* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Lab 73 Measuring Phase Difference
Survey
Document related concepts
Voltage optimisation wikipedia , lookup
Mains electricity wikipedia , lookup
Buck converter wikipedia , lookup
Chirp spectrum wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Alternating current wikipedia , lookup
Power inverter wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Power electronics wikipedia , lookup
Opto-isolator wikipedia , lookup
Oscilloscope wikipedia , lookup
Tektronix analog oscilloscopes wikipedia , lookup
Transcript
LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 SET-UP & Figure 1 1 LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 OVERVIEW In this lab, you will measure phase shift. What is phase shift and why is.it important? In order to understand the answer to this question, we must first examine some wave properties. Wave A in Figure 1 shows a sine wave plotted as a function of time. The signal strength (height of the wave above the equilibrium level) is called the amplitude. The distance between two adjacent peaks is called the wavelength. Look at wave B in Figure 2. This is the same sine wave shown in wave A. However this wave seems to be shifted to the right. We say the wave is phase-shifted from the first wave, since the wave's placement in time is called its phase. How do we measure phase? You have learned that a wave passes through 360 degrees when it completes one cycle. By figuring the angle at which the two waves cross the horizontal center line of the wave, we can find the phase angle between them. See Figure 3. Figure 2 shows two sine wave traces. In the top sine wave, we label the first point where the wave crosses the center line as 0 degrees. The bottom sine wave crosses the same center line when the top wave has already passed through 45 degrees. We say that the top wave leads the lower wave, or that the lower wave lags behind the upper wave. You will measure the phase between two waves in this experiment using the following method. After connecting your circuit, you will locate two sine waves on your oscilloscope display. One will be the reference wave. You will locate the point where the reference wave crosses the horizontal center line. Place a ruler on the display so that it is perfectly vertical and passes through the point where the measured wave passes through the horizontal center line. Refer to Figure 3. Follow the ruler up until it passes through the reference wave. By finding the number of degrees that the reference wave has deviated from its zero point, you have found the relative phase shift between the two waves. You may be asking yourself, why is measuring phase important? To answer that question, let us look at some applications where phase measurement is done. In electronic systems, technicians are able to troubleshoot circuits by knowing what the phase shift should be between various points in the circuit. Phase shift gives information about the difference between input and output signals in control circuits. But one of the most important uses of phase shift is in power measurements. 2 LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 You have learned that power in DC electrical circuits is equal to the product of the circuit's voltage and current. P = V I However, when working with AC circuits, there is another factor that must be taken into account. It is quite possible that the voltage and current in an AC circuit are out of phase (there is a phase angle .between them). In this case, the product of voltage and current must be multiplied by a correction factor, which is always less than one. In other words, the true power is less than the V I value when the current and voltage are not in phase. Mathematically, the correction factor is the "cosine" of the phase angle. Calculator Exercise: 1. Be sure your calculator is set to the "degree" mode (not radians or grads). Enter the value 0 (zero) in your calculator. This will represent I and V in phase (zero degrees out of phase). Now press the key for cosine, which is marked "cos". The display shows a value of 1. This tells us that when the current and voltage are in phase, the power is the full value of VI. 2. Now enter 45 in the calculator. This represents I and V out of phase by 1/8 of a cycle. Press the cos key. The display shows the correction factor of 0.707. This means that the power will only be seven-tenths of the full V I value. 3. Now enter 90 in the calculator. This represents I and V one-fourth of a cycle out of phase. When I is greatest, V is zero. When V is greatest, I is zero. Part of the time the voltage is positive while the, current is negative. Press the cos key. The display shows a factor of zero, indicating that there was a complete cancellation of the power produced. Mathematically, this power equation is expressed: P = (V x I) cos ө Where: P = power V = voltage I = current ө (theta) = phase angle between the voltage and current Cos ө is commonly known as the power factor. The case where V and I are out of phase by 90 degrees is shown in figure 4. Knowing the phase angle in power circuits is critical when it is necessary to get the maximum power out. Now that you know something about phase shift, you will find, the phase angle in a sine wave using an RC circuit, a function generator, and an oscilloscope. 3 LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 OBJECTIVES A) Use a dual trace oscilloscope to find the phase difference between two phasq-shifted sine waves. B) Measure the phase difference between two phase-shifted signals. EQUIPMENT Phase-Shift Network Circuit Panel Circuit Panel Easel Function Generator Dual-Trace oscilloscope PROCEDURE PART A: Apparatus Setup: Figure 1 shows the basic setup you will use in this experiment. Refer to this figure and the detailed figures that follow, when assembling your equipment. 