3-Phase Generators                                                                             generators       topics

Large commercial power stations generate 3 alternating currents simultaneously from the same machines each differing in timing from each other by 1/3 of the period. As can be seen from the animation, the three currents, "phases" are mathematically 120⁰ apart from each other in phase. In the animation, the sets of wiring are 120⁰ around the generator.

If the first phase has a voltage given by V=V₀ sinωt, then the second is V=V₀ sin(ωt +120⁰) and the third V=V₀ sin(ωt + 240⁰).

The magnet is usually an electromagnet requiring brushes to provide the current to the rotating magnet's wiring.

The potential differences generated is often fairly low but high currents are created. These are usually sent to a transformer in a switchyard which changes the AC currents to very high voltage and low current for long distance transmission. ( Reason, power =VI, if we are to transmit say 100MW to a place several hundred kilometres away, we don't want ohmic heating of the cables. This is caused by high currents in the cables!  So - low currents, meaning high PDs. )  High voltage transmission towers only carry groups of three cables - the 3 phases. The return cable is the Earth itself.

At the other end, the PDs are transformed to low values and eventually the individual phase cables go to consumers, eg your house! Big energy consumers eg businesses may require all three phases into their places. Households usually need only one phase.

A happy quirk of the three phases is that if loads are very close ot the same, the return cables are all joined, the three currents created add TO NOTHING ! So at the generator and every transformer one cable of every phase is joined and stuck in the ground! This can be seen in Australia at the power poles holding a transformer in the streets providing power to houses. Down one side is a massive cable sunk into the ground. Houses get one phase each with a return line, these return lines are joined into the Earth.

Mathematical exercise: Prove that   sinωt + sin(ωt +120⁰) + sin(ωt + 240⁰) = 0  . Hint, you need the mathematics of "sums to products" to do it!