In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is phase to phase voltage 220? I know phase difference between any two phases differ by 120 degree, so they should add up to give something less than 220V! It should sum up to give 220V if the phase difference were 0 degree or 360degree!
I think you got something wrong, or I didn't expressed it correclty! Say you have two AC sources of 110V, now if I put them in series they should add up to give 220V if their instantaneous phases are same(0 or
360) or if they are 180 phase out they should sum up to zero. I think these two AC sources are just like two phases of 3-phase AC supply which are 120degree phase apart. And they should give a voltage less than 220V when summed up.
In North America, line voltage is 120 volts to ground. With three phase power this gives you 208 volts between phases.
If the phase difference between two circuits is 0 degrees, you will measure zero volts between them.
In normal residential wiring, we have two wires that are 180 degrees out of phase - this gives 240 volts between "phases" (some people object to using the term "phase in this situation...)
Although the electrical distribution system as a whole is three-phase, individual homes are fed from the secondary of a single phase transformer. The secondary of the transformer is center-tapped, with the tap grounded to form the neutral conductor. There is 240 volts between the ends of the secondary.
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
One common but bizarre US wiring practice is to have a 240 volt line-to-line delta three-phase system in which one side of the triangle is center-tapped and is neutral. So 120 single-phase is available for regular outlets, 240 single-phase is available for things that need it, and 240 line-to-line is available for three phase loads. That's fairly common in small commercial buildings. The leg opposite the neutral is call the "bitch leg" or the "stinger."
It's a very simple geometric relationship. If two lines or vectors are 120 degrees apart and are of equal length from their common, crossing end, the distance between the tips of the lines is 2*sin (120/2). = 2*0.866 = 1.732. Now, if the line lengths represents 120 volts from the center or crossing point to the tip, the tips must be 120 * 1.732 apart = 207.8 Volts. OK class, for homework, prove the geometric relationship. BTW it can be proven without trigonometry. Bob
I can't see how that can be a delta. I think what you have described is a six phase "Y", center neutral as usual. It's 120 Volts out any leg and 240 Volt center tapped any leg to it's stinger. And, 208 Volts leg to leg. It's also 120 Volts from any leg to the adjacent stinger. It's not all that bizarre and can be created from any three phase system with a transformer. Bob
It's still common in the US where houses are mixed with small motor installations. That unexpected 208-to-neutral is(was) a problem on water-well controls when you needed 120v for the control-circuit. WAde H
Why would anybody do that? What is the voltage from A to C? A to N?, What is the relation of N to ground. Clearly this kind of a bastard connection would be prone to gross unbalance or to parasitic voltages on the neutral. If its 240V leg to leg, with N in the center of one leg, what is N to B for? What is the voltage B to C? Furthermore if its 240 Volts three phase on all legs, what equipment uses it, most are 208? Since 120 Volts is only available from one leg with a neutral how is power distributed in a building say to lighting or to other 120 Volt circuits maintaining some semblance of balance between the phases. I'm not saying you have never seen this but I never have and it can't be very common. Bob
Well, that certainly makes more sense than some sort of a nutty asymetrical grounding scheme. Oh well, based on the above comments, I guess anything is possible. What does the NEC have to say about it? Bob
Your two single phase 110 sources in series will give 220V or 0 depending on connection. This is the Edison or 3 wire single phase system in the 220V case (and something useless in the other case) which is a common North American configuration. In Europe, where it is not used, it is called a 2 phase system and if one defines an phase system as having n voltages to neutralThere are two basic forms of a 3 phase connection -star in which there is a common neutral or delta where there is no neutral. .You mention a voltage of 110V to neutral so the corresponding voltage between lines is
In a star connected 3 phase system the phase and line currents are the same but line to line voltages are greater than phase to neutral voltages by the factor of root(3). In a delta the phase and line to line voltages are the same but the line (external) currents are root(3) times the phase (internal) currents.
There are advantages and disadvantages to both and advantages for 3 phase over single phase (2 or 3 wire), particularly in rotating machines and transformers.