Capacitors, Magnetic Circuits, and Transformers is a free introductory textbook on the physics of capacitors, coils, and transformers. See the editorial for more information....


The real power consumed in a circuit is the average of the instantaneous values over a given period of time. For a-c circuits, the real power is the average taken over one or more complete cycles. Thus, if a complex voltage wave is expressed by


and a complex current wave by


the real power is then obtained as follows


where the product v(t)i(t) results in the following series


It is not necessary to integrate every term in the series represented in Eq. 5-34 in order to evaluate the series. However, by considering 9 of the several typical terms, the integral can be evaluated in a simpler manner. Let us first consider the first term in the series of Eq. 5-34 as follows


Then consider the product of the nth voltage harmonic and the nth current harmonic when n may have any integral value 1, 2, 3, etc.


There is also in Eq. 5-37 the series of cross-products represented by the term

which can be reduced to


The average value of the term above, taken over a complete cycle, is zero. From this it is apparent that the real power associated with a voltage harmonic of a given order and a current harmonic of a different order is zero. Therefore, the expression for the power obtained by carrying out the integration of Eq. 5-33 becomes


In Eq. 5-38, the angle


or the angle by which the nth current harmonic lags or leads the nth voltage harmonic, and


where PFn is the power factor associated with the nth voltage harmonic and the wth current harmonic. In the case of a sinusoidal voltage and a non-sinusoidal current, the power is the product of the effective value of the voltage, the effective value of the fundamental in the current, and the cosine of the angle by which the fundamental in the current is displaced from the voltage. The same consideration applies to a sinusoidal current and nonsinus-oidal voltage.

Last Update: 2011-01-13