WHY DO WE USE THREE PHASE POWER SUPPLY HEAVY LOADS?

With a single phase, the voltage drops to0 every half cycle. the amount of power you can get out of the line is not constant over time. it has a large120Hz cycle. With large motors this can cause vibration, and also means that largr wires required for same power load.

-Using three phase power supply for same load reduces the absorbed current, which leads to:

1- less voltage drop (about the third) IZ

2-less losses in the cable I2R

3- less current requires less winding insulation rating

moreover; motors require three phase connection to build up high torque and rotating magnatic field. 

if you look at the Sinusoidal waveform of three phases so it will show you the angle diffrance of57.3 (which can be obtained by the 2π RADS) where the rads is =57.3         2π  X RADS=360Degree

so on the Zero or180 degree the voltages will drop to its minimum  for single phase ,but in three phase if it will be on straight line in one phase then it will be on half of sine at second and the at peak in3rd phase.which can give you a strong magnitizam in3rd part of the single phase .

to prevevent the weakness and to avoid the big windings people use the3phase on heavy loads.