Undergraduate Electric Machines Laboratory Innovation ... - IEEE Xplore

Session 5C4

Undergraduate Electric Machines Laboratory llnnovation at the United States Military Academy Karl E. Reinhard

Herbert L. Hess ABSTRACT

New laboratory hardware supporting undergraduate electric machine and power systems instruction has been developed and installed at the Department of Electrical Engineering and Computer Science of the United States Military Academy. The new equipment integrates the transformers, machines, power supplies, loads, and meters necessary to support electric power engineering laboratory instruction into an electric power workstation; the workstation is designed to be a self-contained unit for students to perform laboratory exercises in their entirety. The workstation is built around induction, DC, and synchronous machines sharing a common shaft and three single-phase, tapped transformers. Variable speed drives are available to power each machine. Electrical connections between components are made on external console panels -- designed for easy access and to reinforce student understanding. GPIB capable digital instruments measure AC, DC, and mechanical performance. Laboratory exercises that model, analyze, and verify performance can be performed in significantly less time. Examples are given. INTRODUCTION

Instruction in electric machines has been offered to cadets at the United States Military Academy for nearly a century. Laboratory hardware has historically been motor generator sets, beginning with ten horsepower Westinghouse pad-mounted units circa World War I. Over the years, these were upgraded to realize improvements in performance and size. The last major improvement in 1965 replaced the entire laboratory with bench-mounted, analog instrumented universal machines. While the machines were well crafted, they had reached the end of their economic life.

Paul F. Barber

find equipment meeting the department's requirements. This prompted the electrical engineering faculty to develop a new equipment specification incorporating advances in machine design, power elwtronics, computer technology, and safety features. In 1991, Hampden Engineering Corporation was awarded a contract to design and build the new power workstations. New and more efficient approaches to illustrating machine and transformer behavior in the laboratory are now possible. This paper reports the specifications and application of these test benches. The resulting workstation's cost was approximately $65K. WORKSTATION DESCRIPTION

The workstation consists of a sturdy, wheel-mounted steel frame with a 2m by lm footprint, standing 2m tall, and weighing 1300 kg. A front view of the workstation is shown in Figure 1 with a panel description given in Table 1. Mounted above the worktable is a console that is divided into eleven numbered equipment panels. (Panels are numbered from left to right and tolp to bottom; there are four panels in each of the top two rows and three in the bottom row). Each panel is modullar, flush mounted, and sloped for easy viewing. The induction, dc, and synchronous machines mounted on a common shaft are located below the work table. All machine connections are brought into a single panel in the bottom center of the console. Fixed and variable AC and I>C power supplies are located on the lower left and right panels. Two variable speed drives provide the capability to control the available machines in a wide range of experiments. Fixed digital instrumentation is mounted on AC and DC panels. Interconnection of modules is accomplished by patch cord. Measurement functions include voltage, current, power, torque, and speed.

Power Reauiremenb In 1986,the U.S. Military Academy began a long term program to completely update engineering instructional facilities and equipment. A survey of commercially available electric machine instructional equipment did not 1'

Power enters the bench through a 1201208 Volt, 20 Amp, three phase, five wire plug. The input line is connected to themain power switch on panel #9 (at the lower left

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Figure 1 Electric Machine Laboratory Bench corner of the console. The main supply feeds four independent power supplies: a variable three-phase, four wire, 0-120/208V, 20A, AC supply and three rectifier fed DC supplies. The DC supplies include a fixed 125V, 30A supply; a variable 0-200V, 5A supply; and a variable 0150V, 1.5A supply. Each variable supply is fed through an autotransformer controlled by a RAISE/LOWER switch connected to respective internal drive motors. Each supply, fixed or variable, is protected by its own set of breakers. Patches to machines and instrumentation can be made either by heavy 30A cords terminated with spade lugs or by lighter 15A7instrumentationcords with single banana termination. (For safety reasons, the ubiquitous candle plugs and knife switches of yesteryear are avoided; all external connections are screened by insulation.) There are also two 120 Volt single-phase, transformer isolated convenience outlets on each end of the console.

