first thanks leitmotif , keithkyll , and Jeev
second i am not electerical power engineer , so this forum may be simple to many , but i need to understand this as i need to understand this concept for a forum which is :
http://www.plctalk.net/qanda/showthread.php?t=41364&page=2&pp=15
i still do not understand well why the two phases of delta connection acts as a single phase ??
how then we can obtain two phases or three phase for delta connection ?? as if we take any two phases for delta connection they are called a single phase - not two phases - ??
if we take two phases for star connection - not delta connection - and connected them to the scope , what do we show at the scope for these two phases for star connection ??
do we show single sine wave ?? or do we show two sine waves ?? and why for either answer ??
my questions may be simple to many , but i still need to understand the answres of it well !!
i know very little informatiom for star and delta connection as i know the voltage between phase to phase for star connection is the voltage between phase to nuetral multiplied by the root of three , and the phase current is the line current and vice versa for delta connection , but i need to understand this concept well as i said i need it for the above forum , as i want to know when we consider they are two phases and when we consider it acts as a single phase !!
the explain by leitmotif is good as it opened to me some things , but i still need more !!
another additional question : what is the advantage of use less operated voltage at USA which is 120 VAC while at Egypt and Europe the operated voltage is more as it is 220 VAC , as if the electerical power of any device is same , so the current pass through the device which operated by 120 VAC is the double of the current pass through the device which operated by 220 VAC , as then we need cable with big cross section area and the cost increases , so i want to know - as an additinal question to my orignal question - what is the advantage of use less voltage at USA which is 120 VAC while at Egypt and Europe is more voltage as it is 220 VAC ??
thanks