[SI-LIST] Re: transformer

Atul,
The 4th (neutral) wire of a Y-connected system carries the vector sum of 
the currents in the three "hot" legs.  If those currents are sinusoidal 
and equal, the current in the neutral is zero. In that case only, the 
neutral wire is redundant.

If the load currents are spiky- e.g. a rectifier-capacitor input filter 
typically found in computer power supplies, the neutral current will not 
be zero, nor will it be sinusoidal.  The diode conduction for any phase 
(a few degrees before the wave peak) is not time-coincident with the 
conduction of any diode on any other phase.  The neutral carries the 
sum, as always, but there is no cancellation because there is no 
conduction overlap.

"Power Factor Corrected" supplies have input current flow during a much 
larger fraction of the line period and will/may exhibit some degree of 
current cancellation in the neutral .  The line current is not 
necessarily sinusoidal, but  to the extent that a portion of the 
conduction cycle  overlaps the conduction on another phase, (partial) 
cancellation is possible.

In the 3-wire system, the difference currents flow in each phase wire.

Hope that helps.

Mike

Atul Rastogi wrote:

>** Proprietary **
>
>dear sir ,
>i want to know in three phase four wire system current flows through =
>neutral ?
>in three phase threewire the difference of current will flow where?
>how to connect the earth fault relay to system ?
>
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