Three-Phase AC Circuits

Transcription

1 Salman bin AbdulazizUniversity College of Engineering Electrical Engineering Department EE 2050Electrical Circuit Laboratory Three-Phase AC Circuits Experiment # 5 Objectives: 1. Discuss the differences between three-phase and single-phase voltages. 2. Discuss the characteristics of delta and wye connections. 3. Compute voltage and current values for delta and wye circuits 4. Connect delta and wye circuits and make measurements with measuring instruments. Equipments: 1. Three similar loading resistors 2. Three-phase, multi-function meter. 3. Two AC voltmeters, two AC ammeters. 4. Three-phase auto transformer. Note: Please record any damage, if exists any while performing the experiment. You can also write the difficulties you are confronted with when using the equipment, and your suggestions and critics related with the equipment you used. 1. Background: Most of electrical energy is generated and transmitted using 3-phase system. There are several reasons why three-phase power is superior to single phase power. The power delivered by a single-phase system pulsates, as shown in Fig. 1. The power falls to zero three times during each cycle. The power delivered by a three-phase circuit pulsates also, but it never falls to zero, as shown in Fig. 2. In a three-phase system, the power delivered to the load is the same at any instant. This produces superior operating characteristics for three-phase motors. Experiment # 4 Page 1

2 Fig. 1. Single phase Fig. 2. Three phase A single-phase alternating voltage can be produced by rotating a magnetic field through the conductors of a stationary coil, as shown in Fig. 3. Since alternate polarities of the magnetic field cut through the conductors of the stationary coil, the induced voltage will change polarity at the same speed as the rotation of the magnetic field. Fig. 3. Producing a single-phase AC voltage. If three separate coils are spaced 120 apart, as shown in Fig. 4, three voltages 120 out of phase with each other will be produced when the magnetic field cuts through the coils. Fig. 4. The voltages of a three-phase system are 120 out of phase with each other Experiment # 4 Page 2

3 This is the manner in which a three-phase voltage is produced. There are two basic three-phase connections, the wye or star connection and the delta connection. 1.1 Wye Connection The wye or star connection is made by connecting one end of each of the three-phase windings together as shown in Fig. 5. The voltage measured across a single winding or phase is known as the phase voltage, The voltage measured between the lines is known as the line-to-line voltage or simply as the line voltage, as shown in Fig. 6. Fig. 5. A wye connection is formed by joining one end of each of the windings together Fig. 6. Line and phase voltages are different in a wye connection In a wye connected system, the line voltage is higher than the phase voltage by a factor of the square root of 3 (1.732). Two formulas used to compute the voltage in a wye connected system are: Experiment # 4 Page 3

4 If ammeters have been placed in the phase winding of a wye-connected load and in the line supplying power to the load, as shown in Fig. 7, we notice that the phase current and the line current are the same. One formula used to compute the current in a wye-connected system Fig. 7. Line current and phase current are the same in a wye connection If the Voltmeter connected to the phase winding indicates 120 V, then the voltmeter connected across the lines indicates a value of ( ) 208 V. Regarding current, 10 A of current flow in both the phase and the line. Therefore both ammeters indicate the same reading. As said before, three-phase voltages are 120 apart. If the three voltages are drawn 120 apart as shown in the phasor diagram shown in Fig. 8, it will be seen that the vector sum of these voltages is 208 V. Fig. 8. Phasor diagram method of adding three-phase vectors Experiment # 4 Page 4

5 Another illustration of vector addition is shown in Fig. 9. In this illustration two-phase voltage vectors are added and the resultant is drawn from the starting point of one vector to the end point of the other. 1.2 Delta Connection Fig. 9. Vector sum of the voltages in a three-phase wye connection. In Fig. 10, three separate inductive loads have been connected to form a delta connection. This connection receives its name from the fact that a schematic diagram of this connection resembles the Greek letter delta ( ). Fig. 10. Three-phase delta connection. In Fig. 11, voltmeters have been connected across the lines and across the phase. Ammeters have been connected in the line and in the phase. In the delta connection, line voltage and phase voltage are the same. Notice that both voltmeters indicate a value of 480 V. One formula used to compute the voltage in a delta-connected system Fig. 11. Voltage and current relationships in a delta connection. Experiment # 4 Page 5

