Computer running LabVIEW. DAQ Board (National Instruments AT-MIO- 16DE-10) GPIB Interface Card National Instruments AT-GPIB/TNT

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1 VIRTUAL INSTRUMENTATION IN AN UNDERGRADUATE ELECTRICAL MACHINES LAB Thomas W. Gedra School of Electrical & Computer Engineering Oklahoma State University, Stillwater, OK ABSTRACT At OSU, we have substantially completed a major instrumentation upgrade of the undergraduate energy conversion laboratory. We are now utilizing digital signal acquisition and virtual instrumentation (using National Instruments' LabVIEW software) to collect and visualize data. Data from GPIB digital oscilloscopes can also be acquired in our setup. Dynamometers have been purchased which interface to the digital acquisition hardware, allowing computerized collection of torque, speed, and angular position, as well as computer control of the mechanical load provided by the dynamometers. In this paper, the laboratory setup is described, and a few of the new capabilities, including real-time phasor display of AC data and spectral analysis, are highlighted. Anecdotal evidence of the improved learning environment is also presented. 1. INTRODUCTION Electromechanical Energy Conversion I is a required course for all juniors in the School of Electrical and Computer Engineering at Oklahoma State University. The original intent of the course was coverage of one-and three-phase transformer connections and their equivalent circuits, DC shunt, series and compound machines, synchronous machines, and one- and three-phase induction machines. Recently, we have attempted to introduce students to other areas of power engineering during this course. Both power electronics and power systems analysis are briey introduced during the class, and this coverage is supplemented by two new laboratory experiments. The exposure to these topics appears to have increased student interest in the senior elective classes in power electronics and power systems analysis. These new experiments are described in [1]. Most of the data in the laboratory had been collected using traditional analog voltmeters, ammeters, wattmeters, and digital multimeters. Several years ago, we replaced our aging analog oscilloscopes with modern digital oscilloscopes. Beginning in the summer of 1996, we began a major instrumentation upgrade. Each of the laboratory benches was equipped with digital signal acquisition capabilities, including computer signal acquisition, processing and display software. We have purchased new electrodynamometers, which can function as prime movers as well. We have also improved the safety of our laboratory. The laboratory manual has been completely rewritten to reect the new equipment and the ability to make measurements which were previously not possible. In this paper, we describe the hardware and software im- Figure 1. Front panel of a simple VI. Figure 2. Wiring diagram of a simple VI. provements we have made, and discuss the changes in the laboratory environment which have resulted. 2. VIRTUAL INSTRUMENTATION Virtual instrumentation refers to the use of computer signal acquisition, processing and display. We are using a National Instruments software product called LabVIEW. Each LabVIEW program is called a \virtual instrument," or VI. It consists of two pieces. One is the actual panel of the instrument, containing controls (such as buttons, knobs, etc.) and indicators (meters, LEDs, graphs, etc.). This is the part which is used during the collection and analysis of data. The front panel for a very simple VI is shown in gure 1. On the left is a control, which provides input to the VI from the user. In this example, the control is a knob, although many other controls (sliders, editable boxes, buttons, etc.) are available. The knob can be turned by grabbing it with the mouse. On the right is an indicator, which displays the VI's output to the user. In this example, the indicator is a \tank" indicator, although many other indicators are possible. Controls and indicators can be resized, moved, painted dierent colors, etc. The other part of every VI is the \wiring diagram." This Presented at the 1997 Midwest Section ASEE Conference, Columbia, MO, April 2{4, 1997.