1. Connect the oscilloscope and function generator to an AC power source. Turn the oscilloscope power on. Leave the function generator off. Plug the phase shift PC Board into the circuit panel. See Figure 6. 2. Connect the BNC cable to the high output of the function generator. Attach the test clips to points la and lb on the PC board. 3. Attach an oscilloscope probe to points 2a and 2b on the PC board. Set the probe to x 1. Plug the other end of the probe into channel 1 of the oscilloscope. You will measure the input signal with this probe. 4. Attach the other oscilloscope probe to points 2a and 2b on the PC board. Set the probe to x 1. Plug the other end of this probe into channel 2 of the oscilloscope. You will measure the output signal with this probe. 5. Adjust the oscilloscope to the settings given below. 4 LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 Channel selector: dual Triggering: Channel 1 TIME./DIV: 0.2 msec Volts/Div: 0.5 V/Div (both channels) Measurement Mode: AC (both channels) . 6. Adjust the oscilloscope settings so that the trace is focused and bright enough to be seen clearly on the display. PART B: Measuring the Phase Angle 1. Turn the function generator on. Using the frequency control, set the input signal to approximately 850 Hz. Set channels 1 and 2 of the oscilloscope to AC. Increase the signal strength (of the function generator) until the input signal (channel 1) spans 4 vertical divisions (peak to peak). 2. Position the input signal (channel 1) to be centered on the top half of the oscilloscope display and the output signal (channel 2) to be centered on the bottom half of the oscilloscope display. 3. One cycle of the input signal should span 6 horizontal divisions. If it does not, then adjust the frequency control of the function generator until it does. 4. Set channel 2 of the oscilloscope to AC. This trace should be identical to that of channel 1. Do you see why this is so? Are the traces identical on your oscilloscope display? 5. Disconnect the output probe (channel 2) from the point 2a and reconnect it to the point 3a. (The ground plug may remain in place since the ground is common to the points labeled 2a, 3a, 4a, and 5a.) Has the output trace changed? How? You may have noticed a reduction in amplitude for the output signal. This is to be expected. 6. Center both the input signal and the output signal on horizontal center line of the oscilloscope display. 7. Adjust the sensitivity of channel 2 on the oscilloscope so that the output signal spans 4 vertical divisions. If this is not possible, increase the function generator output and try again. 8. Make certain that the input signal still spans exactly 6 horizontal divisions. Both traces should be centered on the horizontal center line of the oscilloscope display. 9. There are 30 horizontal grid spacing’s for the input signal (6 horizontal divisions x 5 grid spacing’s per division 30 spacing’s). Find where the input signal crosses the horizontal centerline and where the output signal crosses the horizontal centerline. Count the number of grid spacing’s between these two points along the horizontal centerline. The distance corresponds to the phase shift of the output signal relative to the input signal. Convert this value to degrees by using the process shown below. 5 LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 Step 1: Find the fraction of a complete cycle: # of grid spacing’s 30 grid spacing’s/cycle Step 2: Multiply the fraction of a cycle by 360° to obtain the number of degrees. Record your answer in Table 1 for . 10. Sketch what you see on the oscilloscope display using the grid labeled ө1 on the data form. 11. Disconnect the channel 2 probe form point 3a and reconnect it to point 4a. 12. Repeat steps B-6 through B-10, entering your results for 82 in Table 1, and sketching the display in the grid labeled ө2 . 14. Disconnect the Channel 2 probe form point 4a and reconnect it to point 5a. 15. Repeat steps B-6 through B-10, entering the data and sketch in the appropriate places on the data sheet for ө3 . 16. Disconnect the equipment. Continue with the analysis on the report form. 6 LAB 73 MEASURING PHASE DIFFERENCE OBJECTIVES: May 4, 2017 SKETCH OF EQUIPMENT: PHASE SHIFT: # OF GRID SPACES: 1 _________ 2 _________ 3 _________ # OF DEGREES: 1_________ 2_________ 3_________ 4 -4 4 -4 Oscilloscope for phase shift #1 4 4 -4 4 -4 -4 4 -4 Oscilloscope for phase shift #2 Oscilloscope for phase shift #3 7 LAB 73 MEASURING PHASE DIFFERENCE May 4, 2017 LAB 73: ANALYSIS 1. Draw two sine waves where the phase between the two waves is: a. 90 degrees b. 180 degrees 4 4 -4 -4 4 -4 4 -4 2. Assume that you have an AC circuit in which V = equals one volt and I equals one amp. Knowing that P = V·Icos in AC circuits, find out how much power is provided when: a. = 0 degrees, P = _________ c. = 90 degrees, P = ________ b. = 30 degrees, P = _________ d. = 360 degrees, P = _______ 3. Explain what you think the answer to 2-d means. 4. You have a power circuit that gives maximum power (0 degrees phase between voltage and current). A technician plays with the circuit and now you measure the phase shift as 60 degrees. What is the percentage of power that you can now get from the circuit? 5. Consider the circuit used in this experiment. What could you change in the circuit that would change the phase angle between the input and output phase angle? Give at least two factors. 8 LAB #73 MEASURING PHASE DIFFERENCE 9