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AC Fundamentals

Jan 1, 2003 12:00 PM, By R. Fehr, P.E., Engineering Consultant

Alternating current circuits improve the versatility and usefulness of electric power systems.

In the late 1800s, Nikola Tesla demonstrated phenomena with alternating current (AC) electricity that weren't possible with direct current (DC) power — transformer action and the principle of the induction motor. AC still plays a vital role in today's energy generation; much of our electricity is generated by turbine-driven alternators, changed in voltage by iron-core transformers, and consumed by induction motors.

AC waveforms. In the United States, electricity is generated at various voltages that oscillate, or alternate, at 60 times per sec. This oscillating voltage is described as having a frequency of 60 cycles per sec, or 60 Hz. As these generated voltages are applied across an impedance or resistance to alternating current, current will flow as defined by Ohm's Law:

V = I × Z

where V, I, and Z represent voltage, current, and impedance, respectively. The current will have the same frequency as the applied voltage, thus the term alternating current.

Since AC voltages are generated by round rotors revolving inside circular stators, the resulting waveform is a sine wave. The height, or amplitude, of a sine wave is called the peak value. Instead of peak values, the amplitude of a sine wave is often described as an effective, or root-mean-square (rms) value, which is about 71% of the peak value (Figure above). Most analog and digital meters measure rms quantities.

Electric and magnetic fields.

AC voltages vary in amplitude as a function of time, so their electric fields also vary with time. Similarly, AC currents produce time-varying magnetic fields. Time-varying fields make possible electromagnetic induction, the principle upon which transformers and induction motors are based. Induction can't occur in DC circuits because the electromagnetic fields in a DC environment are constant. (The process of electromagnetic induction will be discussed next month.)

Skin effect.

Current density, or the density of electrons flowing in a conductor, is uniform across the cross-section of the conductor in DC circuits. But as the frequency of the current increases, the flowing electrons tend to concentrate at the outer surface of the conductor. At 60 Hz, more than 63% of the electrons travel within the outer 1 cm of an aluminum conductor and within the outer .85 cm of a copper conductor. This phenomenon, known as skin effect, reduces the effective cross-section of a conductor, and is a reason for using hollow conductors for some high-ampacity bus work. It's more pronounced at higher frequencies, so harmonic currents can lead to extreme heating of conductors. Where a significant harmonic content is expected, paralleling several smaller conductors is preferred to using one large conductor.

Although inductive and capacitive circuit elements are affected differently by AC and DC, resistive elements behave the same under both conditions. AC circuit analysis methods are more involved than the corresponding DC methods, but the basics remain the same.

Fehr is an engineering consultant based in Clearwater, Fla.

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