unit 27

Unit 27Three-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.


Three-Phase Advantages2. 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.


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.


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

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