 In a three-phase power supply system, three conductors each carry an alternating current (of the same frequency) but the phase of the voltage on each conductor is displaced from each of the other conductors by 120 degrees. (One third of a 360 degree “cycle”.) Hence, the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the third conductor. Using the voltage on one conductor as the reference, the peak voltage on the other two conductors is delayed by one third and two thirds of one cycle respectively. This phase delay gives constant power transfer over each cycle. It also makes it possible to produce a rotating magnetic field in an electric motor.

With a three phase supply, at any instant, the potential of any phase is exactly equal to and the opposite of the combination (sum) of the other two phases. This means that – if the load on the three phases is “balanced” – the return path for the current in any phase conductor is the other two phase conductors.

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