Bruce Hebbard wrote:
>
> On Mon, 25 Mar 2002, Bill Hooper in USMA:19071 wrote:
>
> . . .
> > The watt is also equal to a volt-ampere but that is not it's original
> > definition. The watt is originally defined as a joule per second. Since the
> > volt (V) is a joule per coulomb (J/C) and the coulomb (C) is defined as an
> > ampere-second (A�s), it is easy to show that the volt-ampere equals the
> > watt:
> > 1 V�A = (1 J/C)�(1 A) = (1 J/A�s)�(1 A) = 1 J�s = 1 W
> . . .
>
> That of course being the case, could someone please explain why serious
> power equipment often uses VA (kVA, MVA, etc.) as distinct from W ?
>
> I recall recently seeing some small computer UPS (or some other
> equipment) that was rated in both VA and W, with *different* numerical
> values given for each. (Is this an RMS-vs-peak kind of thing?) This has
> always perplexed me when I see VA used on utility transformers, etc.
>
> Bruce H.
I almost put something in my response about this, in anticipation of
the question. Now I wish I had. Nah, not really.
In alternating current (ac) circuits, current that is in phase with
voltage does real work (=I2R=V2/R=IV); this is the only case that occurs
in purely resistive circuits. By "in phase" we mean that the current
rises and falls simultaneously with the voltage.
If there are capacitors, inductors, or both present, the current will
be out of phase with the voltage. Whereas resistors provide resistance
to the flow of current, capacitors and inductors present "reactance" to
the flow of current and this causes the current to either lead (in
capacitative circuits) or lag (in the case of inductive circuits) the
voltage. The current will have the same frequency as the voltage but
will rise either early (lead) or late (lag), compared to the voltage.
Reactances depend on both the amount of capacitance or inductance
present as well as the frequency of the driving voltage.
Think of this reactive current as representing the flow of charges to
temporarily store electric fields in capacitors or magnetic fields in
inductors; these fields build in one direction, collapse, build in the
other direction, and collapse all within one cycle of the alternating
current. This it seems as if the energy is merely sloshing around.
In the rare event that the capacitative and the inductive reactances
balance each other (i.e., they are equal in magnitude), then the circuit
behaves as a purely resistive circuit at that on frequency, the
frequency of resonsance. In the more common case where they do not
balance out, then "extra" current flows above what is used to do real
work.
To distinguish these situations, power companies and builders of
motors, generators, etc. often use watts to designate the product of
I(resistive)V where I(resistive) is the component of current in phase
with voltage and doing real work. They then use volt amperes to
designate the product of I(reactive)V where I(reactive) is the component
of current that is 90 out of phase with voltage; this is the current
that flows as a result of the inductors or capacitors present. They also
use volt amperes to represent the product I(total)V where I(total) is
the "hypotenuse" of the triangle that also includes the perpendicular
I(resistive) and I(reactive). As you might guess, I(total) is then the
total or observed current. (There is a similar right-triangle whose legs
are resistance and reactance and whose hypotenuse is impedance; all
three of these terms are measured in ohms.)
What this amounts to is a subtle way of using units to qualify or
designate the quantity. Watts indicate the rate at which energy is used
to do real work. Volt ampereres are used to designate the rate at which
energy "sloshes around" and into and out of reactive components
(inductors and capacitors), and which does NOT do real work. That
reactive current does incur a cost, though, since it must travel through
the same wires, heating them up as it sloshes around and that tiny
fraction is "real work" in the physics sense, but "useless work" in the
engineering sense -- the motor gets hotter but doesn't pump water any
faster.
Gene Mechtly and Bill Hooper, as well as others, may feel that I have
painted this picture rather vaguely and they may have some quibbles
about some of my terminology, but I have tried to present this as much
as possible in laymen's terms. Believe it or not, that's about as simple
an explanation as I can give of the "distinction" made in industry
between watts and volt amperes.
Go, the lesson is ended.
Jim
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone/FAX: 843.225.6789
