IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 1, FEBRUARY 1997 111

Correspondence

Incorporating Inctructional Feedback in ElectricalEngineering Laboratory Experiments—An Example

Vassilios G. Agelidis

Abstract—The advantages of incorporating software tools, other thansimulation packages, in electrical engineering laboratory experiments arediscussed in this correspondence. This approach supports the prerequisitelecture material and allows study and understanding of some practicalissues which are best handled through a laboratory experiment. It isalso shown that only through software can instructional feedback beeasily integrated in the students’ work. Time and effort are optimizedand difficulties in assimilating the material and correlating the resultsare minimized. A detailed laboratory experiment, namely performanceanalysis of an induction motor, which is significantly enhanced withthe support of a software package called Mathcad, is provided as anexample.

I. INTRODUCTION

The ever increasing power and availability of comput-ers—hardware and software—has already begun to revolutionize theway mathematics, science, and engineering subjects are taught andlearned. Furthermore, many opportunities are created for problemanalysis and quantification in ways which would be impossibletoday without the aid of computers [1]–[4].

In recent years, there has been increased emphasis on computer-based technology to assist and advance learning endeavors at all levelsof education and training. There are a number of different types of ed-ucational applications that constitute the teaching role of computers.Under the broad term of Computer-Assisted Learning (CAL) there aremore specific terms which refer to the major types of CAL, such asComputer-Assisted Instruction (CAI), Computer-Managed Instruction(CMI), Computer-Based Instruction (CBI), Computer-Based Training(CBT), Diagnostic and/or Prescriptive Applications just to name afew [4]–[6].

In engineering education, most of the subjects provide opportu-nities for CAL. For instance, simulation, a model or description ofoften complex events or conditions, is an interesting form of CAL. Inparticular, circuit simulation has become an integral part of severalcourses in an electrical engineering undergraduate curriculum. Theadvantages of using simulation in order to investigate the behaviorof electric and electronic circuits are tremendous and they have beenwell documented in the literature [7]–[12].

Another interesting and useful software tool widely available forcomputers is the spreadsheet analysis program. It has been used forthe analysis of various electrical engineering problems [13], [14],and electrical machines [15]. The main disadvantage of the above-mentioned approaches using spreadsheet software tools is that theentries required are code-based.

The objective of this correspondence is to discuss and illustrate,with an example, the enhancement of laboratory facilities in an

Manuscript received June 9, 1994; revised September 18, 1996.The author is with the Centre for Renewable Energy Systems Technology

Australia (CRESTA), School of Electrical and Computer Engineering, CurtinUniversity of Technology, GPO Box U1987, Perth, WA, 6001 Australia.

Publisher Item Identifier S 0018-9359(97)01550-1.

electrical engineering undergraduate program using computer soft-ware. A program called Mathcad is briefly described and its uniquecharacteristics are outlined. An example based on a typical laboratoryexperiment in power engineering, that is, the analysis of a three-phase squirrel-cage induction motor through laboratory tests with theassistance of Mathcad, illustrates the effectiveness of the proposedapproach.

The contents of this paper are organized as follows: Section IIdiscusses the advantages of using software tools in engineeringeducation. Section III briefly presents some distinctive characteris-tics of Mathcad. In Section IV, the laboratory goals are summa-rized. A typical laboratory experiment enhanced with the assistanceof Mathcad is provided in Section V as an example. Finally, inSection VI, the advantages gained by the proposed approach areoutlined.

II. COMPUTER SOFTWARE TOOLS IN ENGINEERING EDUCATION

Computer software tools have applications in literally any intellec-tual endeavor. All of these applications share the basic idea of usingcomputers to analyze and model complex physical and/or mathe-matical problems. Moreover, computer-aided graphic presentation ofthe results improves the understanding of the problem and helps theinterpretation of its solution.

On the other hand, laboratory familiarization of students in engi-neering disciplines form an important part of the learning and trainingprocess. That is the main reason why it is of great interest for anypossible application of computer software to be used for enhancingexisting laboratory facilities.

