Three-Phase Power

It is the standard form of electricity distribution from the power station to the powerpole. Three wires carry three currents that are identical except for their cycles being shifted one third of a cycle from each other. Electrical systems that use more than one phase are called polyphase systems.

Power Station

Power stations usually have one generator for each phase. For each bank of generators, three wires carry the current to the grid, along with a neutral wire which is earthed at the station. Electrical towers carry the three lines side by side, with the neutral wires above them to provide protection from lightning. The frequency of the currents is between 50Hz and 60Hz, depending on the country.

Single Phase

Large industrial equipment requires three phases, but most household appliances only require a single phase. If there are only two wires running from a house to the powerpole, the house has single-phase power. The two wires can be connected to two phases, or to one phase and the neutral. The phase connections are alternated between the three wires on the powerpole to balance the load. A socket that has three phase power will have four pins, while single phase sockets have two or three pins.

Advantages

Three phases is the minimum required to have positive voltage for the entire cycle. Single phase power has negative voltage for half the cycle, while two-phase power has negative voltage at least twice per cycle. Using more than three phases is costly and offers little advantage. Three phases provide a smoother current flow than a single phase, and reduced harsh running in large machines. Induction motors which use three phases are smaller, cheaper, efficient, and have a high starting torque than single phase motors.