Part 9 – Three-Phase AC and Calculations

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

Three-phase Circuit Graphs highlighting Volts Volts Volts 0 Pure direct current is perfectly smooth: no ripple mmm Rectified single phase AC is a series of pulses: high ripple DC Ripple 0 Rectified three phase AC is a series of overlapping pulses: low ripple DC.

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

Activity: Three-phase Circuits

Click the arrows to view images of three-phase circuits.

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

Activity: AC Generator Connected Three-Phase Star

Click the image hotspots to learn more about AC generator connected three-phase star.

Activity: Three-Phase Voltage Vector

Click the tick marks on the slider to learn more about three-phase voltage vector.

Activity: Star Winding

Click the tick marks on the slider to learn more about star windings.

Calculating Phase Voltage

OD = OA cos 30°

OD = 0.866 x OA

And

OE = 2 x OD

= 2 x 0.866 x OA

= 1.732 OA

Or

OE = √3 OA

 

Three-Phase Voltage Isosceles Triangle highlighting B'D 1 60° 300 Phase volts E Figure 50
Three-Phase Voltage Isosceles Triangle
Voltage

Line volts  =  √3 Phase volts

ELine  =  √3 x EPhase

Current
Line Current  =  Phase Current
ILine  =  IPhase

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.

AC Generator Connected Three-Phase Delta highlighting BIz B IB C Ic
AC Generator Connected Three-Phase Delta

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.

Phase/Current Relation highlighting Line A = √3 Phase AA Phase A C Line A 1200 B
Phase/Current Relation

Current

Line Current = √3  Phase Current

ILine = √3 x IPhase

Voltage

Line Voltage = Phase Voltage

ELine = EPhase 

P1 No P2 P1 P2 P3 P3 Star Connection Connection Types Delta Connection

License

PEG-3722 Electrotechnology Copyright © by Josee Beaulieu. All Rights Reserved.

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