Three-phase power help needed...?
Answer:
You can understand a lot of the nitty gritty from considering an equilateral triangle, in which the center point is also drawn and connected to the vertices by lines. Label the vertices A, B, and C, and the center point O. Also, mark the midpoint of the bottom leg BC and label it M. There are two common ways of hooking things up, but they have in common that the voltage BO is 120 degrees out of phase with AO, and the voltage CO is 120 degrees out of phase with both of the other two.
Way #1: wye connection. If AO = BO = CO = 120 volts, then AB = BC = AC = 208 volts. This is not commonly used at low voltage, although high voltage transmission uses it (with multiples of the voltage, of course). Some motors are designed to run on 208 volts, 3 phase.
Way #2: delta connection. Point O is not connected; so only voltages AB, BC, and AC are defined. It is common to let point M be the neutral point, with voltages AB, etc. being 240 volts. Then MB and MC are each 120 volts, and standard house wiring rules apply for these, but if you have a 3-phase 240 volt motor, you can connect it to A, B, and C, and be in business. (A little geometry will show you that voltage AM is 208.)
Variations of this theme apply. In Saudi Arabia, typical house wiring delivers 135 volts at outlets. I found this odd until I looked at a breaker panel and discovered that the wiring was 3 phase; obviously, it was 240 volts, and in that scheme, voltages AO, etc. are 135 volts.
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i think you are talking about the household power supply 208/120v wye power supply. 120v is the phase voltage, and 208v is the line voltage. the relationship is 208=root(3)*120
in three phase Y connected circuits the difference between any line and the grounded neutral is around 240v, and between any two lines in delta connection is 1.73 times 240=415v, there in no such things like 208v, the explanation is that the lines are equally displaced from the neutral,so in equilateral triangle the difference is the square root of three(1.73) time the phase difference 240v, be careful do`nt touch two live lines, otherwise you will get severe and even fatal shock
Below is an answer explaining why three-phase power is handier for motors especially.
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3-phase electricity is generated by using a rotating magnetic armature to induce current in the stationary windings of the generator. It's then easy to replicate this rotating field by winding your 3-phase motor much like the generator that made the electricity.
In a three phase system, three 120 volt windings can be connected in the "Wye" configuration, all having one of their ends connected in the center. The remaining 3 wires on the outside are the phases or "Hots", and will measure 208 Volts (120V * sqrt (3)) between any two (See rhsaunders' answer about Delta and Wye connections, no need to do it twice). This is called a 120/208V system. That alone is enough info to tell it's 3-phase 'cause of the square root of 3 relationship.
Single phase is just a voltage taken from any two wires of a three phase system. It can be two phases, or a phase and a neutral. Now it's just a voltage, and doesn't have any of the rotating characteristics of 3-phase.
Your house, which is single phase, is powered by a transformer with two wires on the high voltage side (Phase-phase or phase-neutral) and 3 wires on the low voltage side. These three wires are not 3-phase since there is only one winding on the low side. The three wires are the ends of the winding and the center. The overall voltage is 240 volts from end to end. Either end measured to center (The neutral) is 120V. This is called a 120/240V system, and you know it's single phase cause there's no square root of 3 relationship. See this old answer:
http://answers.yahoo.com/question/index;...
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