Table 1 Console Panel Descriptions Variable Frequency Drive 2- Three Phase 3.3 KVA RL,C Load Bank 3. Three Single Phase Transformers 4* Regenerative SCR Based DC Drive 5. AC Meters: Voltage, Current, and Power 6 . Circuit Breaker Connections 7* Circuit Breaker Connections (Same as 6) DC Meters, Tachometer, and Torque Meter 9* AC and DC Power Supplies 10. Motor Terminals and Controlling Rheostats 11. Variable and Fixed DC Supplies

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Machine and Transformer Panels

The machines are directly wired to terminals on panel #10 (lower center panel). In addition to the power terminals, each machine has an annature-mounted search coil

terminated to banana jacks on the machine panel. Connections are diagrammed to reflect the physical configuration of the machines. One of the pedagogical problems inherent in any machine bench is obscuring connections to the detriment of understanding; in this system, there are no hidden connections from any machine terminal to power. Additionally, there are no hidden or default connections to instrumentation, save the


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RPM and torque metering.

Each machine may be operated as either a generator or motor.

capacitance is available in discrete increments of 50 VA. Inductance, also available up to 1 kVA per phase, is selected from continuously tapped coils. As with the transformer, various series, parallel, and other combinations can be appropriately connected.

Induction machine A 3 horsepower, 208 Volt, 60 Hertz, 1750 RPM wound rotor induction machine is mounted at the left end of the shaft. Both ends of each stator winding are brought to the machine console panel, making either wye or delta connection convenient. Three rotor winding terminals are provided; they may be patched to either a three phase, four wire rheostat or shorted. DC Machine The dc machine is a 2 kW, 125 Volt, 19 Amp, 1800 RPM, trundle-mounted machine. Connections are brought up to the center of the motor panel. There are three field windings: a shunt field winding, a series field winding designed for use in concert with the shunt winding to create a compound machine, and a series field winding designed for operation by itself. Separate rheostats are available for adjusting the shunt and series field currents. A strain gage load cell connected between the motor case and bench frame senses the reaction torque. Synchronous Machine On the right end of the shaft, a three-phase wound field synchronous machine is mounted and rated at 3 horsepower, 1201208 volts, 8.4 amps, 60 Hertz and field current rated 2.3 amps. Its terminals are connected to the motor panel in much the same manner as the other machines. Field resistance control is available through a rheostat that can be patched into the field circuit. There is an inductive start capability to bring the shaft to synchronous speed.

Variable Sneed Drive Modu les Two variable speed drive modules are provided on this lab bench. The drives are commercial units that have had their front panels and terminal connections modified to fit the space and format of the front console. Input to either drive is 230 Volts, three phase, 60 Hertz. The AC variable frequency drive is controlled from Panel #1 and is a 3 horsepower, constant Volts/Hertz, Pulse Width Modulated, variable speed unit and compatible with the induction and synchronous machines. A wide selection of adjustments common to such a drive are mounted on the panel, including SPEED, VOLTS/HERTZ, MAX and MIN ITtEQ, ACCEL, DECEL, SLIP COMP, OVERLOAD 'TRIP, LOAD and SPEED estimation, CURRENT LIMIT, STABILITY, VAR TORQUE, and BOOST. Six-step operation is also an option. Test points from the drive itself are brought out to terminals on the front panel, as well as s o h a r e diagnostics on an LED display. Closing the speed loop can also be done from the panel through the use of speed feedback from the tachometer.

Transformer Three single phase, 240/120V, lKVA transformers are connected to terminals on panel #3. The transformers are not visible from the front of the machine, but are easily accessible from a rear door. The circuit connections are silk screened on the transformer panel. Each transformer has two primary windings, each rated at 120 volts, 4 amps. One of the primary windings is tapped at 18.6 and 87.8 volts. Each transformer also has two secondary windings rated at 60 volts and 8 amps. One of the secondary windings is tapped at 12 and 24 volts and the other is tapped at 9.3 volts and 43.9 volts. This enables the transformers to be connected in many possible configurations including Scott, zigzag, and a variety of series and parallel configurations.

