Q.why do we need three phase power for heavy load?????

A.With a single phase, the voltage drops to 0 every half-cycle. Therefore, the amount of power you can get out of the line is not constant over time, it has a very large 120-hz ripple. With a large motor, this can cause vibration, and also means that larger wires are needed for the same power load. The same is true for so-called "2- phase" power, 2 voltages 180 degrees apart. They both cross zero at the same time, which is why this is normally also referred to as single- phase. It isn't any better than single phase. With 3-phase power, however, there is never a time when all the voltages go to 0. In fact, if you are extracting power from all 3 phases, (like in a motor) the power stays constant throughout the whole cycle. This gives a vibration-free drive (and also allows for simpler motor design because 3- phase power has an inherent "direction" which 2-phase or 1-phase power does not).