2 DAQ Interface Module Computer running LabVIEW V1 V2 V3 V4 I1 I2 I3 I4 Torque Speed Load Aux Voltage Dividers Current Shunts Isolation Modules (8x5B40) Gain Scaling Logic AI0 AI1 AI2 AI3 AI4 AI5 AI6 AI7 AI8 AI9 AO0 AO1 Digital Port PA DAQ Board (National Instruments AT-MIO- 16DE-10) Canon BJC-620 Color Printer Ethernet connection to other machines Hewlett-Packard model 54501A Digitizing Oscilloscope GPIB Interface Card National Instruments AT-GPIB/TNT shows how the data is collected, processed, and routed. But the wiring diagram is not just a visual representation of the computer program: it is the program. Programming in LabVIEW is entirely graphical. Many of the same programming features available in \normal" programming languages, such as case statements, for loops, signal processing, etc., are available in LabVIEW as well. Figure 2 shows the wiring diagram corresponding to the front panel shown in gure 1. The small box which says DBL on the left is the \terminal" from the control on the from panel, and the box on the right is the terminal for the indicator. The DBL indicates that the terminal is a double precision oating point number. A number of other data types are supported, such as boolean and string variables, as well as arrays and clusters of variables. In gure 2, the wiring diagram shows that the value input from the control is subtracted from 10 and then output to the indicator. A number of VI's were written for this laboratory (about 15). Most of the VI's are customized to be used in a particular experiment, although a few are general purpose VI's. Some experiments require more than one VI, especially the power system operations lab, which requires 4 (one for each bench). Also, a number of subvi's (subprograms which are themselves virtual instruments) were written. These subvi's are used, for example, to collect data and apply calibration coecients, to perform phasor analysis, and to draw the phasor diagrams. 3. LABORATORY HARDWARE 3.1. Signal Conditioning, Isolation, and Digital Acquisition A diagram of the digital acquisition system is shown in gure 3. The system at each of the four benches consists of Figure 3. Conguration of digital acquisition system. a signal isolation and conditioning module, a digital acquisition (DAQ) card in the computer, a GPIB card in the computer (for use with the digital oscilloscopes discussed below), and, of course, the computer itself, and the software LabVIEW to control data acquisition and to process and display the data. A color printer is also available for printing VI's. The computers are networked together, and to a Windows NT server, where the VI's are actually stored. Storing the VI's on a central server guarantees that each lab group uses the same VI, and it also provides security by preventing students from modifying the master copy of any VI (although students are free to copy the master VI's and modify the copies if they wish). The isolation ampliers allow both input terminals of each measurement channel to be oating (not ground referenced). In addition, they provide up to 1500Vrms protection between the computer and the externally applied signals. The signal conditioning circuitry scales the input voltage signals to the appropriate level for use by the isolation ampliers. In the case of current inputs, shunt resistors convert the current owing through the circuit to a voltage appropriate for the isolation ampliers. There are 8 channels which are conditioned and isolated: 4 voltage channels and 4 current channels. After conditioning and isolation, the signals are passed on to the digital acquisition (DAQ) board, located in the computer. In addition to the buered channels, there is provision for two unbuered channels, which are called \torque" and \speed," in anticipation of their use with the dynamometer's analog torque and speed outputs (see below). Under LabVIEW control, the DAQ card can acquire any or all of these channels and pass the collected data on to LabVIEW for analysis and display. 2

3 R1 jx1 R2 jx2 + I1 Ie I1 + + I2 + V1 Gc -jbm E1 E2 V2 - Ic Im Each of the conditioned input channels has gain control hardware as well. Each voltage channel has a total of 4 ranges, for a full-scale range of 280Vrms, 140V, 70V and 30V. Each current channel has 2 ranges, for a full-scale range of 10Arms and 2A. Full automatic gain control, on a channel-by-channel basis, will soon be available, probably by July LabVIEW will provide digital outputs to select individual channel gains as appropriate to the signal level being acquired by that channel. The DAQ card also has two analog outputs, which are unbuered. These can also be controlled using LabVIEW. One of these will be used to control the mechanical loading of the dynamometer (see below) Digital Oscilloscopes and GPIB Each lab bench is equipped with an HP 54501A 100 MHz 4- channel digital oscilloscope. These scopes are each equipped with a GPIB (general-purpose instrumentation bus) interface (IEEE standard 488). A GPIB card in each host computer allows LabVIEW to communicate with the scope. Each scope can operate independently, or can be controlled by LabVIEW, which can set timebase, trigger and other parameters and retrieve data from the scope for subsequent processing and display on the host computer. The use of a GPIB interface allows for the future expansion of the lab's capabilities, since many instruments and signal sources are available with GPIB interfaces Dynamometer/Prime Mover Each station is equipped with a LabVolt 8610 Dynamometer/Prime Mover. This enables the student to mechanically drive generators and to load motors, while measuring the speed and torque. The dynamometer/prime mover has analog speed and torque outputs which can be directly connected to the digital data acquisition system. When operated as a dynamometer, the 8610's mechanical loading can be controlled by an externally applied voltage. This is ideal for use with the digital data acquisition system, since the DAQ board can generate this external voltage under LabVIEW control. The dynamometers are not yet in general use, because we are still building some special motor mounts to allow our laboratory machines to be connected to the dynamometers. However, several demonstrations using the dynamometers have been quite encouraging. We anticipate that the new motor mounts will be available by July, Ideal Transformer N1:N2 Figure 4. Equivalent circuit of a transformer. 