IEEE Power & Energy Magazine - July/August 2020 - 10

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j.paserba@ieee.org. Letters may be edited for publication.

The ac Mystery
I found the article "75 Years of Drift:
The Changing Meaning of Phasor" in
the "History" column of the January/
February 2020 issue of IEEE Power &
Energy Magazine (vol. 18, no. 1) very
interesting. It's good to see a history article that deals primarily with concepts.
I'd like to mention the possibility that
Steinmetz was concerned with the fact
that using a phasor carries with it the implication that the frequency of ac is constant. In the days before generators were
driven by turbines, this was not the case.
Because the reciprocating steam engine
was the most common prime mover, the
angular velocity of the alternator's rotor
changed throughout the cycle. AC was a
great mystery. It did not appear to follow
the rules that governed dc. Capacitive
and inductive reactance were not well
understood-in many cases, not understood at all. P.N. Nunn, one of the ac
pioneers who designed and installed the
Telluride project, said, "In 1890, ac was
just plain freak; it did not follow Ohm's
law and 'clogged' itself in its circuits."
Digital Object Identifier 10.1109/MPE.2020.2985433
Date of current version: 17 June 2020

ieee power & energy magazine

In any case, the big deal in those days
was the lack of any simple ac-calculating
techniques. This may
be Steinmetz's most important gift to electrical
engineering. Before his
development of the complex analysis of electric
circuits, one was faced
with solving the general
equation for electric circuits, which we now associate with dc circuits
only. This was the only
means available. For
more information, see
Alternating Current, by
Frederick Bedell and Albert Crehore (New York: W.J. Johnston
Co., 1893, pp. 83 and 160).
-Bob Barnett

The generation of a sine function in
their book is clearly based on the same
method descr ibed by
Thomson and Tait (my
article cited the book as
it was published in 1867,
though the method may
have been known earlier). But that work gives
no hint of the method's
use in electric power.
I note that Bedell and
Crehore were usually
very careful to say that
the equation "represents"
some physical quantity.
As I am sure you are
aware, that makes conceptual sense. To write that a phasor
measurement unit (PMU) "measures a
phasor" does not make sense. A PMU
measures a signal that we assume is well
represented by a sinusoid, and we then
express that sinusoid in phasor notation.
I fear that as our grid gets smarter
and we get further removed from the
awareness that we are forcing the representation to be sinusoidal, we will
encounter problems. If the signal that
we feed to our PMU is not well represented by a sinusoid, we may very well
get a result that has little or no meaning. And we may not even know it. This
is a matter that goes beyond the question of constant frequency.
-Harold Kirkham

One was faced
with solving
the general
equation for
electric circuits,
which we now
associate with
dc circuits only.

Author's Response
First, I'd like to thank you for your interest in my article and for sharing your
thoughts. You are quite correct about
the fact that the underlying concept in
these mathematical representations demands a constant frequency. Most of
us would be quite unable (or perhaps
unwilling) to try to model a system in
which the effect of the pulsations of the
reciprocating steam engine appeared in
the generated power.
I wonder if Bedell and Crehore were
the first to apply what we would call the
phasor representation to our problem.

p&e

july/august 2020

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