Download Lab 1

ECE 385
Lab 1-AC Power Measurements: Single-Phase
Circuits
Due: 20-September-07
Group 2
Author: David Gitz
Scot Shelton
Objective: To learn about the different test equipment in the Power Lab, develop
knowledge with ac instrumentation, and to apply the real and reactive power in ac
circuits.
Theory:
Equations:
v(t )  2VRMS cos(t  V )
i (t )  2 I RMS cos(t  I )
s(t )  v(t )  i(t )
P  s(t ) cos(V  I )
Q  s(t ) sin( V  I )
AC Instruments:
-AC Voltage/Current Meters measure the rms value whereas dc meters measure the
average value of the voltage or current. This is why a DC voltmeter would read 0 volts
on an AC voltage, but are equivalent when measuring constant quantities.
-The wattmeter only measures real power, not reactive or apparent power.
Procedure:
1. Become familiar with the H-RLC-100 module.
2. Configure the circuit as shown in Fig 2. Use the console instrumentation and
variable ac source. Set the ammeter scale to 0.5 A, the voltmeter scale to 150 V,
and the wattmeter to 150 W.
3. Connect the first 5 segments of the resistor load (turn the first five switches up on
the resistor panel). Turn the knob of the reactive load to the fully lagging
position. Power the console and the variable ac source up-increase gradually the
source voltage to 120 V.
4. Record the current and power measurements.
5. Decrease the reactive load by turning the knob two notches towards the zero (pure
resistive) position. Refine the source voltage to 120 V and record the current and
power. Repeat this, each time turning the reactive load knob two notches until the
zero position.
6. Repeat the above procedure, this time turning the knob in the leading position by
two notched at a time until you cover the full range. Each time readjust the
source voltage to 120 V and record the current and power. Make sure to
distinguish in your records the measurements taken in the lagging range from
those taken in the leading range of the knob.
7. Power down the console, disconnect and store the wires and store the equipment.
Results:
Lagging Power
Factor
I (Current)
V (Volts)
P (Watts)
Load Apparent
(VAR)
Load Reactive (VA)
PF
I (Current)
V (Volts)
P (Watts)
Load Apparent
(VAR)
Load Reactive (VA)
PF
I (Current)
V (Volts)
P (Watts)
Load Apparent
(VAR)
Load Reactive (VA)
PF
Zero Power Factor
I (Current)
V (Volts)
P (Watts)
Load Apparent
(VAR)
Load Reactive (VA)
PF
0.48
120
37
Leading Power Factor
I (Current)
V (Volts)
P (Watts)
0.38
120
41
57.6
44.14476
0.642361
Load Apparent (VAR)
Load Reactive (VA)
PF
45.6
19.95896
-0.89912
0.41
120
36
I (Current)
V (Volts)
P (Watts)
0.42
120
31
49.2
33.53565
0.731707
Load Apparent (VAR)
Load Reactive (VA)
PF
50.4
39.73865
-0.61508
0.36
120
35
I (Current)
V (Volts)
P (Watts)
0.46
120
31
43.2
25.32272
0.810185
Load Apparent (VAR)
Load Reactive (VA)
PF
55.2
45.67319
-0.56159
0.33
120
32
39.6
23.32724
0.808081
Conclusions: Due to machine error, we noticed a real power spike during the 5th trial
and a Source Current drop in the 6th trial. These two issues seem to be related but the
actual cause is unknown. Regardless, it is apparent what the relationships are to real,
reactive, and apparent power are in a realistic setting.
Questions/Problems:
Discussion Questions:
As the knob moves from fully lagging to fully leading, the reactive power decreases until
we reach the zero point factor. During this period however, the power factor increases.
Once the knob hits the leading region, the power factor is negative and getting smaller.
The reactive power during this period is increasing. The current RMS is decreasing
during the lagging period until it reaches pure resistive load and then it increases while in
the leading region. The real power is decreasing during the entire process.
Discussion:
1. How does the reactive power and the power factor change as the position of the know
move from fully lagging to fully leading?
As the knob is turned counterclockwise from fully lagging, the Reactive Power starts at
44.15 VA, decreases to 19.96 VA, then climbs to 45.67 VA. The power factor starts at
.64, increases to .81, then jumps to -.899 and then increases to -.56.
2. How does the current rms change?
We were not able to determine if the current measurements were peak or rms. Assuming
that they were in rms, the current started at .41 A, decreased to .33 A, then increased to
.46 A.
3. How is the real power affected through the same positions?
It is not directly apparent what the correlation between Real Power and the knob position
is. According to our measurements, the Real Power first decreases from 37 W to 32 W,
increases sharply to 41 W, then decreases again to 31 W.
According to the Phasor Diagram, when the Current has any power factor other than zero,
the reactive current increases and the real current decreases. This corresponds to a
decrease in real power. This makes sense because as the phasor angle increases, less of it
is in the direction of the horizontal and more in the vertical axis.
4. What do the real and imaginary components of the current represent in fig 2?
In figure 2, the real current is represented by the current going through the variable
resistor, and the imaginary current goes through the LC branch.
Graphs:
Real Power
45
40
35
Real Power
30
25
Real Power
20
15
10
5
0
Trial
Knob Position (CCW turning)
MatLab Code:
PF = [-.8991 -.615 -.561 .6423 .7317 .8080 .8102];
C = [.38 .42 .46 .48 .41 .33 .36];
plot(PF,C)