Editing Polyphase system (section)

==Number of phases==
In the very early days of commercial electric power, some installations used [[Two-phase electric power|two-phase]] four-wire systems for motors. The chief advantage of these 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.<ref>Terrell Croft, ''American Electricians' Handbook, Sixth Edition'', McGraw Hill, 1948, pp. 54–57</ref>

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%{{cn|date=April 2024}}. This is less important in modern machines that create a nearly ideal rotating field using [[Coil_winding_technology#Characterization_of_distributed_winding|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-T transformer|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 electric power|split-phase]].