How to test new transformer output current.

[mmpaaker]

Hi,

I just wish to know how to test a transformer output ampere without connecting any load.
in here available mostly Chinese made transformers and it not giving actual amperage, some transformers are mention 12v 5A but actually its around 2.5A.
can anyone tell me how to check it without any load ( I want to check output ampere)

Regards.

[IanB]

I think you have to measure the output voltage of the transformer and gradually increase the load while watching the voltage. The voltage will drop as you increase the current. When you have increased the current to the point where the output voltage equals the rated voltage, then you have to run the transformer like that for a while and see what happens to the transformer temperature. If the transformer starts to get hot the current is too much and you need to reduce it.

You cannot do this test without loading the transformer. You might make a guess by looking at the physical size and weight of the transformer, but this will not give you an accurate answer.

[HB9EVI]

You have at least the nominal voltage printed on the transformer? Then you could get a guess about the VA-rating over the open circuit voltage

[David Hess]

I usually estimate the output current per winding by measuring the wire diameter.

[IanB]

I usually estimate the output current per winding by measuring the wire diameter.

However, the wire diameter is not always accessible without dismantling the transformer.

[duak]

As a very, very rough guide to the current rating of a transformer, the DC resistance of the secondary winding should be about 2% of the secondary voltage divided by the rated current.  This comes from the assumptions that typical transformers are designed to be about 95% efficient or 5% lossy, that most of the losses are from resistive heating and that the primary losses are about the same as the secondary.  This assumes magnetic losses are about 1%.  Please note that these are very approximate and are affected by many factors, such as size, core type, material operating temperatures, duty factor, lifetime, etc.

ie. Isec = Vsec/(50Rsec)

eg. 12 V secondary has a resistance of 0.1 ohms.  The above equation gives 2.4 A.

Ultimately, the transformer has to be tested to confirm this but it is a start.

Cheers,

[mmpaaker]

I usually estimate the output current per winding by measuring the wire diameter.

have recently rewind a MOT transformer ( secondary side only) with AWG 10.5 copper cable finally I got 8-0-8v now I know how many volts on it. but how to know how much ampere I can get from this transformer.

Regards.

[ArthurDent]

"have recently rewind a MOT transformer ( secondary side only) with AWG 10.5 copper cable finally I got 8-0-8v..."

Many years ago I used to wind some transformers I needed and a rough estimate of the wire size needed for a certain could be approximated by dividing the circular mils of the wire size by some fudge factor of about 750. I have recently seen that taking the mm square of the wire size and multiplying that times about 2.5 will also give you an estimate.

For AWG 10 wire cir mils is 10383/750 is about 14 amps and mm square is 5.26x2.5 is about 13. These should be reasonable ball park figures. Run the transformer with this load and check for heat rise to make sure the bugger doesn't overheat.

[splin]

It's probably good for 15 to 18A continuous, perhaps 20A for short periods. Ultimately it's limited by the maximum allowed winding temperature, of say 100C (of the hottest winding) which you can calculate by measuring the increase in resistance of the primary and secondary windings after running at load for 20 minutes or so. Resistance increases by .4% per degree C. That gives you the average temperature rise of each winding so assume that the hottest part could be up to 10C higher than average (probably less than that).

Main problem with MOTs is that are designed to run at full load only and have around half the number of primary windings to stop the core saturating with no load leading to horrible idle current and power consumption of perhaps 100W. Under full design load (1400VA or more), the IR drop in the primary reduces the effective primary voltage and thus the core saturation will reduce considerably (if not completely) so the efficiency is not so bad as it might seem.

You can either: 1) add more primary windings (even 10% would help) or
2) run 2 similar transformers in series or
3) use a 240V MOT on a 110V supply.