The DC drive is a 3 horsepower fully regenerative dual bridge SCR based unit with closed loop speed and torque regulation. It is Pulse Width Modulated and rated 0 180 volts DC, 15 amps armature and 200 volts, 2 amps field. For feedback, shaft position is determined by a 64-line inductance encoder on the shaft of the synchronous machine. Intemally, the encoder signal is digitally differentiated into a speed reading for display on panel #8 and feedback to both drives. The range: is 0-2400 RPM with direction indication. There are a wide range of adjustments and options available on the drive, but only the ON/OFF, speed and torque knobs, and feedback selection switch (armature or encoder) are on the panel. The tachometer feedback control is a pi&xlarly useful labor and time-saving feature in laboratory exercises requiring constant shaft speed. The panel displays a detailed block diagram of the dc drive.

Load Module

Instrumentation Panels

Panel #2 has three single phase RLC loads, rated at 120 volts. Resistance is selected by toggle switch to 1 kW per phase in 50 watt increments. Similarly, up to 1 kVA of

AC instruments, mounted on panel # 5 , consist of two autoscaled 300V voltmeters, two 20A autoscaled ammeters, and two 3 kW wattmeters. A. useful ammeter

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design feature is the option for direct or current clamp connection. Current clamps eliminate the need to break the circuit -- strongly enhancing flexibility and safety. The DC instrument panel consists of two 150V autoscaled voltmeters and three autoscaled 20A ammeters. The DC ammeters have a direct connection or solid state current clamp option. Torque (-15 to +15Nm) and speed (0-2400 RPM, direction indicated) are digitally displayed. The torque meter can be zero adjusted. All the meter signals are sampled and placed on an IEEE488 bus. Commands to raise or lower the variable voltage values may also be sent on the same bus. Each lab bench is supported by a dedicated 386-based PC for data gathering and analysis. Procurement of LabView software was recently funded to streamline this process. A four-channel, digital storage oscilloscope and a threephase digital wattmeter were purchased to supplement the mounted instrumentation. Both instruments are GPIB capable to allow automated data collection. Test points are available for other ancillary instrumentation as desired, e.g., hand held multimeters or fi-equency counters. LABORATORY EXERCISES AND APPLICATIONS

Six laboratory benches were delivered in the Spring of 1993 to the power laboratory. The laboratory supports two courses: a machines course for electrical engineering majors and a machines, opamp, and control course populated mostly by mechanical engineers. Experiments on these machines comprise the major share of the experimental work in both courses. We have no graduate program, so research opportunities are restricted to faculty working alone or an occasional semester-long undergraduate project. Emphasis to date has been on integrating these new machines into the existing laboratory framework. We initially saw the electrical engineering students as the prime users and were pleasantly surprised when the lion's share of the demand came .fromthe mechanical engineers. The intuitive console presentation allows us to address both groups effectively. In both courses, emphasis has been on machine properties and performance. With careful planning, laboratory exercises are kept simple enough that wiring the experiment does not capture a disproportionate share of the time available for learning. The following discussion presents brief examples of how the machines are employed.

Transformer Exercises

The transformer is an excellent vehicle to introduce energy conversion. Most machine benches omit the transformer, a situation that we found unfortunate. In our case, the transformer also serves to introduce the student to the equipment, making the bench less daunting in the long run. The cadet sees the large and (fi-om frrst glance) complicated panel but quickly learns that we ordinarily deal with only a small portion of any system at a time. This reinforces the premise that large problems may be solved by intelligent use of a set of small models. In the transformer exercise, cadets in two or three person teams perform open circuit and short circuits tests, determine an equivalent circuit model, and verify the model (comparing it with ideal and equivalent circuit models) via load test. With power, load, and meter connections readily available, wiring is easy and the lab can be completed in a two hour allotted time. Both courses require this transformer exercise. AC Machine

