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3-Phase Power Systems
Polyphase electrical systems supply alternating current electrical power in overlapping phases. The most common example are three-phase power systems used in industrial applications.
Polyphase systems have two or more phases. The voltage on each phase is a sine wave, with a fixed time offset, or phase shift, between the phases. Modern utility power generation and distribution is almost universally three phase, with the phases separated by 120° or one third of an AC cycle. It is commonly used in industry, as it is ideally suited to powering the 3-phase induction motor.
A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order. A 3-wire system with two phase conductors 180 degrees apart is still only single phase.
A few older installations in the U.S. used two phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor. Most of these have been upgraded to three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using transformers.
Polyphase power is particularly useful in AC motors, such as the induction motor, where it generates a rotating magnetic field. When a three-phase supply completes one full cycle, the magnetic field of a two-pole motor has rotated through 360° in physical space; motors with more pairs of poles require more power supply cycles to complete one physical revolution of the magnetic field, and so these motors run more slowly. Nikola Tesla invented the first practical induction motors using a rotating magnetic field - previously all commercial motors were DC, with expensive commutators, high-maintenance brushes, and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, and are self-starting.
Higher phase numbers than 3 have been used. A common practice for rectifier installations and in HVDC converters is to provide six phases, with 60 degree phase spacing , to reduce harmonic generation in the ac supply system and to provide smoother direct current. Experimental high-phase-order transmission lines have been built with up to 12 phases. These allow application of EHV-style design rules at lower voltages, and would permit increased power transfer in the same transmission line corridor width.
Residences and small businesses are usually supplied with a single phase taken from one of the three utility phases. Individual customers are distributed among the three phases to balance the loads.
Many larger apartment buildings are fed with 3-phase transformers, with two of the three phases fed to each unit. The phase offset between the two hot wires is therefore only 120 degrees, so that while the voltage from each "hot" wire to neutral is 120V, the voltage between hot wires is only 208V. Some appliances rated for 240V will run satisfactorily on 208V, although heating equipment will only output 75% of its 240 V rating. Special autotransformers can boost 208V to 240V if necessary.