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. The chief advantage of the latter was that the winding configuration was the same as for a single-phase capacitor-start motor and, by using a four-wire system, conceptually the phases were independent and easy to analyse with mathematical tools available at the time. Two-phase systems can also be implemented using three wires (two "hot" plus a common neutral). However this introduces asymmetry; the voltage drop in the neutral makes the phases not exactly 90 degrees apart. Two-phase systems have been replaced with three-phase systems. The move from two to three phases was originally motivated by making a more ideal rotating field for AC motors: Mikhail Dolivo-Dobrovolsky calculated that, for simple winding configurations of the time, the magnetic field fluctuation can be reduced from 40% to 15%. This is less important in modern machines that create a nearly ideal rotating field using sinusoidally distributed windings, but three-phase systems retain other advantages. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer, which can also produce three-phase power from a two-phase input. A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order. A 3-wire system with two phase conductors 180 degrees apart is still only single phase. Such systems are sometimes described as split-phase.
Motors
Polyphase power is particularly useful in AC motors, such as the induction motor, where it generates a rotating magnetic field. When a three-or-more-phase supply completes one full cycle, the magnetic field of a two-poles-per-phase motor has rotated through 360° in physical space; motors with more than two poles per phase require more power supply cycles to complete one physical revolution of the magnetic field and so these motors run more slowly. Induction motors using a rotating magnetic field were independently invented by Galileo Ferraris and Nikola Tesla and developed in a three-phase form by Mikhail Dolivo-Dobrovolsky in 1889. Previously all commercial motors were DC, with expensive commutators, high-maintenance brushes and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, are self-starting and have little vibration compared with single-phase motors.
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