On Apr 27, 4:01 pm, Roy Lewallen wrote:
K7ITM wrote:

OK, noted, but your definition doesn't match what I was taught and
what is in the Wikipedia definition athttp://en.wikipedia.org/wiki/Phasor_(electronics).
What I was taught, and what I see at that URL, is that the PHASOR is
ONLY the representation of phase and amplitude--that is, ONLY the
A*exp(j*phi). To me, what you guys are calling a phasor is just a
rotating vector describing the whole signal. To me, the value of
using a phasor representation is that it takes time out of the
picture. See alsohttp://people.clarkson.edu/~svoboda/eta/phasors/Phasor10.html,
which defines the phasor very clearly as NOT being a function of time
(assuming things are in steady-state). But in my online search, I
also find other sites that, although they don't bother to actually
define the phasor, show it as a rotating vector. Grrrr. I'll try to
remember to check the couple of books I have that would talk about
phasors to see if I'm misrepresenting them, but I'm pretty sure they
are equally explicit in defining a phasor as a representation of ONLY
the phase and magnitude of the sinusoidal signal, and NOT as a vector
that rotates synchronously with the sinewave.

Tom,

I'm sure a lot of people forget the derivation of a phasor after using
it for a while, just as they do so many other things.

Again, a phasor is a complex representation of a real sinusoidal
function and, as such, definitely has a time varying component. That the
component isn't written doesn't mean it's not there. By all means, check
your texts. I'm sure that any decent circuit analysis text has a
serviceable development of the subject.

I always cringe when I see wikipedia quoted as a reference -- I was
referred to an entry regarding transmission lines some time ago, and it
contained some pretty major misconceptions. That leads me to mistrust it
when looking up a topic which I don't have a good grasp of. I don't have
a full understanding of the process by which it's written, but it seems
that all participants in this newsgroup are equally qualified to create
or modify a wikipedia entry. How could that result in a reliable reference?

Roy Lewallen, W7EL


Hi Roy,

Well, I did not forget the derivation. In Balabanian, "Fundamentals
of Circuit Theory," (a book I have but didn't actually study from) he
uses "sinor" instead of "phasor" but says they are the same, then in a
convoluted way gets around to saying that it's just the phase and
magnitude, and not the real(exp(jwt)) part. Smith, "Circuits,
Devices, and Systems," (most likely the book from which I learned
about phasors) is much clearer about it. Under "Phasor
Representation" in my edition,

"If an instantaneous voltage is described by a sinusoidal function of
time such as
v(t) = V cos (wt + theta)
then v(t) can be interpreted as the "real part of" a complex function
or
v(t) = Re {V exp[j*(wt + theta)]} = Re
{[V*exp(j*theta)]*[exp(j*wt)]} (eqn 3-18)
In the second form of eqn 3-18, the complex function in braces is
separated into two parts; the first is a complex constant, the second
is a function of tiem which implies rotation in the complex plane.
The FIRST PART we DEFINE [Tom's emphasis...] as the phasor (bold) V (/
bold), where
(bold) V (/bold) = V*exp(j*theta)
....
The phasor V is called a "transform" of the voltage v(t); it is
obtained by transforming a function fo time into a complex constant
which retains the essential information. ... "

OK, so your definition is different from mine. So far, I've found two
actual definitions of the phasor on-line, and both agree with my books
and my own useage. But if it's common useage to consider a phasor to
be a rotating vector, I'll defer to that at least in this discussion.
So far, though, I haven't found a reason to give up my definition of a
phasor. ;-)

Cheers,
Tom