a-c

Alternating current (AC) is electric current that changes direction periodically, flowing forward and backward through a conductor in a repeating cycle. In most industrial and domestic networks, this cycle occurs 50 or 60 times per second, depending on the regional standard. AC is the dominant form of electrical power distribution worldwide because it can be stepped up and down in voltage efficiently using transformers, making long-distance transmission practical and economical.

The voltage and current in an AC circuit vary sinusoidally over time. In a 60 Hz system, the waveform completes 60 full cycles per second; in 50 Hz systems used in Europe, Asia, and Africa, the frequency is correspondingly lower. This oscillating nature distinguishes AC fundamentally from direct current (DC), where electrons flow steadily in one direction. AC power is typically measured in watts for real power, volt-amperes reactive (VAR) for reactive power, and volt-amperes (VA) for apparent power.

AC generation relies on electromagnetic induction. A conductor rotates through a magnetic field, inducing a voltage that naturally alternates as different poles of the field pass the winding. This principle makes AC generation simpler and more robust than equivalent DC systems, particularly at large scales. Most power plants, whether thermal, hydro, or wind-based, generate AC directly.

Practical complications and control

AC systems introduce challenges absent from DC circuits. Inductive and capacitive reactance cause current and voltage to fall out of phase, reducing the real power delivered relative to apparent power. Power factor correction, achieved through capacitor banks or synchronous condensers, addresses this problem in industrial settings. AC circuits also exhibit skin effect at higher frequencies, where current concentrates at the conductor surface rather than flowing uniformly through the cross-section, effectively increasing resistance.

Three-phase AC, standard in industrial distribution, uses three conductors carrying current 120 degrees out of phase with one another. This configuration provides more stable power delivery and better utilization of copper compared to single-phase AC. Motors, in particular, run more efficiently on three-phase supply because the rotating magnetic field they produce is inherently smoother and requires no starting mechanism.