why we use to transmit the power in three phases intead of
single phase ?

Answer Posted / paul

I do not think the previous answer is correct. Three phase is more efficient (Power In v Power Out) in respect to AC motors, and is also more cost economical in regards to power transmission as less copper is required for the same delivered power (given the 3 phase line voltage is taken as being he same as the equivalent single phase voltage therefore allowing the use of cable with the same voltage rating), however..

The transmission power loss (and therefor transmission efficiency) in a three phase systems is the same as in a single phase system (again, assuming same total load power and line voltage).

The previous response has omitted the fact that, even though there is no neutral wire the return current in a three phase system will be subject to I^2*R losses in the associated conductors. Taking R as the loop resistance (supply and return), a single phase system power loss can be calculated as I^2*R. For a balanced three phase system, the single phase equivalent losses will be ((I/root3)^2)*R. Therefore total losses = 3*((I/root3)^2)*R = I^2*R.

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