Equipment: Power Supply, DAI, Variable resistance (8311), Variable inductance (8321)

Transcription

1 Lab 4: 3-phase circuits. Objective: to study voltage-current relationships in 3-phase circuits; to learn to make delta and Y connections; to calculate and measure real, apparent, and reactive powers. Equipment: Power Supply, DAI, Variable resistance (8311), Variable inductance (8321) Theory: The total 3-phase power supplied to a 3-phase load is a sum of powers dissipated by each of the load resistors and is constant if the load is balanced. This property makes 3-phase circuits quite attractive. Therefore, it is important to ensure that the load is balanced. Since our PS is Y-connected, the ratio of line-to-line voltage to line-to-neutral must be approximately equal to 3. Denoting the impedance angle as θ, the real three-phase power can be calculated as: 2 E P= 3E cos 3 load lineiline θ = R The reactive three-phase power can be calculated as: 2 E Q= 3E sin 3 load lineiline θ = X The apparent three-phase power can be calculated as: S = E I = P + Q 3 line line 2 2 The power factor is PF = cosθ = P S See class notes for definitions of real, reactive, and apparent powers. Page 1

2 Experiment: Part I resistive load 1) Using the Power Supply (PS) and the DAI module (Metering window), measure the following line-to-line voltages: Figure 04 1 To do this, connect three AC voltmeters to the indicated terminals of PS, turn the PS ON and adjust its output voltage to its highest value (turn the voltage adjusting knob to its most right position). Record these values of line-to-line voltage. 2) Turn the power OFF and reconnect three voltmeters to measure the voltage from each line to the neutral. Measure and record these line-to-neutral voltages. 3) Start the Oscilloscope and observe three line-to-neutral voltages. What do you conclude regarding the phase shift between them? Export your Oscilloscope data to a file. 4) Construct the following circuit using the Variable resistance module: Page 2

3 Figure 04 2 Notice that this circuit represents a Y -connected load. Set the load resistance to 600 Ω and apply a voltage of 120 V (line-to-neutral). In the metering window, turn ON three power meters and set them to Watts. Record the values to a Data table. 5) Connect the Δ -circuit as shown in Figure 04-3: Figure 04 3 To test that the circuit is connected correctly, turn all load resistors OFF (all switches in lower positions), apply an input voltage of approximately 30 V and then turn ON the load switches for 600 Ω resistors one after another. While doing this, the corresponding ammeters must start reading non-zero currents. When all three load resistors (of equal ratings) are connected, the currents reported by three ammeters should be approximately equal. Note: in this configuration, ammeters read the load currents. Page 3

4 Once the correct wiring is verified, apply an AC voltage of 120 V (while controlling the input voltages by the voltmeters) and record values of voltages, currents, and powers to your Data table. Turn the PS OFF. 6) Reconnect the AC ammeters to read the line currents as depicted in Figure Figure 04 4 Leave the voltmeters in the circuit to read the line-to-line voltages as in Figure 4-3. Apply an AC voltage of 120 V and record values of voltages, currents, and powers to your Data table. Turn the PS OFF. 7) Construct a Y -connected load similar to one depicted in Figure 04-2 but without any meters. Connect the load s neutral to the neutral of PS through the ammeter. Set three load resistances to 600 Ω and apply an input voltage of approximately 30 V. The current through the neutral wire must be approximately zero since the load is balanced. Next, unbalance the load by inserting a 1200 Ω resistor in parallel to one of the load resistors. Do you observe any changes in the value of current? Part II inductive load 8) Construct the Y -connected load as indicated in Figure 04-5: Page 4

5 Figure 04 5 Set the power meters in the Metering window to measure VARs, set each inductance section to reactance of 600 Ω and apply the AC voltage of 120 V line-to-neutral. Record the values of line currents, voltages across the inductances, and reactive powers for each inductive load to your Data table. Turn the PS OFF. 9) Construct the circuit shown in Figure 04-6: Figure 04 6 Page 5

6 After verifying the correct wiring, set the resistance of each load to 600 Ω and the reactance of each inductive load to 600 Ω. Apply an AC voltage of 120 V line-to-neural and record the measured values of line currents, voltages across the inductances, and reactive powers to your Data table. Turn the PS OFF. 10) Reconnect voltmeters as shown in Figure 04-7 Figure 04 7 Apply an AC voltage of 120 V line-to-neural and record the measured values of line currents, voltages across the resistors, and real powers dissipated in the three resistors to your Data table. Turn the PS OFF. Save the Data table and disassemble your circuit. In your report: 1. Calculate the ratio of the average line-to-line voltage to the average line-to-neutral voltage for measurements in Parts 1 and 2. Does this ratio approximately equal to 3? 2. Using Matlab and the data exported from the Oscilloscope, plot (on the same axes) the three line-to-neutral voltages. What is the approximate phase difference between them? 3. Using Matlab and the Data table you recorded in Part 4, report the line currents and the voltages across the load resistors you have measured. Are the voltages and currents reasonably well balanced? Calculate the power dissipated by each load and compare it to the measured values. Discuss possible sources of discrepancy. Calculate the total 3-phase power. Page 6

7 4. Using Matlab and the Data table you recorded in Part 5, report the load currents and the line-to-line voltages you have measured. Are the voltages and currents reasonably well balanced? Calculate the power dissipated by each load and compare it to the measured values. Discuss possible sources of discrepancy. Calculate the total 3-phase power. 5. Using Matlab and the Data table you recorded in Part 6, report the line currents you have measured. Calculate the average load current and the average line current. Calculate the ratio of the average line current to the average load current. Is this ratio approximately equal to 3? 6. Describe your observations in Part 7. What do you conclude regarding unbalancing the load? 7. Using Matlab and the Data table you recorded in Part 8, report the line currents and the voltages across the loads that you have measured. Are the voltages and currents reasonably well balanced? Calculate the reactive power for each of the inductive loads and compare it to the measured values. Discuss possible sources of discrepancy. Calculate the total 3-phase reactive power. 8. Using Matlab and the Data table you recorded in Parts 9 and 10, report the line currents and the voltages across the inductances and across the resistors that you have measured. Are the voltages and currents reasonably well balanced? Calculate the reactive power for each of the inductive loads and compare it to the measured values. Calculate the real power for each of the resistive loads and compare it to the measured values. Discuss possible sources of discrepancy. Calculate the total 3-phase real power. Calculate the total 3-phase reactive power. Calculate the total 3-phase apparent power. Calculate the power factor using the total 3-phase real and apparent powers. Page 7