Unit 27 Three-Phase Circuits - PBworksokanagancollegefoundationrefrigeration.pbworks.com/w/file/fetch...Unit 27 Three-Phase Circuits Objectives: • Discuss the differences between

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.


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.



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.



3. A three-phase system needs three

conductors; however, each conductor is

only 75% the size of the equivalent kVA

rated single-phase conductors.


Three-phase power never falls to zero.


Three-phase voltages with 120 degrees

of phase shift.


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.


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.


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


Wye Connections

• In a wye system, the line current is equal

to the phase current.

• ILine = IPhase


Line and phase voltages in a wye connection.


Line and phase currents in a wye connection.


Vector sum of typical wye system voltages.


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


Delta Connections

• In a delta system, the line current is

equal to the phase current.

• ELine = EPhase


Delta system voltage and current relationships.


Delta system division of currents.


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.


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.


Example #1 given values.








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.


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.

Unit 27 Three-Phase Circuits - PBworksokanagancollegefoundationrefrigeration.pbworks.com/w/file/fetch...Unit 27 Three-Phase Circuits Objectives: • Discuss the differences between

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