voltage - Why do we need 3 phase power supply?

Why do we need 3 phase? For the reason that voltage will never be zero in 3 phase? OR we need high voltage?

Answer

There are several reasons 3-phase is desirable over 1-phase power. One advantage of 3-ph over 1-ph has to do with instantaneous power (i.e. power generated or consumed at any instant in time within the power cycle).

For example, consider a heating element (power resistor) in a 1-phase circuit. Voltage and current are in phase. Both cross zero twice during a cycle as they go positive and then negative.

Their product is power which is a sinusoid at 2x the fundamental frequency (multiply two sine waves and you get a new sine wave with twice the frequency). The power dissipated by the heating element has a sinusoidal waveshape that sits above the zero line (because the two V & I sinusoids, when multiplied together, always give a positive value).

The power sinusoid also reaches zero twice during the fundamental cycle at the same time V or I cross zero. The resistor doesn't produce heat (consume power) at these zero-crossing instants in time (resistor remains hot due to its thermal mass). Now replace the resistor with a 1-ph induction motor.

For similar reasons, despite V & I being out of phase, there are times in its power cycle when the motor doesn't produce mechanical power (it remains spinning due to its own and its load's inertia). In a 3-ph system, the phases are staggered by 120 electrical degrees. If a 3-ph heating element is connected (Y or delta; doesn't matter for the purpose of this discussion), each individual resistor "sees" a zero crossing but collectively the three are producing heat at all times. There is no instant in time when heat is not being produced.

Similarly, with a running 3-ph motor, there is no instant in time when it isn't producing mechanical power. The result is a simplified motor (no starting winding needed as is the case with a 1-phase motor), smaller frame for same horse-power because instantaneous power is never zero. Unlike the 1-ph motor, the 3-ph motor doesn't need the larger bulk to "coast through" a zero crossing.