When a single turn coil is rotating counter-clockwise direction with a uniform angular velocity of ‘ω’ radian per second in a uniform magnetic field then the single turn of the coil will produce an emf and the system is known as single-phase supply. Single-phase supply is normally used for lighting load and fractional horsepower motor loads.
When three identical coils are so placed that they are having 120° electrical apart between the three coils. Then the system is said to be a three-phase system.
Taking A, B & C are the starting terminal of 1st, 2nd 3rd coils & A’, B’, & C’ are the finishing terminals of 1st 2nd 3rd coils. Taking ‘AA’ coil as a reference point. The three emf’s are represented as follows:
eAA’ = Em sin ωt
eBB’ = Em sin (wt-120°)
eAA’ = Em sin (wt-240°)
= Em sin (wt+120°)
Medium voltage 3 phase supply is used for motor loads and high voltage 3 phase supply is used for transmission of electrical power from generating station to sub-station.