The most common frequency of alternating current is 60 cycles per second (usually termed 60 cycles) or more commonly, 60 Hertz (Hz). The latter unit is used in recognition of Heinrich Hertz, a German physicist who proved the existence and transmission of electric oscillations.
This designation indicates AC current goes through exactly 60 complete cycles of current alternations in each second. As shown in Fig. 1 (original article), the current wave starts at zero, reaches a peak on the positive side of the zero axis, returns to zero, continues to another peak on the negative side of the zero axis, and then returns to zero again. One positive and one negative loop represent one cycle or Hz. Thus, 60 Hz current goes through 60 complete sets of these positive and negative loops in one second.
If AC current reverses itself 60 times a second, how can it be measured since equal positive and negative values will cancel each other, with a net result of zero amperes? The answer is: The value of AC current is not based on its average value. Instead, AC ammeters really measure the heating effect of the AC current. The ampere scale on an AC ammeter is calibrated in effective amperes, also called root-mean-square (rms) amperes.
To fully understand this concept, let's briefly talk about current and resistance. We know when DC current is passed through a given resistance, heat is produced. Well, AC current also produces heat when it is passed through this same resistance. In both cases, this heating effect is proportional to I2R. That is, this heating effect varies with the square of the current (I2) for a specific resistance (R). The larger the current, the greater the heat produced in a given circuit. Therefore, an AC ampere can be defined as that current, flowing through a given ohmic resistance, which will produce heat at the same rate as a DC ampere.
Fig. 2A (original article) shows DC current is constant while Fig. 2B (original article) shows the effective or true rms AC current is equal in heating effect to 1A DC. Note this current is above the zero axis.
The I2 wave is generated by squaring each instantaneous value of AC current in both the positive and negative loops. Since a negative quantity squared becomes a positive quantity, the I2 wave for the negative loop of AC current appears above the zero axis. The average value of this wave for one cycle equals 1A. The square of 1 equals 1. Therefore, the 1A of effective or true rms AC current shown in Fig. 2B is equivalent to the 1A of DC current shown in Fig. 2A. When squared, the DC current will produce the same heating effect as the 1A of effective (rms) AC current squared.
Important points to remember. If the above discussion is somewhat confusing, just remember the following points. * AC amperes, unless specifically mentioned otherwise in any literature or discussions, are always effective or rms amperes. Motor, electric heating unit, transformer, switch, busway, fuse, circuit breaker, and wire and cable ratings are all given in rms amperes. The calculated currents obtained by using standard electrical equations (See Back To Basics, January '93 issue) to determine loads are also rms amperes. *The peak instantaneous current of a pure, undistorted AC current sine wave is equal to 1.414 times its rms amperes. In other words, the ratio of its peak instantaneous value to its rms value is 1.414.
The ratio of any waveform's peak instantaneous value to its rms value is called its crest factor. Thus, the crest factor of a pure, undistorted sine wave 1.414. Crest factor is important when discussing waveforms distorted by harmonic currents generated by non-linear loads. (See February '93 issue.)