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Unit 27
Three-Phase Circuits
Unit 27 Three-Phase Circuits
Objectives:
• Discuss the differences between three-phase and single-phase voltages.
• Discuss the characteristics of delta and wye connections.
• Compute voltage and current values for delta and wye connections.
• Compute the amount of capacitance needed to correct the power factor of a three-phase motor.
Unit 27 Three-Phase Circuits
Three-Phase Advantages
1. The horsepower rating of three-phase
motors and the kVA rating of three-phase
transformers are 150% greater than single-
phase motors or transformers of similar
frame size.
Unit 27 Three-Phase Circuits
Three-Phase Advantages
2. The power delivered by a single-phase system pulsates and falls to zero. The three-phase power never falls to zero. The power delivered to the load in a three-phase system is the same at any instant. This produces superior operating characteristics for three-phase motors.
Unit 27 Three-Phase Circuits
Three-Phase Advantages
3. A three-phase system needs three
conductors; however, each conductor is
only 75% the size of the equivalent kVA
rated single-phase conductors.
Unit 27 Three-Phase Circuits
Three-phase power never falls to zero.
Unit 27 Three-Phase Circuits
Three-phase voltages with 120 degrees
of phase shift.
Unit 27 Three-Phase Circuits
Basic Properties
• Three-phase systems have either three or four conductors.
• There are three-phase conductors identified as A, B, and C.
• The three phases are 120 degrees out of phase with each other (360 divided by 3).
• There is sometimes a fourth conductor, which is the neutral.
Unit 27 Three-Phase Circuits
Wye Connections
• The wye, or star, connection is made by connecting one end of each of the phase windings together in a common node.
• Each phase winding has a voltage drop known as the phase voltage.
• The line voltage is measured from phase conductor to a different phase conductor.
Unit 27 Three-Phase Circuits
Wye Connections
• In a wye system, the line voltage is
higher than the phase voltage by a factor
of the square root of 3 (1.732).
• ELine = EPhase x 1.732
• EPhase = ELine / 1.732
Unit 27 Three-Phase Circuits
Wye Connections
• In a wye system, the line current is equal
to the phase current.
• ILine = IPhase
Unit 27 Three-Phase Circuits
Line and phase voltages in a wye connection.
Unit 27 Three-Phase Circuits
Line and phase currents in a wye connection.
Unit 27 Three-Phase Circuits
Vector sum of typical wye system voltages.
Unit 27 Three-Phase Circuits
Delta Connections
• In a delta system, the line current is
higher than the phase current by a factor
of the square root of 3 (1.732).
• ILine = IPhase x 1.732
• IPhase = ILine / 1.732
Unit 27 Three-Phase Circuits
Delta Connections
• In a delta system, the line current is
equal to the phase current.
• ELine = EPhase
Unit 27 Three-Phase Circuits
Delta system voltage and current relationships.
Unit 27 Three-Phase Circuits
Delta system division of currents.
Unit 27 Three-Phase Circuits
Three-Phase Power
• Three-phase power can be computed in two ways, using line values or phase values.
• VA = 3 x ELine x ILine
• VA = 3 x EPhase x IPhase
• Note that this is the same on wye or delta systems.
Unit 27 Three-Phase Circuits
Three-Phase Power
• Computing watts requires using the
power factor (PF).
• P = 3 x ELine x ILine x PF
• P = 3 x EPhase x IPhase x PF
• Note that this is the same on wye or
delta systems.
Unit 27 Three-Phase Circuits
Example #1 given values.
Unit 27 Three-Phase Circuits
Example #2 given values.
Unit 27 Three-Phase Circuits
Example #3 given values.
Unit 27 Three-Phase Circuits
Example #4 given values.
Unit 27 Three-Phase Circuits
Review:
1. The voltages of a three-phase system are 120° out of phase with each other.
2. The two types of three-phase connections are wye and delta.
3. Wye connections are characterized by the fact that one terminal of each of the devices is connected together.
Unit 27 Three-Phase Circuits
Review:
4. In a wye connection, the phase voltage is less than the line voltage by a factor of 1.732. The phase current and the line current are the same.
5. In a delta connection, the phase voltage is the same as the line voltage. The phase current is less than the line current by a factor of 1.732.