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What Is Three-Phase Electric Power?

Allan Robinson
Allan Robinson

Three-phase electric power is a system of delivering electricity that involves three alternating currents. These currents have the same frequency and reach their peak value at equal intervals throughout the electrical currents cycle. This allows the circuit to deliver relatively constant power throughout the cycle. Its primary advantage is the fact that it requires less conductor material to deliver a given voltage than single and dual phase systems.

An electrical circuit must have at least three phases in order to possess a number of desirable properties for electrical systems. All of the conductors carry the same amount of current and may therefore be the same size. Each of the phases tends to cancel the others out, producing a balanced load. This constant transfer of power tends to reduce vibrations, prolonging the life of the electrical equipment. The design of a three-phase electrical motor is also simpler, since this system produces a magnetic field in one direction. Nikola Tesla developed three-phase electric power in 1887.


In order to produce electricity, an electrical generator is used to convert mechanical energy into electrical energy. The generator in a three-phase system is designed so that each of the three circuits reach their peak 120 degrees apart from the other two circuits. A large electrical generator produces electricity with a potential difference of at least a few hundred volts, and may produce as many as 30,000 volts. A transformer typically increases this voltage dramatically to reduce the power loss during transmission, and the power network usually transforms the voltage multiple times before it is available to the end user.

Three-phase electrical power is the most common method used in electrical grids throughout the world. This power typically splits off from the distribution board before being fed into homes and other buildings. In some places, including the US, household circuits are typically single phase, and three-phase electric power may not even enter the house. Three-phase power is sometimes used to feed the larger appliances in a home that require high voltage, such as dryers and stoves. This system is also popular in large motors and other applications that carry heavy electrical loads.

A circuit that uses three-phase electric power can have a neutral wire, which allows the circuit to support single-phase applications that use a lower voltage. If the circuit only needs to produce high-voltage, it will not usually use a neutral wire, however.

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