Three Phase AC Circuits
Three-phase systems are commonly used in generation, transmission and distribution of electric power. Power in a
three-phase system is constant rather than pulsating and three-phase motors start and run much better than singlephase motors. A three-phase system is a generator-load pair in which the generator produces three sinusoidal
There are two types of system available in electric circuit, single phase and three phase system. In single phase
circuit, there will be only one phase, i.e the current will flow through only one wire and there will be one return
path called neutral line to complete the circuit. So in single phase minimum amount of power can be transported.
Here the generating station and load station will also be single phase. This is an old system using from previous
In polyphase system, that more than one phase can be used for generating, transmitting and for load system. Three
phase circuit is the polyphase system where three phases are send together from the generator to the load. Each
phase are having a phase difference of 120°, i.e 120° angle electrically. So from the total of 360°, three phases are
equally divided into 120° each. The power in three phase system is continuous as all the three phases are involved
in generating the total power. The sinusoidal waves for 3 phase system is shown below The three phases can be
used as single phase each. So if the load is single phase, then one phase can be taken from the three phase circuit
and the neutral can be used as ground to complete the circuit..
The phase voltages va(t), vb(t) and vc(t) are as follows
v a Vm cos t
cost 240 ,
v b V m cos t 120
v c Vm
the corresponding phasors are
Vb Vm e j120
Vc Vm e j240 .