polyphase
Of, pertaining to, or involving multiple alternating currents that have the same frequency but differ in phase.
polyphase: three currents marching to the same beat, out of step
A polyphase electrical system uses multiple alternating currents of identical frequency but shifted in time relative to one another. In industrial practice, this means two or more AC circuits reaching their peak voltages at different moments. The phase difference is measured in degrees; in three-phase systems, the standard in modern power distribution, each current lags the previous one by 120 degrees. This staggered timing is not a flaw but the whole point of the design.
Three-phase power dominates industrial settings because it delivers more power through fewer conductors than single-phase circuits and does so with minimal ripple or pulsation. A three-phase motor draws current continuously as the rotating magnetic field created by the three offset currents interacts with the rotor. Single-phase motors, by contrast, require starting mechanisms like capacitors or auxiliary windings because the magnetic field pulses rather than rotates smoothly. For identical power output, a three-phase system uses about 75 percent of the copper that a single-phase system would require.
Two-phase and six-phase systems exist but are rare. Two-phase power was used in early twentieth-century AC distribution but fell out of favor. Six-phase rectifiers appear in high-power industrial applications where smoother DC conversion is needed. Twelve-phase systems are found in some specialized HVDC (high-voltage direct current) converter stations.
Practical consequences
Polyphase motors are smaller, more efficient, and more durable than equivalent single-phase machines. A three-phase induction motor rated at 10 horsepower will run cooler and last longer than a single-phase motor of the same output. This efficiency advantage compounds across thousands of motors in a factory or plant. Polyphase systems also permit balanced loading across utility generation and distribution networks, reducing losses and voltage sag.
The term "polyphase" emerged as AC power distribution became standardized in the 1880s and 1890s. Nikola Tesla's polyphase induction motor patents proved the superiority of offset phase currents for rotating machinery. The word is transparent: many (poly) phases, each one a complete cycle offset from the others. When you encounter reference to a device or system without explicit mention of phase, it is usually single-phase at household voltage or polyphase at industrial voltage.