4. CAPABILITIES OF THE LABORATORY Currently, there are 12 experiments which are conducted during the course of a semester. Their titles are: 1. Getting Familiar with the Laboratory 2. Measuring Impedance and Power 3. Transformer Excitation 4. Transformer Equivalent Circuit 5. Three-Phase Transformer Connections 6. DC Machine Connections 7. DC Generators 8. DC Motor Load Characteristics 9. Three-Phase Synchronous Machines 10. Induction Machines 11. Introduction to Power Electronics 12. Power System Operations Typically, each experiment has its own customized VI, specially tailored to its data collection requirements. Several experiments (such as 6, 7 and 8) share a generalpurpose VI. On the other hand, a few experiments (such as 11 and 12) require several dierent VI's for a single experiment. In this section, we will give some examples of specic measurements which can be easily made and/or visualized using the virtual instrumentation concept. One laboratory experiment which illustrates several of the capabilities of virtual instrumentation is Experiment 3, Transformer Excitation Transformer Excitation Experiment In the transformer excitation experiment, voltage is applied to the primary winding of a single-phase, two-winding transformer. The primary current is measured, as is the secondary voltage (with no load present at the secondary). The so-called exact equivalent circuit of such a transformer is shown in gure 4. This equivalent circuit neglects the nonlinear aspects of the core loss, instead modeling it as a linear shunt resistor, with conductance G c. With no load on the secondary, several observations can be made from the equivalent circuit. First, the primary current will be due solely to the exciting current I ~ e. Second, the secondary voltage should lag the primary voltage slightly due to the voltage drop across the series element R 1 + jx 1. Finally, the secondary voltage V ~ 2 will equal the ideal induced voltage E ~ 2, since the secondary current I ~ 2 = 0, and so one can obtain the ux in the core of the transformer by Z (t) = (0) + 1 t v 2() d; (1) N 2 0 3

4 Figure 5. Time-domain waveform collection. where N 2 is the number of turns in the secondary winding and (0) is the initial ux at time t = 0 in the core. The linear equivalent circuit in gure 4 describes the operation of the transformer fairly well for small input voltages. But as the input voltage rises, the nonlinear eects of hysteresis begin to become more evident. The exciting current becomes increasingly nonsinusoidal as the B?H curve saturates, and the current begins to contain large amounts of third and higher-order odd harmonics. Each of the eects mentioned above can be seen in real time as a result of the virtual instrument designed for this experiment. Figure 5 shows the time-domain voltage and current waveforms collected from the transformer as it is operated at rated voltage. By looking at the voltage waveforms waveforms, it can be seen that the secondary voltage V 2 is slightly larger than the primary voltage V 1 (due to a turns ratio slightly greater than 1) and lags V 1 slightly. It can also be seen from the primary current waveform I 1 is quite nonsinusoidal, exhibiting sharp peaks as the transformer core begins to saturate at large values of ux. The student can also see a graph of the instantaneous power owing into the transformer core (not shown in this paper for reasons of space), which is obtained simply by multiplying the instantaneous voltage and current waveforms together and displaying them graphically using LabVIEW. The ability to view these basic waveforms does not require virtual instrumentation or advanced digital signal acquisition: all that is required is a good oscilloscope. However, virtual instrumentation can provide other perspectives on the data which cannot be obtained by a scope. One example of this is the phasor diagram. Throughout any class on electric power, phasor diagrams are used extensively to explain the behavior of linear systems under sinusoidal, steady-state conditions. Since the entire current and voltage waveforms are acquired, one can extract the 60Hz component of these waveforms (using Fourier analysis) and the results can be displayed on a polar plot as a phasor diagram (for the 60Hz component). Such a display is illustrated in gure 6 for the transformer data. This display is color-coded, and clearly demonstrates the phase relations between V ~ 1, V ~ 2, and I ~ 1. Current readings are rst multiplied by a \current scale factor" (a quantity with dimensions of ohms) to obtain a voltage before being displayed on the phasor diagram. This phasor diagram shows that, indeed, ~V 2 lags V ~ 1 by several degrees, and shows that the exciting current ~I 1 is primarily reactive, lagging ~V 1 by close to 90. This diagram, like all other quantities, are updated several times per second, yielding a real-time display of phasor quantities. Figure 6 also shows some of the other quantities which can be easily computed using LabVIEW. Since we have full information on the voltage and current waveforms, we can also calculate the apparent power, real power, reactive power, power factor, power factor angle, etc. These quantities are displayed in gure 6 for the V 1=I 1 voltage-current pair. Again, since one has full