A three-phase circuit is an electrical distribution method that uses three alternating currents to supply power. This type of power distribution is the most widely used in the world for transferring power from generating systems to electrical supply grids. A three-phase circuit is also commonly used on large motors, pumps, and other pieces of mechanical equipment. Most households receive electricity in the form of single-phase circuits, though some may have special three-phase circuit breakers installed for appliances such as washing machines or stoves.
To understand how a three-phase circuit works, it is helpful to understand how power is generated. Power plants use fuel sources such as oil or gas to power large-scale generators. These generators rely on a revolving magnetic field to produce waves of electrical current. For single-phase power, picture each full rotation of the revolving field creating a single wave of electrical current. That is, for each 360-degree turn, one wave of electrical current is set in motion and distributed through the power grid.
To produce three-phase power, three separate waves of current are generated for each revolution of the magnetic field. For every 120 degrees of revolution, a wave is released, for a total of three waves per 360-degree turn. These three waves travel together throughout the power grid, and will stay in a set unless separated by mechanical means. The three waves produce an alternating current, meaning they peak at different times. Unlike a single-phase circuit, a three-phase system will always produce a continuous level of electrical current because there will always be one wave in peak position.
As three-phase power reaches the circuits in a building, it creates 208 Volts of electricity. When this three-phase power reaches a house, it first passes through a transformer to transform it to single-phase power, which produces 120 Volts of electricity. A standard 120-Volt household appliance can be plugged into a three-phase circuit, but will operate less efficiently than if it were plugged into a single-phase circuit.
Three-phase power distribution is not only efficient, but offers superior performance for most applications. It provides a smooth and even electrical supply so that motors and equipment can operate more effectively. This smooth electrical supply leads to less vibration and wear for mechanical equipment, and also helps to extend the life of these devices. Three-phase circuits are also able to supply larger quantities of power than single-phase systems, making it particularly beneficial for large-scale applications.