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

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    \$\begingroup\$ No one needs any power. Three-phase power is convenient for a number of reason, but it's not a necessity. \$\endgroup\$ Commented Oct 14, 2014 at 9:46
  • \$\begingroup\$ @ConnorWolf Is power/current/voltage coming to house single phase? Is it delta configuration? what happens when voltage drops to zero in single phase? Is this single phase? \$\endgroup\$ Commented Oct 14, 2014 at 17:51
  • \$\begingroup\$ The power coming to your hose is whatever you pay for and/or the utility is willing to provide. \$\endgroup\$ Commented Oct 15, 2014 at 1:04

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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.

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Comparing 3-phase with single phase transmission, 3 phase has a couple of significant advantages:

1) More efficient use of conductors : given the same peak voltage between conductors and same current in each conductor, 3-phase delivers 3x the power with 1.5x the copper (3 wires instead of 2). This doubles the usefulness of each pound of copper (aluminium etc) Over long distances, this is a significant cost saving.

2) Three phase motors run smoothly with no additional complexity to define their running direction. Power delivery is approximately constant as the rotor follows the rotating field (with some slip, in an induction motor) with no torque variation or vibration. Reversing can be as simple as interchanging any two phases.

The advantage of smoothness also applies to the 3 phase generators : they absorb power from e.g. the turbine smoothly : a large single phase generator would probably be shaken apart from the torque variations.

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Total power in three phase system is almost constant and so creates less vibration.

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Total power in single phase system is pulsating and so creates more vibrations.

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3-phase is easy to generate, easy to transmit, and easy to manipulate in many different ways.

In a delta configuration it makes for very easy long distance transmission, since there is no "ground" needed - the ground equivalent functionality for one phase is provided by the other two phases.

In a star configuration it provides three individual single-phase power circuits for feeding to consumers.

Switching between delta and star arrangements is as simple as using a transformer with the primary and secondary windings in the right arrangement.

It's has to be by far the most flexible of all the arrangements you can come up with.

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