information about the current and voltage waveforms, it is possible to compute the full spectra of each. Fourier analysis is used to extract the 60Hz component of each waveform for phasor analysis and display on the phasor plot, and at the same time, the components of each waveform at other frequencies is also computed, and is displayed as shown in gure 7. The spectra of the various input signals can be displayed in decibels relative to the dominant harmonic or in volts and amperes on a logarithmic scale. In this gure, it is clear that the voltage waveforms are fairly pure 60Hz sinusoids, which agrees with the time waveforms shown in gure 5. However, Fourier analysis also shows the presence of a small amount of fth harmonic (300Hz) resulting from the local power supply. This amount of harmonic distortion is obviously too small to be seen by looking solely at the time waveforms. From the spectral analysis one can also see large third, fth, and other odd harmonic components of the exciting current. That the exciting current is nonsinusoidal is clear from the time waveforms in gure 5, but the specic breakdown by harmonic requires a spectral analysis. Figure 7 also shows how the user can position cursors or crosshairs on the plot in order to obtain accurate measurements from the plot. Here, the cursors show that the fth harmonic in the voltage spectrum is 31.79dB below the 60Hz fundamental, but that the third harmonic in the current spectrum is only 7.11dB below the fundamental. Since the core ux can be computed from equation (1) under no-load conditions, we can use the capabilities of virtual instrumentation to numerically integrate the secondary voltage to obtain the core ux (since we know the number of turns in our secondary winding). This can then be displayed as a function of the excitation current to obtain the hysteresis curve of the transformer as shown in gure 8. The area enclosed by the hysteresis curve is proportional to the core hysteresis loss, which in our equivalent circuit is modeled by the quantity G c. One can obtain the saturation curve of the transformer by articially removing the eect of the core loss conductance, G c, by computing ^ m(t) = i 1adj (t) = i 1(t)?e 1(t) ^Gc = i 1(t)? N1 N 2 ^Gcv 2(t); (2) where ^Gc is our guess of the true value of the core loss conductance. The VI for this experiment allows the student to select a value of ^Gc, and the resulting plot of versus I m is displayed. By adjusting this value, the saturation curve of the transformer is obtained, as shown in gure 9, and at the same time the value of G c is estimated by the student's selection of ^Gc. Again, note the use of cursors in gures 8 and 9. Here the cursors are being used to measure 4

5 Figure 7. Frequency domain analysis. Figure 6. Example of a phasor plot. the slope of the ux versus current curve. Combined with a knowledge of the transformer's cross-sectional core area and the number of turns in the windings, this slope can be used to estimate the permeability of the transformer's magnetic core Other Capabilities Although the transformer excitation experiment shows many of the capabilities of the digital acquisition/virtual instrumentation we have implemented, there are many others. One example is the use of the GPIB card and drivers to control GPIB-based instruments, such as our HP 54501A oscilloscopes. An example VI for controlling such a scope is shown in gure 10. At this time, this VI is used primarily for capturing fast transients, the acquisition of which requires both high speed and triggering capability. The scopes and the scope VI's are used, for example, to capture the trigger pulses of triacs in Experiment 11, Introduction to Power Electronics. In Experiment 12, Power System Operations, a small power system is simulated using small machines and loads from the laboratory. Each of the simulated substations has its own VI, which contain strip charts for frequency and voltage, and real and reactive imports, just as in a real utility energy control center. The dynamometers that we have purchased will be fully integrated with the LabVIEW virtual instrumentation environment. The ability to acquire speed and torque directly into LabVIEW is highly desirable, and mechanical power can be immediately calculated and displayed in real time. The ability to control the mechanical loading of the dynamometer under LabVIEW control is also an important feature. It enables one to provide closed-loop control of the system under test: one can easily write a VI which maintains constant torque, constant speed, or constant power as some other parameter is being varied. For example, one could vary the voltage supplied to an induction motor under a constant power mechanical load in order to determine the VAr consumption as a function of voltage Improvements in Safety and Convenience While the use of virtual instrumentation is the most clearly visible component of our lab upgrade, we have also been improving the safety and convenience of the laboratory facility. We have installed shunt trip breakers which allow emergency shutdown of the high-voltage power supplies using \panic buttons," one at each end of the lab. The computers and other instruments are supplied by a separate circuit, and so stay on when the shunt trip is used. We have also been gradually replacing all of our banana jacks on all the benches with a type of banana jack which accepts a shrouded banana plug, and we have been replacing the old jumper cables (banana to banana) with new safety jumpers (shrouded banana to shrouded banana). It is nearly impossible to touch the metal parts of such a shrouded banana plug, whereas it is quite easy to do so with the normal (unshrouded) variety. Furthermore, if a jumper cable falls out of the jack it is plugged into, this no longer results in a short circuit and blown fuse, as it did with the unshrouded connectors. We have also installed a number of indicator lamps (actually LED's) which help to debug a circuit which doesn't work. Each of the power supplies has a green pilot light which glows if the supply is on and its fuse is intact, so a blown fuse in a power supply can be immediately identied. For other components, such as the rheostats, a red lamp connected across the fuse illuminates if the fuse blows while the circuit power is on, immediately showing the location of the blown fuse. Details such as the indicator lamps might seem trivial, but it certainly decreases the amount of time spent trying to help students debug their circuits. 5

