Unit 26 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 26 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 26 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 26 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 26 Three-Phase Circuits
Three-phase power never falls to zero.
Unit 26 Three-Phase Circuits
Three-phase voltages with 120 degrees
of phase shift.
Unit 26 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 26 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 26 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 26 Three-Phase Circuits
•
•
Wye Connections
In a wye system, the line current is equal
to the phase current.
ILine = IPhase
Unit 26 Three-Phase Circuits
Line and phase voltages in a wye connection.
Unit 26 Three-Phase Circuits
Line and phase currents in a wye connection.
Unit 26 Three-Phase Circuits
Vector sum of typical wye system voltages.
Unit 26 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 26 Three-Phase Circuits
•
•
Delta Connections
In a delta system, the line current is
equal to the phase current.
ELine = EPhase
Unit 26 Three-Phase Circuits
Delta system voltage and current relationships.
Unit 26 Three-Phase Circuits
Delta system division of currents.
Unit 26 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 26 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 26 Three-Phase Circuits
Example #1 given values.
Unit 26 Three-Phase Circuits
Example #2 given values.
Unit 26 Three-Phase Circuits
Example #3 given values.
Unit 26 Three-Phase Circuits
Example #4 given values.
Unit 26 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 26 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.