nonthreephase

Nonthreephase is a term used to describe electrical power systems that do not use exactly three phases. In practice, it usually refers to single-phase systems, but it can also describe two-phase arrangements or higher-order multiphase configurations that have four, five, or more phases. The central idea is that power is carried by a set of sinusoidal voltages that are not three evenly spaced phases.

Single-phase systems supply most residential and small commercial loads. They use one alternating voltage, typically distributed via transformers to step voltage up or down. A notable drawback is that instantaneous power varies with the sine wave, which can cause voltage variations under uneven loads; large motors often require starting devices or a separate three-phase supply.

Two-phase systems were explored in early AC power development but are now rare. They offered some theoretical advantages but were superseded by three-phase systems that provide smoother power delivery and more efficient transformer usage.

Four- or higher-phase configurations are uncommon in modern practice, appearing mainly in specialized equipment or experimental settings. In most cases, non-threephase arrangements are used where only single-phase supply is available or where an adapter or converter makes one-phase power usable for three-phase equipment.

For power distribution, three-phase networks remain dominant because of efficiency and standardization. Non-threephase systems persist for small loads, regional isolated networks, or legacy installations.

See also: Single-phase, Three-phase electric power, Multiphase power.