Wattage
vs. VA (VoltAmps) and what's the difference...
POWER:
WATTS vs. VA
Power
in a DC circuit is expressed by the formula (P=IE).
Where (P) is the power in Watts, (I) is the
current in Amps, and (E) is the voltage in Volts.
We all feel safe and comfortable with this
power formula.
So what’s this (VA) designation we keep seeing
on transformers and AC power supplies? The key factor in the last sentence is “AC”.
For most applications of AC Watts can still
be used as long as there isn’t a lot of the AC “stuff”
going on known as reactance. We all know that when
AC gets involved, things in electronics get more complicated.
(VA) is the symbol for Apparent Power. Apparent Power
takes into consideration all that complicated AC “stuff”
I just mentioned.
The power in an AC circuit dissipated by a purely resistive
load is called “true power” and is expressed by a
formula that looks a little like the one we just mentioned
for DC. For AC the “true power” is given by the formula (P = E_{rms}
I_{rms}).
The “rms” simply means that with AC we have
to use the average values of the variables for a sine
wave. OK,
so far so good.
Now, what about the AC “stuff”.
Reactance occurs when there is a significant
amount of Inductance or Capacitance in a circuit.
Both these components cause the voltage and the current
to become out of phase with one another.
Yes reactance does draw a strange sort of power
that doesn’t create any heat and doesn’t do any work,
but it is power nonetheless and can zap a little transformer
if it gets too high. The formula for this reactive
power is P_{reac
}= E_{rms} I_{rms} sin
q. q
is the angle between current and voltage, given with
reference to the voltage.
OK, so how do we come up with the Apparent Power (VA),
the total power we can draw out of our little transformer
without smoking it?
If P_{a} represents the apparent power
in an AC circuit, P_{t} represents the true
power, and P_{r} represents the reactive power,
then (P_{a}^{2} = P_{t}^{2}
+ P_{r}^{2}). Or:
P_{a} = ÖP_{t}^{2}
+ P_{r}^{2}
Unless you try to run your air conditioner off a wall
plug transformer (large inductive component), or you
want to send a lightning bolt across the room (large
capacitive component), for most purposes, and I will
let you define most, VA
and Watts can be thought of as the same. You see, the
transformer manufactures just didn’t want to get sued
if you try to start your car with a small 12 V DC transformer.
Of course you wouldn’t try that in the first
place but I am sure someone would.
Disclaimer
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