Unit 27 Three-Phase Circuits

Transcription

1 Unit 27 Three-Phase Circuits

2 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.

3 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.

4 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.

5 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.

6 Three-phase power never falls to zero.

7 Three-phase voltages with 120 degrees of phase shift.

8 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.

9 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.

10 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). E Line = E Phase x E Phase = E Line / 1.732

11 Wye Connections In a wye system, the line current is equal to the phase current. I Line = I Phase

12 Line and phase voltages in a wye connection.

13 Line and phase currents in a wye connection.

14 Vector sum of typical wye system voltages.

15 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). I Line = I Phase x I Phase = I Line / 1.732

16 Delta Connections In a delta system, the line current is equal to the phase current. E Line = E Phase

17 Delta system voltage and current relationships.

18 Delta system division of currents.

19 Three-Phase Power Three-phase power can be computed in two ways, using line values or phase values. VA = 3 x E Line x I Line VA = 3 x E Phase x I Phase Note that this is the same on wye or delta systems.

20 Three-Phase Power Computing watts requires using the power factor (PF). P = 3 x E Line x I Line x PF P = 3 x E Phase x I Phase x PF Note that this is the same on wye or delta systems.

21 Example #1 given values.

22 Example #2 given values.

23 Example #3 given values.

24 Example #4 given values.

25 Review: Unit 27 Three-Phase Circuits 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.

26 Review: Unit 27 Three-Phase Circuits 4. In a wye connection, the phase voltage is less than the line voltage by a factor of 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