discribe about 3 phase 3 wires and 3 phase 4 wires distribution system?

Answer 1

A 3-phase system requires 3 power leads A transformer or load connected delta (A to B, B to C, C to A) has no provision for a neutral connection, so there is no point in running a 4th wire. (note: a ground wire IS required by code)..a Wye, or Y connection (A to N, B to N, C to N)
has provision for a neutral wire.

There are 4 possible 3-phase connections, Δ - Δ, Δ - Y, Y - Δ, and Y - Y. Only the Y - Y connection can support a neutral, and thus 3-phase-4-wire wiring. In this system, the neutral wire carries the current resulting from unequal balancing of the phase loads.

Answer 2

assuming that u know about the AC systems....

a 3 phase 3 wire system is that where three phases supply and are connected at a point called neutral. in this only 3 wires will be there , all 3 wires are phases and are supplies.. the 3 phases are haveing voltage and curent wavefoms(phasors) each of 120 degrees apart but are equal in magnitude and frequency.
this is possible only when all the three phases are balanced or have equal impedance.
but if the impedance offered in one of the phases is different then that phase will have current or voltage of an unequal magnitude than the other two. there is possibility that the phase angle difference between voltage and current will be different from the other two and the voltage and current phasors are not 120 degrees away from the other two phasors.
in the balanced 3 phase 3 wire system the sum of currents at the neutral point is 0. but if there is any imbalance is there then the sum would not be zero. the remaining current will cause more imbalance in the system if it is not removed. for that purpose another wire is connected to the neutral point. this forms the 3 phase 4 wire system.
it is not true to say that all 3 phase 4 wire systems are unbalanced again. the 4th wire may be connected just for a safety reason also to accomodate any imbalance.

well all this explanation is valid for a star connected system, but for a delta connected system, 3 phase 4 wire system is not possible. that is the reason star connection is more preferred for distribution that a delta connection.

Answer 3

Three phase 3 wire is often called a 'delta' connection since the load(s) are distributed between each pair of phases. The 4 wire systems are also called 'Y' systems since the loads are distributed between each phase and a 4'th 'common' phase. The 4 wire system is intrinsically a bit safer since the common phase can also be made electrically neutral (ground). Doug

Answer 4

Try checking the wiki for a bit of an overview at
http://en.wikipedia.org/wiki/3-phase

you could also check this electrical engineering website
http://www.acontractorslicense.com/books/vpg-chapter.html