6 Notice that the line current and phase current are different, however. The line current of a delta connection is higher than the phase current by a factor of the square root of 3 (1.732). Formulas for determining the current in a delta connection are: The vectorial sum of two phase currents gives the line current as shown in Fig. 12. Fig. 12. Vector addition is used to compute the sum of the currents in a delta connection. 1.3 Three-Phase Power Students sometimes become confused when computing power in three phase circuits. One reason for this confusion is that there are actually two formulas that can be used. If line values of voltage and current are known, the apparent power (VA) can be computed using the formula: If the phase values of voltage and current are known, the apparent power can be computed using the formula: Notice that in the first formula, the line values of voltage and current are multiplied by the square root of 3. In the second formula, the phase values of voltage and current are multiplied by 3. The first formula is used more often because it is generally more convenient to obtain line values of voltage and current, which can be measured with a voltmeter and clamp-on ammeter. Experiment # 4 Page 6

7 2. Procedure: Three Phase Balanced Loading Three Phase Supply L 1 L 2 L3 N 3- PHASE MULTI FUNCTION METER Three-phase, multi function meter Fig. 13 Three-phase balanced load 1. Connect the circuit as shown in Fig Set the balanced load as three florescent lambs. 3. For the 3-phase supply, measure the three line voltages (V L1L2 ), (V L2L3 ) and (V L3L1 ). Also measure the three phase voltages (V L1N ), (V L2N ) and (V L3N ) V L1L2 = volt V L2L3 = volt V L3L1 = volt V L1N = volt V L2N = volt V L3N = volt 4. Are the three line voltages same? If not, then why? Experiment # 4 Page 7

8 5. Comment on the difference between the two sets of measurements taken in step Due to loading, Record the readings of the three line/phase currents (I L1 ), (I L2 ) and (I L3 ). I L1 = amp I L2 = amp I L3 = amp 7. Are the three line/phase currents same? If not, then why? 8. The florescent lambs are considered as RL load and there is phase difference between the phase voltage and phase current which is characterized by the load power factor PF. So measure the three values of PF and record it as follows PF L1N = PF L1N = PF L1N = 9. Calculate the phase impedance (Z L1N ), (Z L2N ) and (Z L3N ) Z L1N = R L1N + j X L1N = Ω Z L2N = R L2N + j X L2N = Ω Z L3N = R L3N + j X L3N = Ω 10. Record the active power per phase (P m ) using the multi function meter P ml1 =. Watt Experiment # 4 Page 8

9 P ml2 =. Watt P ml3 =. Watt 11. Calculate the active power per phase (P C ) based on the formula P CL1 = 3 I 2 L1N R L1N P CL1 = 3 I 2 L1N R L1N =. Watt P CL2 = 3 I 2 L2N R L2N =. Watt P CL1 = 3 I 2 L3N R L3N =. Watt 12. Comment on the measured and calculated active power 13. Record the reactive power per phase (Q m ) using the multi function meter Q ml1 =. VAR Q ml2 =. VAR Q ml3 =. VAR 14. Calculate the reactive power per phase (Q C ) based on the formula Q CL1 = 3 I 2 L1N X L1N Q CL1 = 3 I 2 L1N X L1N =. VAR Q CL2 = 3 I 2 L2N X L2N =. VAR Q CL1 = 3 I 2 L3N X L3N =. VAR 15. Comment on the measured and calculated reactive power 16. Record the apparent (Total) power (S m ) using the multi function meter Experiment # 4 Page 9

10 S m =. VA 17. Calculate the apparent (Total) power (S C ) based on the formula S C = 3 V L1N I L1 S C =. VA 18. Calculate the apparent (Total) power (S C ) based on the formula S C = 3 V L1L2 I L1 S C =. VA 19. Comment on the measured power (step 16) and calculated apparent power (steps 17 & 18) 20. Can you find another method to calculate the apparent power S C? Experiment # 4 Page 10