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



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.