polyphase circuit
A group of alternating current circuits having two or more interrelated voltages, usually of equal amplitudes, phase differences, and periods, etc; if a neutral conductor exists, the voltages referenced to the neutral are equal in amplitude and phase; the most common version is that of 3-phase, equal in amplitude with phases 120 degrees apart.
polyphase circuit: three or more AC circuits offset in time
A polyphase circuit is a set of alternating current conductors carrying voltages that are deliberately offset from one another in phase, usually by equal intervals. The most common form is three-phase, where three sinusoidal voltages are separated by 120 degrees. Each phase reaches its peak voltage at a different moment; this staggered timing, not the number of wires alone, defines the system. Single-phase AC is a reference point for comparison, but most industrial power is polyphase because the geometry of the phase offsets creates mechanical advantages that single phase cannot match.
Three-phase systems typically carry voltages of 208 V, 277 V, 380 V, 400 V, or 480 V between phase conductors, with 120/240 V single-phase derived from two phases where local codes allow. A three-phase system can be wired in delta (three conductors, higher voltage, no neutral) or wye (four conductors including neutral, lower voltage reference, safer for mixed loads). The neutral conductor, if present, sits at the geometric center of the three phase voltages and sees zero current under balanced load. Real-world distribution often uses delta on the transmission side and wye on the utilization side, with a transformer at the junction.
Why polyphase matters in industrial work
Three-phase motors are smaller, lighter, and more efficient than single-phase motors of equivalent power because the three offset voltages drive rotor torque smoothly without the dead spots that plague single-phase designs. A three-phase induction motor produces constant torque; a single-phase motor produces pulsating torque and requires a capacitor and auxiliary winding to start. Industrial machinery, pumps, compressors, and crushers almost always use three-phase motors. Power distribution systems favor polyphase because three balanced phases can deliver more power through the same conductor cross-section than single phase can, and the neutral current in a balanced wye system is theoretically zero, so less copper is needed.
Imbalance is the main failure mode in polyphase circuits. When loads are unequal across phases, or when a single phase opens, the remaining phases must carry extra current; motors overheat, protection relays trip, and equipment fails. Phase imbalance as small as 3 to 5 percent can shorten motor life significantly. Testing requires a three-phase power analyzer to measure voltage and current on each phase and check the phase angle separation; a basic multimeter cannot diagnose polyphase problems because it reads only between two points at one instant.
The term polyphase comes from the multiple offset waveforms, each called a phase. Two-phase systems (90 degrees apart) were used historically, especially in parts of North America, but are now rare; four-phase or six-phase systems exist in specialized applications like HVDC conversion and some welding rectifiers, but the economic and practical advantage of three-phase has made it the global industrial standard since the 1890s.