Post By peterh5322
Single-phase and Three-phases Explained
Newbie said: "I'm sure that someone with more knowledge than I will be along to explain the differences between 1ph and 3ph."
Single-phase power in North America consists of two phasors which are aligned as follows: Line 1 starts at the 180 degree position of a circle and ends at the circle's origin; Line 2 starts at the circle's origin and ends at the 0 degree position. These two, therefore, add to 240 volts (230 and 220 volts are earlier standards which implement the same concept). The circle has a radius of 0.5 units and a diameter of 1.0 units, which corresponds to 120 and 240 volts, respectively. As the phasors simply move up and down in the same plane (0 degrees is directly opposite the 180 degree position), single-phase does not provide a rotating field, therefore single-phase motors always require a starting means, which could be a "shaded pole" or a starting winding or possibly other means.
Three-phase power in North America consists of three phasors which are aligned as follows: Line 1 starts at the 0 degree position of a circle and ends at the 120 degree position of the circle; Line 2 starts at the 120 degree position of a circle and ends at the 240 degree position of the circle; Line 3 starts at the 240 degree position of a circle and ends at the 360 degree position of the circle (the 360 degree position is the same as the 0 degree position, so the circle is complete.) The circle has a radius of 0.578 units which corresponds to 138.72 volts. However, by the so-called wye-delta transformation, this corresponds to 240 volts. As the phasors move around the circle, three-phase motors require no starting circuit.
For a graphical representation, see this ...
Three-Phase vs. Single-Phase, Illustrated
Peter has the formal training, the experience, and the credentials. His explanation is precise and correct.
Originally Posted by peterh5322
But for those who do not have the background, try this:
Somewhere in the dawn of the electrical age (and the history is online) it was noticed that a simple single-phase generator and motor were rather 'rougher' than (good) direct-current. To smooth that, one of the things tried was to align a second generator or set of coils 'in between' the angles of the first. Much the same as adding cylinders to a steam or internal combustion engine, whether on the same crankshaft or not.
TWO phase was born. If some was good, more was better, so a THIRD set was tried.
It went on up from there, but at the end of the day, extra wires were needed between source and load as phases were added, and three-phase gave the most benefit from the least extra 'everything'.
Least extra switching contacts, least extra transformer and long-distance transmission-line complexity. And least extra complexity in motors and other loads. Not 'zero' cost... but very good value-for-money.
So 3-phase survived and prospered.
12 phase and more survived for a time in positioning/repeater devices such as the 'Selsyn', with scary thick wire bundles and nearly assured screw-ups when connecting them.
2-phase has survived in a different form. As wrong-name-only in the mistaken mind of such English, and the Chinese who copied them - who do not understand that our US-practice of 240 Volt, center-tap to neutral, 120V either side does NOT create a 'new' phase angle any different than its parent had.
Three-phase is great stuff. Smooth, less heat waste, lower noise, can permit physically smaller motors when done right, needs no start/run capacitors or their switching, and is cheap and easy to rectify into smooth DC.
Only downside is that distances as they were in America as electricity was rolled out across the landscape, utility companies were not up for even the easily justifiable extra cost to make it universal, so it became a large-building or industrial only service. And it is nowadays all over the place - reaches supermarkets but not the average small shop or residence sitting close by. Utility charges extra to run it, and more yet when they bill it. Often prohibitively so even for short distances, and scary big bucks when not so short. Most just do not want to be bothered at all for small-holders, either.
More densely populated Europe, also having fewer local copper and iron-ore deposits, was generally more sensible about that, hence it is common even to the residence, or very, very close to it, 'coz it needs fewer scarce 'strategic' materials to get the job done in an easily balanced manner than single-phase.
Last edited by thermite; 07-21-2012 at 02:28 PM.
Post Deleted. Thanks for clarifying.
Last edited by Philabuster; 07-21-2012 at 04:05 PM.
Reason: posted inaccurate info
It would be if that was what was actually happening. But all such discussions belong elsewhere.
Originally Posted by Philabuster
This thread is only about helping understand what single-phase and 3-phase really ARE. Origin, similarities and differences kept simple so those not already aware can better understand what to look for when they go to the gadzillions of OTHER PM threads and external sources. Wikipedia has some very good articles, BTW.
Not the place here for yet-another discussion of best and worst ways to convert.
Those discussions can turn 'religious' all too rapidly, and the simpler underlying physics get dropped on the floor.
I think there might be some confusion by those trying to understand 3 phase. The statement "TWO phase was born. If some was good, more was better, so a THIRD set was tried." Don't make sense! 2 phase power has 4 wires. At first it was generated by connecting two Alternator together 90 degrees apart. Germans developed the 3 phase 120 degree 3 wire shift and soon it took over. I'm not going to get into which was better.
The 'sense' has to do with bang-for-the-buck. The economics.
Originally Posted by Froneck
Simply count the wires needed for 2-phase and 3-phase vs each other or single-phase and extrapolate the costs over distance.
IOW the extra smoothness of 3-phase is relatively speaking a 'free ride', and when 4 and more phases were looked at, not enough extra 'DC like' smoothness gained for general use to justify any further investment in conductors or complexity over significant distance.
Simply put, 3-phase is the all-around 'sweet spot' value-for-money wise.