The synchronous machine laboratory models the steady state performance of the round rotor machine by open circuit and short circuit tests. Cadets develop an equivalent circuit model for the machine operating in the generator mode and verify the model against machine performance under load. They use the variable speed DC drive to control the DC motor which in turn drives the synchronous generator; the synchronous speed can be selected and maintained using the drive speed control. In this exercise, cadets are introduced to the three phase wattmeter and its capabilities. Induction machines can be treated in a similar manner. A full range of possibilities for modeling and verification of performance predictions is available. Behaviors under steady state and line start conditions are easy to observe. The AC drive provides important insight into induction machine behavior. At present, these are covered in demonstrations while laboratory exercises are being developed. Another exercise under development is to have the cadets build and control a small power system by interconnecting several workstations. Synchronous machines may be connected to the load banks to simulate line and load reactance. Meters indicate speed, voltage, and real and reactive power flow. Cadets assemble a power system of two or three generating stations, excite the generators, and bring them on line. Requiring that the I


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cadets shift generation from machine to machine while maintaining both load frequency and voltage manually will be a powerful hands-on exercise and a real education for both electrical and mechanical engineers. DC Machine

Investigation of DC Machine capabilities are enhanced by this laboratory bench. Series, shunt, compound, or separately excited operation are readily available from a set of variable DC supplies, rheostats, and the DC drive. A wide range of generator characteristics are available from the dc generator in torque mode. Precise constant speed operation is available with the synchronous machine on the same shaft. The DC machines laboratory exercise is of the same nature -- perform appropriate tests, develop a model, and verify that model's predictions with performance data. Electrical or mechanical drive and load are available, depending on whether generator or motor operation is desired. Behavior of different configurations of the DC machine are modeled and compared.

REFERENCES [11 Hampden Engineering, "Hampden Model H-EWC-3OOAX Mobile Electrical Work Center,"Hampden Bulletin 165-201, 1993.

[2] D.W. Novotny, R.D. Lorenz, T.A. Lipo, and D.M. Divan, "The Electrical Machines and Power Electronics Labo"tory Modemization at the University of Wisconsin," Proceedings of the American Power Conference, 1989. [3] N. Mohan, "Notes from NSF Workshop on Teaching Electric Drives,"August 4-6, 1994.

***** Herbert L. Hess Herb Hess received the BS degree in Electrical Engineering in 1977 from the U.S. Military Academy, where he was also an Instructor from 1983-1988. His MS degree is from the MIT in 1982. After receiving the Ph.D. degree from the University of Wisconsin in 1993, he joined the Electrical Engineering Department at the University of Idaho, where he is an Assistant Professor at its Boise Engineering Education progyam. He is a licensed professional engineer.

Karl E. Reinhard

CONCLUSION

The recent acquisition of state of the art laboratory test benches greatly improves instruction in electric machines and transformers at the United States Military Academy. These benches primarily support the introductory machines course and a machines and control course populated heavily by ME students. The thoughtful bench design permits laboratory exercises to range further with much of the tedium of data collection removed. Additional exercises and demonstrations to exploit the full potential of these capabilities are being designed to illustrate machine behavior that was formerly difficult and time-consuming to show. These test benches support both the depth necessary for the electrical engineering students and breadth required for students in other engineering disciplines. ACKNOWLEDGMENTS

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Dr. Charles A. Gross, Visiting Professor USMA '86 '88 from Auburn University played a key role in conceptualizing the curriculum and lab update. Hampden Engineering, East Longmeadow, Massachusetts, designed and built the laboratory benches per specification.

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Karl E. Reinhard is currently a Major on active duty in the U.S. Army and is serving as an Assistant Professor in the Department of Electrical Engineerbig and Computer Science. He received the B.S. degree in 1982 from the U.S. Military Academy and was commissioned in the Ordnance Corps. He earned the M.S.degree from the University of Texas-Austin in 1992 and joined the Department of Electrical Engineering and Computer Science. He is responsible for the department's electric power courses. He is a licensed professional engineer.

Paul F. Barber Paul Barber earned a BS from the U.S. Military Academy in 1965 and served for 28 years in the IJ.S. Army Corps of Engineers. He earned MS degress in Civil and Electrical Engineering from the University of Illinois in 1974 and a Ph.D. degree in Electric Power Engineering from Rensselaer Polytechnic Institute in 1988. He taught for 12 years on the Electrical Engineering faculty of the U.S. Military Academy at West Point. He is a registered professional engineer and is currently a senior engineer and manager with Citizens Power and Light Corporation in Boston, MA.

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