Can I find transformer regulation?

[fish2005]

I have a big transformer: 170VA, 120V/240V primary and 2x29V secondary.  This is all ratting information that I have.
I need a 24V (at full load - 48W) secondary and is possible to remove some windings for this. My problem: I don't know the transformer regulation, so I don't know what is the no load voltage that I need.
If I remove wire until the no load voltage is 24V, when I hook up the consumer, the voltage will be smaller. Once we connect a load across this winding, current will flow, and there will be a voltage drop across the winding. Hence, the voltage measured On-Load will be smaller than the voltage measure at No-Load.
My application (24V 48W) is very sensible so I cant make an experiment whith it.
Some ideas or formula for this?

 

[audioguru2]

If you counted its secondary turns you would have a formula to calculate how many turns to remove, but the secondary is probably wound in layers and can't be counted accurately.
So just remove turns until the loaded voltage is correct. If you are using both windings in parallel, make the number of turns removed from each winding the same. ;D

 

[fish2005]

Thank's for your answer.
You say:
"So just remove turns until the loaded voltage is correct."
There is the problem.  It's verry simple to remove turns, check whith multimeter and have desired NO LOAD voltage. But can I calculate the NO LOAD voltage for a given FULL LOAD voltage. I need 24VAC at 48W - what is the no load voltage?
My application (24V 48W) is sensible to voltage variations so I can't make an experiment whith it. There is a formula or solution for this problem ?

 

[audioguru2]

The resistance of its primary will cause a fixed voltage drop when loaded and the resistance of the secondaries would also cause a voltage drop . A formula for removing turns could be worked out if you knew the number of turns.

 

[Alun]

Or better still unwind it counting the number of turns then rewind it with thicker wire, this will reduce the secondary resistance, boost the ouput current and improve the regulation.

 

[fish2005]

This is easy. The transformer have 2 separate windings for 29V (is a 2x29V). So I can make 2 taps in windings and conect them in paralel. But I can't do this for primary. In my knowlege there is some losses  in core, too.
I dont know.
Maybe I need some kind of experimental consumer, something at 24V 48W, I don't know what.