down power supply. How can I determine the maximum power of a transformer if there is no data on it. Based on what I have read, I understand that with an increase in the output current, the core begins to saturate, as a result the transformer overheats and the voltage is distorted, am I right? I read a lot on the Internet, but did not find anything certain about the calculations. I can calculate the cross-sectional area of ​​the core, the resistance of the windings and the voltage on the windings, also I can guess what material the core is made of. Can anyone explain what happens when the core is saturated and which calculation would be more practical.

Answer 1

Based on what I have read, I understand that with an increase in the output current, the core begins to saturate

That is wholly incorrect. Saturation occurs with an unloaded secondary winding and has nothing to do with load currents.

the core begins to saturate, as a result the transformer overheats and the voltage is distorted, am I right?

Primary current distorts badly as the core saturates but output voltage usually remains pretty close in shape to the input voltage. Obviously, if you push things too far things get worse but, let me re-emphasize; this has nothing to do with secondary load currents.

Can anyone explain what happens when the core is saturated

The core saturates due to operating the transformer on too high an input voltage or too low an operating frequency or both. When the core saturates, primary inductance drops and a distorted primary current arises but, pretty much the flux produced in the core follows the same waveform shape as the applied voltage. This is why the output voltage is largely unaffected in shape.

which calculation would be more practical.

It depends on what you wish to calculate.

How can I determine the maximum power of a transformer if there is no data on it.

That's an age old problem in electronics for non-engineers and hobbyists. The best plan and one I can thoroughly recommend is this:

$$\color{red}{\text{Buy new parts supported with a data sheet and full supplier support.}}$$

But, I'm bound to say that because I'm an EE and I understand transformers (notwithstanding the fact that most transformers are relied upon for safety).

Answer 2

How can I determine the maximum power of a transformer if there is no data on it.

You do weight it and then compare with transformers of the same construction, UI, EI, ... Typically, they are built in standard sizes, so the weight discrepancy comes with extra terminals, mounting accessories, winding covers.

Of course you do measure also the core cross section and find some formulas on the net.

If there is enough place that you could insert few turns of enameled wire, for example 10 turns, you could use it to determine the primary and secondary turns:

• use variac to apply a low voltage on primary
• measure primary, secondary, tertiary voltage
• calculate turns with a known values of tertiary turns NR and voltage

The rest is already answered by Andy.