Three-Phase Systems:

A three-phase (3 ø ) system is a combination of three single-phase systems.  Within a 3 ø balanced system, power comes from a 3 ø AC generator which generates three separate and equal voltages, every of that is 120° out of phase along with the other voltages that was show in the below figure.

Figure: Three-Phase AC

Three-phase equipment (motors, transformers and many more) weighs less than single-phase equipment of the similar power rating.  They have a huge range of voltages and could be used for single-phase loads. Three-phase  equipment  is  weighs  less,  smaller  in  size  and  is  more  efficient  than single-phase equipment.

Three-phase systems could be connected in two variant ways.  It is called a wye if the three common ends of every phase are connected at a common point and the other three ends are connected to a 3ø line, or Y-, connection display in below figure.  it is called a delta, or ?-, connection if the three phases are connected in series to form a closed loop.

Figure: 3ø AC power connection