6 Figure 8. Transformer core ux versus exciting current. 5. DISCUSSION The laboratory equipment upgrade has made a great improvement in the quality of the lab experience for our undergraduates. Many measurements are now possible which were impossible or extremely tedious with the old equipment. Students can see, in real time, the eect of an experimental variable on real power, reactive power, and the phasor and spectral representation of AC signals. Previously, reactive power had to be calculated from current, voltage, and real power using power triangle calculations. Such calculations were repetitive and boring, and were done only after the lab period was over, when the student was writing up the lab report. Phasor diagrams, too, could only be drawn by hand after the lab period was over. The ability to see the changes in these quantities during the experiment is much preferable. Some might argue that LabVIEW does too much for the students, that they need to do these calculations the old fashioned way, etc. In my opinion, this is a valid point only if students never learn to draw a phasor diagram or learn to do power triangle calculations. We make sure that they can do this, both in the lecture part of the course and in the lab reports. Certainly no one would argue that a spectral analysis is invalid unless the Fourier transform were calculated by hand. It is also true that this is the way information will be gathered in the future in the work environment where the students are employed. Many employers already use LabVIEW or a similar system for data acquisition, processing, display, and for system control. While the use of sophisticated test equipment can result in erroneous answers or interpretations if the user does not understand how the measurement is being made, this is no reason not to use modern equipment and techniques. Instead, it is a reason to make sure that the students learn to understand what the measurements mean and what assumptions are being made. Figure 9. Transformer saturation curve. The immediate feedback provided by the LabVIEW environment cannot be overemphasized as a learning tool. We had initially thought that most of the laboratory experiments would take less time because of the ease with which the measurements can be made. In fact, we have observed the opposite. Students can immediately see the eect of loading a motor, changing a supply voltage, or changing a connection. They spend a fair amount of time studying the various data displays which we provide them. They often ask questions during the lab: Why does this phasor change? Why is the third harmonic so large? The labs may take a little longer, but the students have a much better opportunity to understand the experiment. Without these data displays, the student would walk away from the lab with a set of numbers and little clear understanding of what went on. The idea that the students will gure it all out in a ash of inspiration while writing up their lab reports (often late at night!) is fairly optimistic, and in any case, no substitute for the experience we can now provide them. The improved laboratory setup also appears to be generating an increased level of interest in the power area. Students think the setup is modern, and they think it's just plain neat. This is an important consideration. While students may still complain about having to write the lab reports, they seem to enjoy the lab much more than students in the past who had to use the old analog instruments. To many people not in the power area, power represents old technology, and an area in which not much happens. While those working in the power area know that nothing could be further than the truth, the myth still continues among the unenlightened. Our laboratory setup at least makes a start at dispelling that myth. 6

7 6. CONCLUSIONS Improvements in this undergraduate laboratory are generating an improved undergraduate experience for the students, as well as increasing the level of interest in the power area. The use of virtual instrumentation provides a modern and interesting way for students to perform experiments. We are continuing the upgrades, and plan to have the dynamometers fully functional by July, Experiments and demonstrations for the senior elective courses in power electronics and power systems analysis are also being planned. I hope that our experiences can be used as a model for others interested in improving their laboratories. ACKNOWLEDGMENTS This work was funded in part by the National Science Foundation under grant ECS Contributions for the laboratory equipment upgrade have been made by Oklahoma State University, the National Science Foundation, the PSO/Albrecht Naeter Professorship at OSU, Analog Devices, and National Instruments. I'd also like to thank Kristi Storm and Kevin McBride, two undergraduate students who have been heavily involved in the upgrade over the past year, and who have contributed much to its success. REFERENCES [1] Thomas W. Gedra. Innovations in OSU's undergraduate machines course. In Proceedings of the 1996 Midwest Symposium on Circuits and Systems, August Iowa State University, Ames, IA. Figure 10. A VI to control a digital scope. 7