Polyphase system

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A polyphase system is means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful for transmitting power to electric motors. The most common example is the three-phase power system used for most industrial applications.

One voltage cycle of a three-phase system.

1 Phases
2 Motors
3 Higher phase order
4 Single phase loads on a polyphase system
5 See also
6 Further reading

[edit] Phases

Main articles: Phase and Phase shifting

In the very early days of commercial electric power, some installations 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, 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 a Scott-connected transformer.

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.

[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, are self-starting and have little vibrations.

[edit] Higher phase order

Higher phase numbers than three 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 Extra High Voltage (EHV) 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. Single-phase loads such as lighting may be connected from an energized phase to the circuit neutral, allowing the load in a large building to be balanced over the three supply phases. The phase offset of the line-to-neutral voltages is 120 degrees; the voltage between any two live wires is always $\sqrt{3}$ times between a live and neutral wire. See the article Mains power systems for a list of single-phase distribution voltages around the world; the three-phase line-to-line voltage will be $\sqrt{3}$ times these values.

In North America, residential apartment blocks may have 120 volt (line to neutral), 208 Volt (line to line) distribution. Major single-phase appliances such as ovens or cook tops intended for the 240 Volt split phase system usually used in single-family dwellings may not operate well when connected to 208 Volts; heating appliances will develop only 3/4 of their rated power, and electric motors will not operate correctly with 13% lower voltage applied.