watts vs VA Re: Primary- vs. secondary-current feedback

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 02:28 AM 11/22/2006, Tesla list wrote:
> Original poster: Callum Brieske <callum@xxxxxxxxxxxxxxxx>
>
> Tesla list wrote:
> Perhaps this should be in another thread, But you post reminded me 
> to ask, What is the difference between VA and Watts? The formula for 
> power is P=VA so aren't they the same?
>
> -Callum


*apparent* power is RMS V * RMS I  (that is, if you hook up a 
voltmeter and ammeter, that's what you'll get)

However, if the current isn't in phase with the voltage (that is, if 
you have a load that is partly inductive or capacitive), then the 
actual power (in terms of something that can turn into heat or 
mechanical work) will be less than the apparent power.  Active power 
is what a wattmeter measures.

The ratio of active to apparent power is the "power factor"... =1 for 
a resistive load

Apparent power is in VoltAmps (VA)
Active power is in Watts
Reactive power is in VAR (volt amp reactive)..

Since transformer ratings depend on things like the voltage (flux in 
the core) and current (heating of the windings), fairly independent 
of the power factor, they are rated in VA.