<P> Three - phase electric power requires less conductor mass for the same voltage and overall amount of power, compared with a two - phase four - wire circuit of the same carrying capacity . It has replaced two - phase power for commercial distribution of electrical energy, but two - phase circuits are still found in certain control systems . These power pulsations tend to cause increased mechanical noise in transformer and motor laminations due to magnetostriction and torsional vibration in generator and motor drive shafts . </P> <P> Two - phase circuits typically use two separate pairs of current - carrying conductors . Alternatively, three wires may be used, but the common conductor carries the vector sum of the phase currents, which requires a larger conductor . Three - phase can share conductors so that the three phases can be carried on three conductors of the same size . In electrical power distribution, a requirement of only three conductors, rather than four, represented a considerable distribution - wire cost savings due to the expense of conductors and installation . </P> <P> Two - phase power can be derived from a three - phase source using two transformers in a Scott connection: One transformer primary is connected across two phases of the supply . The second transformer is connected to a center - tap of the first transformer, and is wound for 86.6% of the phase - to - phase voltage on the three - phase system . The secondaries of the transformers will have two phases 90 degrees apart in time, and a balanced two - phase load will be evenly balanced over the three supply phases . </P>

What is a 5 wire 2 phase system