D_Arsonval said:
I would be confused by a statement saying "...there is no such thing as a two phase system..." as well.
For I've worked on two phase electric motors in my past.
John
Google it for something to do:
https://en.wikipedia.org/wiki/Two-phase_electric_power
True, but a motor is not a power distribution system.
The key idea is that the currents do not disappear. Current supplied by the source must be returned to the source. That is
Kirchoff's Current Law applied to a system.
In a single phase system, we have two wires. Current comes from the source through one wire, and returns via a second wire. Two wires, one phase.
In a three phase system, the currents in the three phases are shifted 120 degrees relative to each other, so that when they are combined, the three add up to zero. Thus no return wire is needed. Three phases, three wires.
Clever people can invent hundreds of other combinations of wires and phase shifts and give them many names. (Earth returns excluded.) Here are just a handful of them.
1-Phase, 2-Wire 120 V with neutral 120 – US 3Y-208
1-Phase, 2-Wire 230 V with neutral 230 – EU, Others 3Y-400
1-Phase, 2-Wire 208 V (No neutral) – 208 US 3D-240
1-Phase, 2-Wire 240 V (No neutral) – 240 US 3D-240
1-Phase, 3-Wire 120/240 V 120 240 US 3Y-208
3-Phase, 3-Wire 208 V Delta (No neutral) – 208 US 3D-240
3-Phase, 3-Wire 230 V Delta (No neutral) – 230 Norway 3D-240
3-Phase, 3-Wire 400 V Delta (No neutral) – 400 EU, Others 3D-400
3-Phase, 3-Wire 480 V Delta (No neutral) – 480 US 3D-480
3-Phase, 3-Wire 600 V Delta (No neutral) – 600 US, Canada none1
3-Phase, 4-Wire 208Y/120 V 120 208 US 3Y-208, 3D-240
3-Phase, 4-Wire 400Y/230 V 230 400 EU, Others 3Y-400, 3D-400
3-Phase, 4-Wire 415Y/240 V 230 415 Australia 3Y-400, 3D-400
3-Phase, 4-Wire 480Y/277 V 277 480 US 3Y-480, 3D-480
3-Phase, 4-Wire 600Y/347 V 347 600 US, Canada 3Y-600
3-Phase
4-Wire Delta 120/208/240
Wild Phase 120,
208 240 US 3D-240
3-Phase
4-Wire Delta 240/415/480
Wild Phase 240,
415 480 US 3D-480
3-Phase
Corner-rounded Delta 208/240 – 240 US 3D-240
3-Phase
Corner-Grounded Delta 415/480 – 480 US 3D-480Some are called two phase, so in that respect you are correct But in the end, they must reduce to one of the two above. Either there is a wire for return current (sometimes called the neutral
), or the currents add to zero.
http://www.ccontrolsys.com/w/Two_Phase_Electrical_Service said:
Residential electric service in the United States (120/240 VAC) is sometimes called two-phase service but this is NOT correct. It is only single-phase, since both line voltages are derived from a single phase of a distribution transformer with a center tapped neutral and are 180° out of phase with each other.
Two-phase service is an obsolete style of electrical power distribution where two phases are provided that are 90° out of phase with each other. There were two line wires and one neutral, so two-phase service was commonly a two-phase three-wire service.
Another variation used four line wires and one neutral for a two-phase five-wire service. This was sometimes incorrectly called four-phase five-wire service.
We are not aware of any two-phase service still in use.
