In #electric circuits, there are two types of systems. They are single-phase systems and three-phase systems. In single-phase systems, the current flows through a single wire and to complete the circuit there is a return path called a #neutral line. In single-phase systems, there is only one phase.
A three-phase system has three #phases. The three phases are sent together from the generator to the load. Each phase will have a phase difference of 120°. All the three phases in a three-phase system are involved in generating the #power so that the power is continuous.
Line voltage: The #voltage between two phases in three-phase circuits is termed line voltage.
Phase voltage: Phase voltage is the voltage between one phase and the neutral line.
Three-phase #sinusoidal waveforms are shown below.
Generation of three-phase voltage:
In three-phase circuits, there are three equal voltages or EMFs whose #frequency is the same but there is a phase difference of 120°. A three-phase AC #generator produces these three voltages. A three-phase AC generator has three identical #windings which are electrically 120° degrees apart.
Consider three coils RR', YY', and BB'. The angle between the windings is 120° as illustrated in the figure below.
When the coils are rotated with a uniform velocity of w radians per second, an #emf is induced in the coils that are displaced by 120°. Assuming the voltage to be sinusoidal, the instantaneous values of the voltages of the three phases can be written as
V(RR') = Vm sin(wt)
V(YY') = Vm sin(wt - 120°)
V(BB') = Vm sin(wt - 240°)
The algebraic sum of the three voltages at any instant must be zero.
Phase sequence:
The phase sequence is the sequence in which the 3 phase supply reaches the maximum positive value.
Generally, #RYB is the phase sequence.
Types of connections in three-phase systems
Three-phase systems can be connected in two ways. They are
Star connection
Delta connection
1. Star connection:
The following figure shows the star connection
In #star connection, four wires are used out of which three are phase wires and one is the neutral wire. The neutral wire is taken from the star point. As it has a neutral point it is preferred for power #transmission over long distances.
If the current in all three phases is equal, then the current is said to be a balanced current. If the current in any one of the phases is different then it is said to be an unbalanced current. The neutral line does not have a significant role in the case of balanced current but in the case of unbalanced current, it has a significant role. It protects the transformer by making the unbalanced current flow towards the ground.
Line voltage in star connection is √3 times the phase voltage. The current is the same for line and phase.
Delta connection:
The delta connection is shown below
In #delta connection, three wires are used which is there is no neutral line in delta connection. As there is no neutral line the effect of unbalanced current cannot be eliminated so these are preferred for short distances. The ground can be used if neutral is required in the load station.
The line and phase voltages are equal in delta connection. Line current is given by √3 times the phase current.
There are four different ways in which star and delta connections can be arranged. They are
Star-Star connection
Star-Delta connection
Delta-Star connection
Delta-Delta connection