In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at one third of a cycle from each other. Taking one current as the reference, the other two currents are delayed in time by one third and two thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.
Three-phase systems may have a neutral wire. A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage single-phase loads. In high-voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
Three-phase has properties that make it very desirable in electric power systems:
The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.
Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.
Most household loads are single-phase. In North American single-family dwellings, three-phase power generally does not enter the home; multiple-unit apartment blocks may have three-phase power but three-phase power is not used for household appliances. Utilities that supply three-phase power for lower-load-density area homes typically distribute only one phase to individual loads. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.
Wiring for the three phases is typically identified by color codes which vary by country. Connection of the phases in the right order is required to ensure correct rotation of three-phase motors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time; a direct interconnection between two different phases is a short-circuit.
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