Star Connection in a 3 Phase System

Star Connection in Three-Phase Systems

In a star (Y) connection, the similar ends (either starts or finishes) of the three windings are joined at a common point called the star or neutral point. The three line conductors extend from the remaining free terminals to form the phase connections.

For a three-phase, three-wire system, only the three line conductors are connected to the external circuit. Alternatively, a four-wire system includes a neutral conductor drawn from the star point, as illustrated in the diagram below:

Star Connection Analysis with Phase and Line Quantities

With reference to the figure above, the finish terminals (a2, b2, c2) of the three windings are joined to form the star (neutral) point. The three line conductors (labeled R, Y, B) extend from the remaining free terminals, as illustrated.

• Phase current (Iₚₕ): Current through each winding phase.

• Line current (Iₗ): Current through each line conductor.

• Phase voltage (Eₚₕ): Voltage across each winding phase.

• Line voltage (Eₗ): Voltage between two line conductors.

Phase Voltage vs. Line Voltage in Star Connection

The star connection configuration is depicted in the figure below:

Star Connection in Balanced Three-Phase Systems

In a balanced system, the three phases (R, Y, B) carry equal currents. Consequently, the phase voltages E_NR, E_NY, and E_NB are equal in magnitude but displaced by 120° electrical from each other.

Phasor Diagram of Star Connection

The phasor diagram for the star connection is shown below:

The arrowheads on the EMFs and current indicate direction and not their actual direction at any instant.

Now,

Hence, in star connection line voltage is root 3 times of phase voltage.

Hence, in a 3 Phase system of star connections, the line current is equal to phase current.