# Three phase A.C supply

i was reading three phase ac supply and this statement is quite confusing ''because of the 120 degrees phase shift the voltage across any two lines is 400 volts (the voltage across each of the lines connected to the terminals is 230 volts)
can any body please explain it???????

stewartcs
i was reading three phase ac supply and this statement is quite confusing ''because of the 120 degrees phase shift the voltage across any two lines is 400 volts (the voltage across each of the lines connected to the terminals is 230 volts)
can any body please explain it???????

What do you find confusing about it? How they came up with the 400 volts?

CS

mgb_phys
Homework Helper
Assuming you mean european 230v/415V - 3phase (the answer is slightly different for US 120v 3phase but by coincidence some of the numbers are close enough to be confusing.)

A single phase measured relative to the ground varies between 0 and about 325V - with an RMS value of 230V =peak voltage divided by root(2).
The maximum voltage difference between any two phases is about 400V = RMS voltage * root(3)

the line voltage is the voltage "difference" of two phase voltages... that means one phase voltage should be reversed and added to another phase.. than u'll have 2x230xcos(30) as the line voltage ($$\approx$$400).... (check for the attachment, then u'll get a better idea)

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the line voltage is the voltage "difference" of two phase voltages... that means one phase voltage should be reversed and added to another phase.. than u'll have 2x230xcos(30) as the line voltage ($$\approx$$400).... (check for the attachment, then u'll get a better idea)

rookie question
how do you came up with 2(230)cos(30) and why 30????????

Dale
Mentor
2020 Award
Asok drew a picture showing very nicely exactly where the 30º came from. Please look at the picture and if you don't understand that then ask a specific question about it.