Polyphase system
means of distributing alternating-current electrical power
A polyphase system (the term coined by Silvanus Thompson) is a means of distributing alternating-current (AC) electrical power that utilizes more than one AC phase, which refers to the phase offset value (in degrees) between AC in multiple conducting wires; phases may also refer to the corresponding terminals and conductors, as in color codes. Polyphase systems have two or more energized electrical conductors carrying alternating currents with a defined phase between the voltage waves in each conductor. Early systems used 4 wire two-phase with a 90° phase angle, but modern systems almost universally use three-phase voltage, with a phase angle of 120° (or 2π/3 radians).
Polyphase systems are particularly useful for transmitting power to electric motors which rely on alternating current to rotate. Three-phase power is used for industrial applications and for power transmission. Compared to a single-phase, two-wire system, a three-phase three-wire system transmits three times as much power for the same conductor size and voltage, using only 1.5 times as many conductors, making it twice as efficient in conductor utilization. Also, symmetric polyphase motors such as three-phase motors will draw a constant power and provide constant torque, while a single-phase motor will draw a pulsating power and provide pulsating torque with an associate vibration.
Systems with more than three phases are often used for rectifier and power conversion systems, and have been studied for power transmission.
Number of phases
In the very early days of commercial electric power, transmission systems used multiple wires primarily to avoid the safety hazards of high voltage; two common standards used three (two-phase+neutral) and five (four phases+neutral) wires. These systems were obviated by the development of the electrical transformer, but some installations retained a two-phase four-wire system for motors.
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