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I hope u were trying to ask why we have make our circuit ground and supply ground common......
If so it is for the return path for the current.
If not placed ur circuit becomes floated and no current flows through the circuit
In standard house lines there are 3 wires: Hot, Neutral, and Ground. Ground is the chassis ground and neutral is your system ground. In some cases neutral is connected to chassis ground, but it generally isn't. The purpose of this is to have a direct path to ground if a fault to ground on the chassis occurs (a wire comes loose, a cable drops on the box, etc).
As for a two terminal line, the white one (neutral) is your "ground". MAKE SURE that you measure its potential against a grounded source (like the 3rd prong in an outlet) to ensure that it is actually neutral (neutral will have 0VAC difference, hot (black) will have 220VAC difference).
For a transformer, just hook up both of your lines on the primary side and you will have a voltage on the secondary side. All a single phase transformer needs is one current carrying line and a neutral.
check out ur AC supply......
what is ur freuqency?
I think it is 120V AC source.In this u have main path for current and other ground.
U can connect ur main path to one end of ur transformer and other to ground at primary side.
Generally we have 3 wires in our house hold where one is current path, one ground and other nuetral.
Voltage between main path and ground is 230V.They are 180deg out of phase
Voltage between main path and nuetral is half of it.
These are single phase systems
U have three phase systems where 3 liness differ in phase by 120deg.