APPLICATION OF VIRTUAL INSTRUMENTATION IN A
POWER ENGINEERING LABORATORY
Qamar H. Arsalan, Amanda Filbeck, Thomas W. Gedra
School of Electrical & Computer Engineering, Oklahoma State University, Stillwater OK, 74078
ABSTRACT
This paper presents a few of the OSU–Stillwater Power Engineering Laboratory capabilities. Digital signal acquisition and virtual instrumentation (National Instruments’ L AB V IEW software)
are used to help students understand the concepts by visualizing
the data. Some features of virtual instrumentation are real-time
display of waveforms, phasor diagrams, real and reactive power,
spectral analysis as well as the ability to interpret the data graphically. Some techniques for improved laboratory practice as well as
the laboratory capabilities concerning experiments of transformers
and rotating machines are presented.
1. INTRODUCTION
In traditional power engineering laboratories, the data is collected
using traditional analog voltmeters, ammeters, wattmeters, digital
multimeters, and oscilloscopes. Students spend a large portion of
time connecting the standard hardwired instrumentation, which allows them to measure only RMS voltage, RMS current, and real
power. The quantities not measured, such as reactive power and
phase angle, are calculated as part of the lab write-up using the
power triangle and trigonometric identities. Since such quantities
are calculated after the lab, students cannot observe the effect on
these quantities due to changes of other experimental parameters.
With the use of virtual instrumentation, students can see the realtime effects of an experimental variable on the real power, reactive
power, and phasor quantities and the spectral representation of AC
signals. For example, they can immediately see the result of loading a motor, changing a supply voltage, or changing a connection.
Our laboratory has the capability of performing the actual experiments in the field of power engineering. In the study of electrical machines, computer simulation has been proposed by some educators. However, many others agree that simulation is not a substitute for actual experiments [1]. This paper will emphasize some
of the capabilities of utilizing virtual instrumentation to study single and three-phase transformers, DC generators, and synchronous
and induction machines.
2. LAB CAPABILITIES
Each computer is equipped with a digital acquisition (DAQ) board.
There are 8 measurement channels: 4 voltage and 4 current channels which are connected to the digital acquisition board via conditioning and isolating circuitry [2]. The DAQ card acquires data
from any of the channels and passes it to L AB V IEW, a National
Instruments software product, for analysis and display.
A LabVolt 8610 prime mover/dynamometer is placed at each
station, enabling students to mechanically drive generators and to
load motors. The prime mover/dynamometer has analog speed and
torque outputs which are directly connected to the DAQ card [3].
Figure 1: Hysteresis curve of a transformer.
The combination of digital acquisition, computer processing
and display is referred to as “virtual instrumentation” (VI). The
term “virtual” refers to the fact that the display and processing of
information is controlled by software, and can easily be reconfigured to suit various purposes. It is important to emphasize that
“virtual instruments” are real instruments: no simulation is involved. Real-world data is collected, processed, and displayed.
Each VI consists of two pieces. The part of the virtual instrument that the user deals with the most is the “front panel.” The
front panel of the VI is just like the front panel of a standard hardware instrument: it may contain knobs, buttons, and switches for
controlling the instrument. These controls are used to indicate instrument status while LEDs, meters, graphs, and numeric outputs
are used to display the measured data. The other part of every
VI is the “wiring diagram.” This shows how the data is collected,
processed, and routed. The wiring diagram is not just a visual
representation of the computer program: it is the program. Programming in L AB V IEW is entirely graphical. The OSU Power
Engineering Lab has several experiments already set up, and the
flexibility to create new ones.
2.1. Single and Three phase Transformers
To study the characteristics of a single-phase transformer, voltage is applied to the primary of a single-phase two-winding transformer. Using a current and a voltage channel, the input current
and voltage are measured. For small input voltages, the relation between flux and exciting current is linear. However, as the primary
voltage increases, the nonlinear effects of hysteresis and saturation begin to become more evident, which are the cause of third-
A
3-phase
B
supply
C
N
+
+
+
-
I1
I2
I3
V4
-
H1
X1
H2
H1
X2
X1
H2
H1
X2
X1
H2
X2
+
V1 +
V2 +
V3 -
+
N’
a
b
c
n
Figure 2: Ungrounded Y-Y connections.
Figure 4: Phasor diagram for Y–grounded Y 3 transformer.
Figure 3: Harmonics for ungrounded Y–Y transformer.
and higher-order odd harmonics of the exciting current. Using the
VI designed for this experiment the hysteresis curve obtained is
shown in figure 1. This curve is obtained by displaying core flux
as a function of the exciting current. Core flux can be calculated
as:
¾ ¾
¼
where ¾ is the number of secondary turns and ¾ is secondary
voltage. The capability of the VI to numerically integrate the secondary voltage has been used.
