• posted

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.

• posted

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?

• posted

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.

daestrom

• posted

transformer

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.

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