Just for the sake of those that cannot follow the link Bob posted, below are a couple of interesting sentences from Wikipedia.
at Niagara Falls
installed in 1895 were the largest generators in the world at the time and were two-phase machines. The advantage of two-phase electrical power was that it allowed for simple, self-starting electric motors. In the early days of electrical engineering
, it was easier to analyze and design two-phase systems where the phases were completely separated.
It was not until the invention of the method of symmetrical components
in 1918 that polyphase power systems had a convenient mathematical tool for describing unbalanced load cases. The revolving magnetic field produced with a two-phase system allowed electric motors to provide torque
from zero motor speed, which was not possible with a single-phase induction motor
(without extra starting means). Induction motors designed for two-phase operation use the same winding configuration as capacitor start
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 all but replaced two-phase power for commercial distribution of electrical energy, but two-phase circuits are still found in certain control systems.