To study the presence of harmonics in a three-phase wye-wye
transformer and the effect grounding the neutral has on the harmonics, three single-phase transformers are connected in a wyewye configuration as shown in figure 2. Connections are made by
shrouded banana plugs, which help not only to provide a safe environment but also to save time. For example, it takes less than
2 minutes to make the connections of figure 1. By using the current and voltage waveforms, the VI can compute the full spectra
of each. Fourier analysis is performed on each of the acquired
waveforms in order to obtain different frequency components. The
fourier analysis of input voltage V1 to the wye-wye transformer is
shown in figure 3. One can see the large 3rd, 5th and other harmonics. For simplicity other voltages have been omitted. Students can
position the cursors on the plots for accurate measurements. Here,
the cursors show that the 3rd harmonic in voltage spectrum is 4.55
dB below the 60Hz fundamental (figure 3). However, when V4 is
replaced by I4 (thus providing a direct connection between N’ and
N), students can see that the 3rd harmonic is 37.19 dB below the
60Hz (graph not shown here). Therefore, students can visualize
that the 3rd harmonic is significantly reduced by connecting the
neutral of wye-wye transformer to ground.
The ability to study the basic waveforms does not require virtual instrumentation. This is also possible by the oscilloscope.
However, virtual instrumentation can provide other perspectives
on the data which the oscilloscope cannot provide. For example,
students can see the phasor diagram on the same VI program. Using the current and the voltage waveforms, the 60Hz component
is extracted and the results can be displayed on a polar plot as a
phasor diagram. Such displays show the phase relations among the
voltages and currents being measured. The phasor diagram, shown
in figure 4, depicts line to neutral voltages and line currents of Y–
grounded Y 3 transformer. Since the phasors are displaced by
120Æ , one can conclude that the connections are correct. Whereas,
based on only RMS measurements of line to neutral voltages, one
cannot tell whether the connections are correct or not. Thus, the
phasor diagram helps students compare different voltages and currents.
2.2. DC Generator Characteristics
One of the VIs developed at OSU makes it very convenient to build
a graph of one variable versus another. Students can record the
data and plot all the corresponding points on the graph. Moreover,
making multiple graphs on the same set of axes is possible, which
is very useful when comparing characteristics of vaious machines.
If a certain point is undesirable or recorded by mistake, then it can
be removed by using the delete button. Students can delete an entire graph or multiple graphs at once. This application is used to
plot the performance characteristics of various electrical machines.
For example, the VI can be used to plot the magnetization curve
of a DC generator, as shown in figure 5. This is a single graph
of armature voltage (Y-axis, in Volts) versus field current (X-axis,
in Amperes) at a constant speed of 1725 rpm. This curve corresponds to the magnetization equation , where is the generated voltage, is the machine constant, is angular velocity, and is the flux per pole which depends on the field
Figure 5: Saturation Curve of DC generator.
current . The curve starts at approximately 12V because of the
residual magnetism in the machine.
The characteristics of different DC generators in loaded condition are shown in figure 6, which depicts a set of 3 graphs of
load voltage (Y-axis, in Volts) versus armature current (X-axis,
in Amperes). The top-most curve represents a separately excited
DC generator. The curve in center represents a cumulatively compounded long shunt DC generator and the third curve represents
a self-excited DC generator. As expected, the separately excited
DC generator has drooping characteristics because of the losses
due to armature resistance, brush contact and armature reaction.
Whereas, in the case of the self-excited shunt DC generator (without compounding), the field excitation is provided by the generator itself, so reduced terminal voltage will supply a reduced field
current which causes further reduction in the voltage. In a cumulatively compounded generator the series and the shunt fields aid one
another. As the load current increases, the current through series
winding increases therefore series field strength increases which
compensates the decrease in shunt field strength. As a result, the
overall magnetic field strength increases, which is the cause of increase in the output voltage of the generator as compared to that of
the self-excited case. Change in the output voltage from low load
to high load is seen because magnetic field strengths of series and
shunt fields do not compensate for each other completely.
2.3. Synchronous Machine
A three-phase wound-rotor induction machine (“sel-syn”) is used
as a synchronous motor, as shown in figure 7. A start/run switch,
when pressed, disconnects the excitation current and shorts all the
three phases of the rotor so that the motor can be started as induction motor. After rated voltage is applied, the start/run switch is
released allowing DC excitation to be supplied to the rotor which
is synchronized with the stator MMF. Using the phasor diagram,
the stator current and terminal voltage are displayed. By varying
the excitation current, the magnitude and angle of the stator current
is varied. From the phasor diagram, students can see that for low
values of field current, the stator current lags the terminal voltage
and for higher values it becomes leading. Students can plot the
Figure 6: DC generator performance.
