AC Fundamentals and Three-Phase Power

Alternating current reverses direction periodically, at fifty or sixty cycles per second depending on the region. It is used for power distribution because transformers can change its voltage easily, and high voltage transmission loses far less energy than low voltage.

Key quantities

• Voltage — electrical pressure, in volts. AC voltage is normally quoted as an RMS value, which is the equivalent DC value for heating effect.
• Current — rate of flow of charge, in amperes.
• Resistance and impedance — opposition to current. In AC circuits, inductance and capacitance add reactance, and the combination is impedance.
• Frequency — cycles per second, in hertz. It sets the synchronous speed of motors and generators.

Three-phase systems

Three separate voltages, each a third of a cycle apart. The advantages over single phase are substantial: constant total power delivery rather than pulsating, a rotating magnetic field that makes the induction motor possible, and more power transmitted for the same conductor material.

Two voltages exist in any three-phase system: line voltage between any two phases, and phase voltage between a phase and neutral. Line voltage is higher than phase voltage by the square root of three, approximately 1.732. A system described as 400 volts line has roughly 230 volts phase to neutral.

Star and delta

Windings are connected in star, where one end of each winding joins at a common neutral point, or delta, where they form a closed loop with no neutral. Star gives a neutral for single-phase loads and a point to earth the system. Delta is used where no neutral is needed and is common on transformer primaries and inside motors.