Lab 16 Parallel RL Circuits On line Leon M 17Mar2020 (1)

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Lab 16 (eBook 25) Parallel RL Circuits (simplified) Name ____________________ Date ___________________ Class ___________________ READING Text, Sections 12–4 through 12–9 OBJECTIVES After performing this experiment, you will be able to: 1. Determine the current phasor diagram for a parallel RL circuit. 2. Measure the phase angle between the current and voltage for a parallel RL circuit. 3. Explain how an actual circuit differs from the ideal model of a circuit. MATERIALS NEEDED Resistors: One 3.3 kΩ, two 47 Ω, One 100 mH inductor REQUIRED LAB PREPARATION (PRELAB) 1. Read all sections of the lab. 2. READ document named “BRIGHTSPACE ON-LINE LAB ASSIGNMENT AND SUBMISSION PROCEDURE” available in Lab 11 folder 3. Read the text book, sections 12-44 through 12-9 4. Find the required resistor and inductor for this lab as indicated in the Materials Needed section above 5. Review the differential probe measurement and phase measurement techniques from Labs 8 and 14. 6. Review the oscilloscope time “base operation” or what is called Horizontal Control depending on the oscilloscope manufacturer. 7. All Voltage and Phase angle measurements are performed with the oscilloscope. No DMM . Fall 2015 Lab 16 P a g e | 1

8. Complete the PreLab questions at the back of this document and hand in to teacher before going to your Lab station the back of this document and hand in to teacher before going to your Lab station SUMMARY OF THEORY The parallel RC circuit was investigated in Experiment 12. Recall that the circuit phasor diagram was drawn with current phasors and the voltage phasor was used as a reference, since voltage is the same across parallel components. In a parallel RL circuit, the current phasors will again be drawn with reference to the voltage phasor. The direction of the current phasor in a resistor is always in the direction of the voltage. Since current lags the voltage in an inductor, the current phasor is drawn at an angle of −90° from the voltage reference. A parallel RL circuit and the associated phasors are shown in Figure 16–1 . Figure 16–1 Practical inductors contain resistance that frequently is large enough to affect the purely reactive inductor phasor drawn in Figure 16–1 . The resistance of an inductor can be thought of as a resistor in series with a pure inductor. The effect on the phasor diagram is to reduce an angle between I L and I R . In a practical circuit this angle will be slightly less than the −90° shown in Figure 16–1 . This experiment illustrates the difference between the approximations of circuit performance based on ideal components and the actual measured values. Recall that in Experiment 15, the phase angle between the source voltage, V S , and the resistor voltage, V R , in a series circuit were measured. The oscilloscope is a voltage-sensitive device, so comparing these voltages is straightforward. In parallel circuits, the phase angle of interest is usually between the total current, I T , and one of the branch currents. To use the oscilloscope to measure the phase angle in a parallel circuit, we must convert the current to a voltage. This was done by inserting a small resistor in the branch where the current is to be measured. The resistor must be small enough not to have a major effect on the circuit. Fall 2015 Lab 16 P a g e | 2

PROCEDURE 1. See document titled” BRIGHTSPACE assignment and submissions process” which is available in Lab 11 folder, 2. These include a. Simulate the circuit in Multisim and record the required results in the appropriate table, b. Paste into a blank sheet an image of your actual breadboard as if you were doing the experiment in the Lab, and c. Submit this completed Lab document, and the simulation files via the assignment folder in Brightspace. 3. Record the results as required in Table 16-1 . Construct the circuit shown in Figure 16–2 . Notice that the reference ground connection is at the low side of the generator . This connection will enable you to use a generator that does not have a “floating” common connection. Using your oscilloscope, set the generator to a voltage of 6.0 V pp at 5.0 kHz. Check both the voltage and frequency with your oscilloscope. Record all voltages and currents in this experiment as peak-to- peak values. Listed Value Measured Value Show as many significant digits as possible on your best measuremen t range Voltage Drop Show V or mV and as many digits as possible Computed Current Ohm’s law, mA R 1 (Ir) 3.3 kΩ R S 1 (It) 47 Ω R S 2 (I L ) 47 Ω L 1 100 mH _______mH Same as R S2 R W ( L 1 resistance) _______ Ω Fall 2015 Lab 16 P a g e | 3

