Three Phase AC Circuit:

Alternating circuits thus far discussed have single phase supply system and it's satisfactory for domestic application as light, fan and heating, etc. But for industrial applications and big size electrical devices, etc. 1-phase system has several limitations. Therefore, poly phase system (2-phase, 3-phase and 6-phase systems) is utilized in electrical engineering. However for transmission, generation and distribution and utilisation of electrical power a 3-phase system has been universally adopted. 2 and 6-phase are derived from the 3-phase system, if needed.

A set of three single phase sinusoidal voltage currents of same magnitude and frequency but have a progressive phase difference of 120^o or 2π /3 radians electrical constitutes balanced 3-phase voltage/current.

Figure illustrates that 3-phase voltage and their instantaneous values are given by,

V_aa′ = V_m sin ω

V_bb′ = V_m sin (ω - 120^o)

= V _m    sin ( ωt -( 2π/3) )

V_cc′ = V_m sin (ω - 240^o)

= V _m sin (ωt - (4π/3))

For a common n-phase system displacement among phases is given by (360^o (electrical)/ n) so for 2-phase system it is 90^o electrical.