Help on Transformer loading please

Well if it is 5A RMS (AC) you need then this is easy, 24V * 5A = 120VA, but I suspect it is actually not as you are probably trying for DC.

This then gets difficult as it depends on the conduction angle, but for a reasonable sized cap bank I would probably go for a 225VA.

In fact I would go for a 225VA anyway (if it will fit) as having some spare capacity is never a bad thing and the 225VA will run cooler then the 120Va.

Regards, Dan.

Reply to
Dan Mills
Loading thread data ...

If I need to go from 415v ac 50hz to 24v ac 50hz and draw say 5 amps what calculation do I need to do to work out the size in va of the transformer I need.

Ive found an aparent power triangle but I cant seem to relate it to my problem using a equations I know.

Is it as simple as working out the max current the wire in the windings can take based on its cross sectional area?

Thanks in advance Col.

Reply to

You don't supply a *lot* of information, but consider the following....

VA stands for Volt-Ampere. That is, literally volts times amps. So if you have a load that requires 24 VAC and 5A of current, then the VA is 24x5 =

120 VA.

Using simple turns ratio, the 5A at 24VAC would equate to about 0.3 A at 415 (again, 415x0.3 = ~120VA). Of course, you might want to consider some losses, but this should get you pretty close.

The apparent power triangle would show the relationship between VA, W and VAR for the load. Transformers are rated by VA, which you already have from the above. Of those 120VA, it is possible some of it is reactive and only a portion is going to 'real' power in the load. But the transformer has to supply the whole 120VA so size for that.


Reply to


Why cant it be that simple in the book, thanks alot I got the same numbers but wasnt sure if I was right. Asking the question what is a VA? got I dont know response from everyone at work ;-).

Cheers again Col.

Reply to

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.