C
B
3-phase
variac
A
+
I1
Stator
It
+
V1 Vt
-
N
T
+
+
10A DC
supply
-
I2
N
If
Rotor
Run
Common
Start
G
L
A
Dynamometer B
1
2
DAQ
Module
T (torque in)
AIGnd
N (speed in)
AIGnd
L (load out)
AOGnd
A (aux out)
AOGnd
Figure 7: Synchronous motor under load.
variation of stator current as the field current is changed i.e. Vcurve. For different load levels, the recorded data provided the
corresponding V-curves as shown in figure 8.
A synchronized stroboscope is used to observe the change in
the power angle as the power level is changed by connecting it to
the synchronous machine as shown in figure 9. This technique can
be used to study the power angle characteristics of the synchronous
machine. Using the prime mover to drive the machine as a generator, the power angle can be seen to be leading, and as the load
is reduced and the 8610 is switched to dynamometer mode, the
machine becomes a synchronous motor, with a correspondingly
lagging power angle.
To convert the synchronous motor into an unloaded synchronous
generator, three-phase variac is disconnected from the stator and
the prime mover is turned on to bring the machine to near 60 Hz.
By varying the speed of the prime mover, the corresponding change
in the generator voltage and frequency can be observed on the VI.
Then, the speed is kept constant and the excitation is varied to observe the corresponding change in the terminal voltage. Using this
data, students can plot the no-load saturation curve for the synchronous generator.
C
B
3-phase
variac
A
N
+
Ic
Ib
Ia
I3
+
-
+
I2
+
Stator
I1
+
V1 Vt
-
+
V2
DAQ
Module
T (torque in)
AIGnd
V4
-
+
V3
-
N (speed in)
AIGnd
-
T
Rotor
N
G
L (load out)
AOGnd
A (aux out)
AOGnd
L
A
Prime
Mover
B
1
2
+
10A
DC
- Supply
Figure 10: Sel-syn machine as phase shifter and variable frequency
source.
Figure 8: V curves for a synchronous motor.
Figure 9: Stroboscope used to measure power angle.
2.4. Induction Machine
The same machine as used in the synchronous machine experiment is used as a phase shifter and variable frequency source by
connecting the three-phase supply to the rotor and moving it with
the prime mover as shown in figure 10. When the shaft is rotated
by hand, one can see the stator voltage phasor rotating on the phasor diagram. By observing the number of rotations completed by
the shaft as the phasor rotates 360 electrical degrees, the number
of poles of the machines can be determined. In our case, the stator
voltage phasor completes 360Æ in half of a rotation of the shaft,
which illustrates that the machine is a 4 pole machine. If the machine is rotated in the forward direction (using the prime mover)
the stator voltage will have a higher frequency than 60 Hz, and
conversely will have a lower frequency if the machine is rotated
backwards. Figure 11 shows that the frequency of stator voltage
V4 is 96.56 Hz, and that for rotor voltage V1 is 60 Hz.
To convert the machine into an induction motor, all of the connections to the rotor are now connected to the stator, while the rotor
windings are short circuited through a current channel to measure
the rotor current. It takes approximately 20 seconds to convert the
phase shifter into an induction motor, which can be subjected to
several experiments.
3. CONCLUSION
The techniques and capabilities of an improved laboratory practice have developed and improved undergraduate experience for
Figure 11: Source and output voltage spectrum for frequency
shifter
the students. The use of virtual instrumentation has not only provided a modern and interesting way for students to perform experiments but also has reduced the amount of time necessary to make
connections. The real-time display of quantities, such as phasor
voltages and currents, real and reactive power, and harmonic content, has made a great improvement in the students’ ability to visualize these quantities and understand their relation to one another.
We are thankful to the National Science Foundation (under
grant ECS–9501648), the PSO/Albrecht Naeter Professorship at
OSU, Analog Devices, and National Instruments for their contributions to the laboratory.
4. REFERENCES
[1] M.W.Daniels and R.A.Shaffer. Re-inventing the electrical
machines curriculum. IEEE Transactions on Education,
41(2):92–100, May 1998.
[2] Thomas W. Gedra.
Computer-aided instrumentation in
’s machines lab. In Proceedings of the Frontiers of
Power Conference, pages VIII.1–VIII.5, Oklahoma State University, Stillwater, OK, October 1997.
[3] Thomas W. Gedra. Virtual instrumentation in an undergraduate electrical machines lab. In Proceedings of the Midwest
Section ASEE Conference, Columbia, MO, April 1997.