Polyphase system
From Wikipedia, the free encyclopedia
A polyphase system is a means of distributing alternating current electrical power. The most common example is the three-phase power system used for most industrial applications.
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[edit] Phases
Main articles: Phase and Phase shifting
Polyphase systems have two or more energized electrical conductors carrying alternating currents with a definite time offset between the peak amplitudes of the wave in each conductor. In modern utility power generation and distribution three phases are used, with the phases separated in time by one third of an AC cycle.
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. Such systems are sometimes described as split phase.
In the very early days of commercial electric power, some installations used true 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, and, by using a four-wire system, conceptually the phases were independent and easy to analyze with mathematical tools available at the time. Two-phase systems have been replaced with three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using the transformers.
[edit] Motors
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 and Michail Dolivo-Dobrovolsky 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.
[edit] Higher phase order
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.
[edit] Single phase loads on a polyphase system
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. The phase offset between any two live (hot) wires is 120 degrees; the voltage between any two live wires is always sqrt(3) (or 2*cos(120deg)) times higher than between a live and neutral wire.
Here is a comparison of voltage supplies in one-phase and three-phase systems in most of the world:
| Voltage between: | USA and Canada | Asia, South America | Europe, Australia | UK |
|---|---|---|---|---|
| live and neutral wires | 120V | 220V | 230V | 240V |
| two live wires | 208V | 380V | 400V | 415V |
In USA and Canada 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.
