Unit 27
Three-Phase Circuits
Unit 27 Three-Phase Circuits
Objectives:
• Discuss the differences between threephase 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 singlephase 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 threephase 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
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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
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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
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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
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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
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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
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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
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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
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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.