Message text written by "Dave D'Epagnier"
>John, mind telling us how rms power is calculated for a single tone?<
I can try, but I'm hoping that cleverer people than me will tell us where
I've got this wrong...
>The average power is calculated as Vrms squared divided by R. This average
power is also how much heat is delivered to the load. So what is rms power,
and how is it calculated...<
As I understand it, an alternating current of (say) 1A is that which, if it
flows in a non-reactive resistance, produces heat energy at an average rate
equal to that produced by 1A DC flowing in the same resistance. Since the
rate of heat production is I^2R, the equivalent alternating current is such
that the square root of the average I^2 gives the value required, which is
therefore called the root-mean-square or RMS value.
>From memory, if i is the instantaneous value of alternating current flowing
in a non-reactive resistance R, the heat developed in time dt is dw=i^2Rdt.
The mean rate of heating (i.e. the mean power) over a complete period of t
is given by:
P = 1/t {T dw = 1/t {t i^2R.dt = I^2R
} 0 }o
(the curly brackets are the best I can do for an integral!)
>what does it physically mean?<
In the case of a sine wave, I suspect it's where the factor 0.707 comes
from (i.e. Vmax or Imax/root2).
Anyone have a copy of Langford-Smith on the bookshelf? My copy appears to
have warped into hyperspace, and IIRC there's a good treatment of the
subject in there somewhere.
73 John
GW4FRX
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|