Electronic – Why is three-phase offset by 120 degrees

mainsthree phase

For three phase electricity the wave is offset by 120 degrees(2\$\pi/3\$ Rad). Why aren't the phases closer together? Is it because it will affect the frequency of the phases? How was this 120 degrees chosen?

3 Phase Wave Form

Best Answer

When there's 120° between phases the sum of the voltages at any time will be zero.

3-phase vector diagram

This means that with a balanced load no current flows in the return line (neutral).

3 current vectors added

Also, if each phase is 230V with respect to the neutral (star operation), then there will be 230V \$\times\$ \$\sqrt{3}\$ = 400V between any two phases (triangle or delta operation), and they're also equally spaced, i.e. at 120° angles.

(images from http://www.electrician2.com/electa1/electa3htm.htm)