How to work out va or watts

[Daemon]

Hi all

I am looking at getting a transformer second hand. How do I work out the va or watts.

Its a 230v to 12v or 24v and I am looking at it for 230v to 24v so I assume it has 2 12v secondaries.

I know the formula is VA=Amps*volts/PF

I am assuming a power factor of 60% so it would be amps*volts/0.6

the thing I don't know is, is the volts in the equation the input or output and what happens if I wire it in parallel(I assume I can just join the secondaries together)

Thanks Sparkies

[jgrei1]

just out of interest, why are you assuming a power factor of .6? what is the load?

regards, jeremy

[Daemon]

I read on the interwebs that a rule of thumb was to allow a 60% power factor.

The load will be 24v lights

[omzig]

You only have to be concerned about the power factor on the load side. Assuming that you are only using filament lights as a load, the power factor is 1.

[Daemon]

Okay.

So is it VA=AMPS*230 or VA=AMPS*12 or VA=AMPS*24 in this case and what happens to the amps when its wired in parallel.

For instance if each 12v secondary is labelled 5 amps would it be VA=5*230 or VA=5*12 and double the amps in parallel to VA=10*24

Thanks
Steve

[kmiller]

Okay.

So is it VA=AMPS*230 or VA=AMPS*12 or VA=AMPS*24

The short answer is 'yes' to all three, depending on which "amps" you are referring to. For example, if the transformer was marked 100VA and had two 12V secondary windings, it would breakdown as follows:

Primary rating in amps = 100/230 = 0.43 primary amps, regardless of the secondary connection.

If the output windings were connected in series for a 24V output, the amp rating on the secondary would be 100/24 = 4.17 amps.

If you paralled the secondary windings for 12 volt operation, the secondary side amp rating would be 100/12 = 8.33 amps.

**IMPORTANT: When connecting the secondary, you must be careful of polarities. If you tried to parallel the two windings, but connected them with opposing polarities, you will need to buy a new transformer that still has the smoke contained in it.

Hope this helps.

[BuzzKill]

I thought VA was just a marketing number that UPS companies used to make their products look like they put out more power than they actually do?

[lafrost]

Hi All,

VA is actually watts. V= Volts and A= Amps, so ohms law states Volts x Amps = Watts. The other main thing to remember is the power in the primary is equal to the power in the secondary. i.e. If I have a 60VA secondary (60 watts) I must have 60 watts in the primary. From this we can work out the primary current, so in a 12v secondary the current is 5 amps, but in the 240v primary the current is .25 amps or 250 milliamps.

Remember the ohms law formula 'PIE' Power = Amps(I) x Volts (E)

I hope this helps.
Laurie

[Daemon]

The short answer is 'yes' to all three, depending on which "amps" you are referring to. For example, if the transformer was marked 100VA and had two 12V secondary windings, it would breakdown as follows:



Thanks. That is clearer now

Cheers
Steve