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in Ω Table 16–1 Figure 16–2 4. Using the oscilloscope , measure the peak-to-peak voltages across R 1 , R S 1 , and R S 2 . Use the two-channel difference method (described in Experiment 8) to measure the voltage across the two ungrounded resistors. Apply Ohm’s law to compute the current in each branch. Record the measured voltage drops and the computed currents in Table 16–1 . Since L 1 is in series with R S2 , enter the same current for both . 5. The currents measured indirectly in step 4 are phasors because the current in the inductor is lagging the current in R 1 by 90°. The current in the inductor is the same as the current in R S 2 , and the total current is through R S 1 . Using the computed peak-to-peak currents from Table 16–1 , draw the current phasors for the circuit on Plot 16–1 . (Ignore the effects of the sense resistors.) Phase Angle Between: Computed Measured I T and I R I R and I L 90° I T and I L Fall 2015 Lab 16 P a g e | 4

Table 16–2 You can use Excel or any other App to create the specified plots Erase the currently shown grids and copy paste the plots on this page Show the scale, units of the phasor diagram axis, including the phase angle in degrees. Plot 16–1 Fall 2015 Lab 16 P a g e | 5

6. The phasor diagram illustrates the relationship between the total current and the current in each branch. Using the measured currents, compute the phase angle between the total current ( I T ) and the current in R 1 ( I R ). Then compute the phase angle between the total current ( I T ), and the current in L 1 ( I L ). Enter the computed phase angles in Table 16–2 . (Note that the computed angles should add up to 90°, the angle between I R and I L . ) 7. In this step, you will measure the phase angle between the generator voltage and current. This angle is approximately equal to the angle between I T and I R as shown in Figure 16–1 . (Why?) Connect the oscilloscope probes as shown in Figure 16–3 . Measure the phase angle using one of the methods of Lab 15 . The signal amplitudes in each channel are quite different, so the vertical sensitivity controls should be adjusted to make each signal appear to have the same amplitude on the scope. Record the measured angle between I T and I R in Table 16–2 . Figure 16–3 8. Replace R S 1 with a jumper. This procedure enables you to reference the low side of R 1 and R S 2 . Measure the angle between I L and I R by connecting the probes as shown in Figure 16–4 . Ideally, this measurement should be Fall 2015 Lab 16 P a g e | 6

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90°, but because of the coil resistance, you will likely find a smaller value. Adjust both channels for the same apparent amplitude on the scope face. Record your measured result in the second line in Table 16–2 . Figure 16–4 9. By subtracting the angle measured in step 7 from the angle measured step 8, you can find the phase angle between the I T and I L . Record this as the measured value on the third line of Table 16–2 . CONCLUSION ---------------------------------------------------------------------- ---------------------------------------------------------------------- - ---------------------------------------------------------------------- - ---------------------------------------------------------------------- - EVALUATION AND REVIEW QUESTIONS 1. If we assume that the currents determined in step 4 are 90° apart, the magnitude of the total current can be computed by applying the Pythagorean theorem to the current phasors. That is Fall 2015 Lab 16 P a g e | 7

(a)Compare the total current measured in R S 1 ( Table 16–1 ) with the current found by applying the Pythagorean Theorem to the current phasors. (b)What factors account for differences between the two currents? 2. How does the coil resistance measured in step 2 affect the angle between the current in the resistor and the current in the inductor? 3. In Experiment 12 (Parallel RC circuits), a 1.0 kΩ resistor was used as a current-sensing resistor. Why would this value be unsatisfactory in this experiment? 4. If the inductor were open, what would happen to each? (a) the total current in the circuit (c) the phase angle between the generator voltage and current (d)the generator voltage Fall 2015 Lab 16 P a g e | 8

5. If the frequency were increased, what would happen to each? (a) the total current in the circuit (b) the phase angle between the generator voltage and current (c) the generator voltage Lab 16, Parallel RL Circuits PRE-LAB Complete the PreLab questions at the back of this document and hand in to teacher before going to your Lab station Name ____________________ Date ___________________ Class ___________________ 1. In an RL parallel circuit, explain the current flow from V S into both R and L the instant V S is turned on. 2. Does the I L lead or lag I R ? Explain your answer. Fall 2015 Lab 16 P a g e | 9

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3. Does the actual resistance of an Inductor coil make a difference in the circuit? 4. What if anything is the effect of the coil resistance and how does it manifest itself? 5. In which quadrant is the current vector diagrams drawn and why? 6. In the fig 16-2, what is the purpose of the two 47Ω resistors? 7. What technique would you use to measure the phase angle of I L vs I T and I R vs I T . Briefly describe the technique. 8. Write the formula for calculating I T given I R and I L . Fall 2015 Lab 16 P a g e | 10