Three-phase electric power is a common method of alternating current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-wire three-phase circuit is usually more economical than an equivalent two-wire single-phase circuit advantages of three phase system over single phase system pdf the same line to ground voltage because it uses less conductor material to transmit a given amount of electrical power.

The two-phase system was independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky, Jonas WenstrÃ¶m, John Hopkinson and Nikola Tesla in the late 1880s. Normalized waveforms of the instantaneous voltages in a three-phase system in one cycle with time increasing to the right.

This cycle repeats with the frequency of the power system. In a symmetric three-phase power supply system, three conductors each carry an alternating current of the same frequency and voltage amplitude relative to a common reference but with a phase difference of one third the period.

The common reference is usually connected to ground and often to a current-carrying conductor called the neutral. Due to the phase difference, the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the remaining conductor. This phase delay gives constant power transfer to a balanced linear load.

In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero. In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors.