What is the diff. between two phase & three phase supply.

Answer Posted / v8snail

In the case you're asking the question about 2 phase power in the farming community (as opposed to the old 90 degree out of phase 2 phase power), although colloquially known as 2 phase it is in fact split phase power. Due to economics of the long transmission lines required for rural electricity distribution, only 2 conductors are usually used at an elevated voltage (often 22 kilovolts) and a transformer utilised at each spur line tapping point (often 1 for each property) which converts the high voltage to 480v with a centre tap effectively giving 2x 240v in phase (for Australia). These two phases can be split between different circuits in a house or workshop, or in the case of high current draining devices or machines (like a wool press, large welder, large heating element or large pump) can be connected in series to give 480V power for devices designed to operate on this voltage, with only half the current required than for 240v single phase. 2 phase motors often have the option in the junction box to wire it up as single phase 240v or 2 phase 480v (a typical wool press will draw about 20 amps on 240v single phase and only 10 amps connected as 480v 2 phase).
3 phase was explained in earlier posts, usually requiring 4 conductors for transmission with 1 being for the neutral/earth return.

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