Download Lab 73 Measuring Phase Difference

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

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

Heterodyne wikipedia , lookup

Switched-mode power supply wikipedia , lookup

Power electronics wikipedia , lookup

Metadyne wikipedia , lookup

Bode plot wikipedia , lookup

Opto-isolator wikipedia , lookup

Islanding wikipedia , lookup

Oscilloscope wikipedia , lookup

Tektronix analog oscilloscopes wikipedia , lookup

Phase-locked loop wikipedia , lookup

Three-phase electric power 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 VI.
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