Letters

Electrical Apparatus, Jun 2008 by Kelly, Michael

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You say phasor, and I say vector

Editor:

Our textbooks define a phasor as a rotating vector and a vector as only having magnitude and direction.

It appears to us that the words are reversed in [a recent EA] article.

Some electrical engineers do use the terms phasor and vector interchangeably. Some older textbooks-such as Reed's 1948 book Alternating Current Circuit Theory-use only the term vector.

However, in our experience, phasors represent a snapshot in time of the relative phase position of variables.

The IEEE 100 Standard Dictionary defines phasor simply as "a complex number."

Some other texts are ambiguous.

One that is not, however, is Foundations of Electric Circuits, by J.R. Cogdell (1999), in which the author states: "A phasor is a complex number containing the amplitude and phase of a sinusoid. . . . Because phasors add like vectors, they are often incorrectly referred to as vectors."

Another author (Chalmers, 1988) distinguishes time phasors without offering clear definitions.

Look at it this way. A sine wave of voltage can be treated as a variable traced out by a rotating vector having the magnitude of the peak voltage, and no particular position in space because of continual rotation.

The corresponding phasor, which we would use in a diagram dealing with phase angles, power factor, and so on, has instead the magnitude of the effective or RMS voltage and is fixed in space along some reference line, or it may be the current that 's the reference.

In any event, the present custom in dealing with diagrams involving phase relationships in a-c circuits is indeed to call them phasor diagrams, although recognizing that the numerical quantities are manipulated "vectorially."Editor

Michael Kelly

University of Missouri

Kansas City