Three-phase Circuits

Three-phase AC is used extensively because of the:

• Savings are involved in the transmission of large amounts of power compared with a single-phase system.
• More even flow of power.
• Three-phase machinery and its control equipment are smaller and more efficient than single-phase equipment of the same power output.

The combination of three single-phase circuits as far as the relationships of current voltage and power are concerned. It reduces the pulsing associated with single-phase circuits

If the amount of rotation required to generate this were divided into thirds The three conductors would be spaced 120 electrical degrees apart. Three windings wound upon a generator armature, spaced 120 electrical degrees apart brought out to separate slip-rings

AC generator design:

• Armature windings producing the power are stationary (stator): high-voltage three-phase generating windings (stator windings) remain stationary.
• Field windings rotate (rotor): only a low-voltage DC current has to be supplied through brushes and slip-rings to the rotating field coils (rotor windings).

The following are two methods for connecting three-phase windings:

1. Star: also known as “wye” connection
2. Delta

Star Winding

Line volts  =  √3 Phase volts

E_Line  =  √3 x E_Phase

Delta

Windings in which the three connections are brought out to form the three lines A, B, and C. No neutral point is available in this arrangement.

Line Voltage = Phase Voltage

The current flowing in each line (line current) is made up of the currents flowing in the two-phase windings connected to that line.

Current

Line Current = √3  Phase Current

I_Line = √3 x I_Phase

Voltage

Line Voltage = Phase Voltage

E_Line = E_Phase