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Re: Primary- vs. secondary-current feedback
Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi,
At 10:30 PM 11/21/2006, you wrote:
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 Brieske
This page sort of tells "why":
http://www.etasys.com/Main/ABCs/CalcVAWATTS.asp
But in general, unless the load is a "perfect" resistor, the
resistance will vary over time. For inductive or capacitive loads,
this variation is just a time offset. The current might lead or lag
behind the voltage. For say a switching power supply, it might not
make any sense at all!
So "VA" generally refers to the RMS voltage multiplied by the RMS
current. This is very nice in sizing how big of transformer or other
equipment a circuit might need since the raw VA number is always worst case.
But "watts" is "real power". If the voltage and current are both
sine waves that are 90 degrees off from each other (pure inductance
or capacitance) then the "real" power is zero since they
cancel... Real power can also be thought of as how much heat the
load dissipates.
This report shows the different signals for a simple flood lamp and a
nasty spark gap motor.
http://hot-streamer.com/temp/Kill-A-Watt/Kill-A-Watt.html
http://hot-streamer.com/temp/Kill-A-Watt/Tek00005.gif
Voltage is yellow, current is blue, and power is red. It is not so
easy figuring out power without a computerized scope, as used in this
case, that figures out 10,000 data points in real time for you.
Even though the voltage is "tame" the current can be "anything". The
current can even go backwards for a "negative" resistance... This is
why everyone wants "True RMS" meters to show the "real" voltage and
current numbers regardless of how screwed up the signals are.
In general, VA is a worst case number and watts is the "real" number
taking into account that the voltage and current signals might be
very screwed up.
Hope this helps ;-)
Cheers,
Terry