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One voltage cycle of a three-phase system. A polyphase system (the term coined by Silvanus Thompson) is a means of distributing alternating-current (AC) electrical power that utilizes more than one AC phase, which refers to the phase offset value (in degrees) between AC in multiple conducting wires; phases may also refer to the corresponding terminals and conductors, as in color codes.
The return path for the current in any phase conductor is the other two phase conductors. Constant power transfer is possible with any number of phases greater than one. However, two-phase systems do not have neutral-current cancellation and thus use conductors less efficiently, and more than three phases complicates infrastructure unnecessarily.
In a three-phase system, at least two transducers are required to measure power when there is no neutral, or three transducers when there is a neutral. [5] Blondel's theorem states that the number of measurement elements required is one less than the number of current-carrying conductors. [6]
A typical one-line diagram with annotated power flows. Red boxes represent circuit breakers, grey lines represent three-phase bus and interconnecting conductors, the orange circle represents an electric generator, the green spiral is an inductor, and the three overlapping blue circles represent a double-wound transformer with a tertiary winding.
These reactive currents, however, cause extra heating losses. The ratio of real power transmitted to the load to apparent power (the product of a circuit's voltage and current, without reference to phase angle) is the power factor. As reactive current increases, reactive power increases and power factor decreases.
For a simple electrical circuit running on direct current, the electrical current and voltage are constant.In that case, the real power (P, measured in watts) is the product of the current (I, measured in amperes) and the voltage from one side of the circuit to the other (V, measured in volts):
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In the International System of Units (SI), electric current is expressed in units of ampere (sometimes called an "amp", symbol A), which is equivalent to one coulomb per second. The ampere is an SI base unit and electric current is a base quantity in the International System of Quantities (ISQ).