The benefits of using computers in engineering education are notso very different from what they are in any other discipline. Thatis, information, through the use of computers, can be developed,accessed, manipulated, and displayed.

For educational purposes, if the most productive mode ofcomputer-assisted instruction is to be achieved, it is highly preferablethat the student–computer interface be interactive and in real time.The latter means that any waiting time for a computer response isreasonably short for a given problem.

In addition, it is highly desirable that input and output be clearlyand easily interpreted. The above-mentioned characteristics of thesoftware tools dictate both the approach and types of subjects thatare suitable for computer-assisted education and training.

However, it is beyond the scope of this correspondence to discussall aspects of software evaluation for educational purposes. Theobjective as stated earlier is to show how engineering laboratoryexperiments can be enhanced with the use of Mathcad. The latterhas a great deal of unique characteristics as presented in the nextsection.

III. FEATURES OF MATHCAD

Mathcad is a commercial software package that combines the livedocument of a spreadsheet with the interface of a word processor [16].

In Mathcad, equations are depicted in an identical way to whichthey appear on a classroom board, expanded fully in two dimensions.

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112 IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 1, FEBRUARY 1997

All the necessary fraction bars, brackets, and other characters aresized accordingly as they would be written on paper.

In order to illustrate the abovementioned points, let us take thesecond-order system as an example. One solution would be asfollows:

x =�b+

pb2 � 4 � a � c

2 � a: (1)

In programming language and spreadsheet analysis program, theequation would look like something like this:

x = (�B + SQRT (B � �2� 4 �A �C))=(2 �A): (2)

In Mathcad, the equation looks the same way as it is seen on aclassroom board or in a reference book:

x :=�b+

pb2 � 4 � a � c

2 � a: (3)

Like a spreadsheet, as soon as a change is made anywhere in thedocument the results are updated immediately after a new entry, andthe graphs are redrawn.

By combining text, graphics, and equations in a single document,Mathcad makes it easy to keep track of the most complex calculation.Finally, by printing the document exactly as it appears on the screen,a permanent and accurate record of the work can be produced.

There are many other characteristics and further documentation canbe found in the respective manual [16].

IV. L ABORATORY GOALS

In general, the laboratory experiments are designed

1) to reinforce and support the lecture-based course;2) to emphasise the importance of corroborating the results of

laboratory measurements which is accomplished through acomparison of expected and measured waveforms;

3) to expose the students to the measurement techniques used inthe industry in general.

Typically, previously studied material placed in a practical contextdictates the pedagogical approach followed.

The motivation of the proposed use of software programs duringthe execution of the work is to provide an instructional accessoryto complement the traditional and usually monotonous laboratoryinstruction.

V. LABORATORY EXPERIMENT—AN EXAMPLE

In order to illustrate the effectiveness of the proposed approach, acommon laboratory experiment is chosen as an example, namely, theanalysis of three-phase induction motors.

They are widely used in the industry due to their low maintenance,relatively low cost of construction, and high reliability. With theadvance of the power electronics industry, the difficulty of their speedcontrol has been overcome due to the wide availability of inexpensiveinverters.

The computations associated with the performance of a three-phaseinduction motor (see the Appendix) necessitate the knowledge of theparameters of the respective equivalent circuit.

Two tests, namely, the no-load and blocked-rotor test are usuallyperformed in a typical laboratory experiment. Also a dc dynamometeris used to supply the rotational losses, so that the stator copper andcore losses can then be estimated.

Usually, all the measurements are obtained with both analog anddigital equipment. This is very important, since the students famil-iarize themselves with various technologies used in the “real” engi-

neering world. When analog equipment is used, the “two-wattmeter”method [18] consists an important part of the experiment.

There is a great deal of difficulty involved in studying an inductionmotor. The computation of all the quantities is usually done for oneoperating point and if any other operating point is to be studied thecomplete and tedious procedure has to be repeated. Obviously, if anyerror is detected, further calculations must be repeated which causessome frustration for students.

Taking advantage of the Mathcad features, all these problemscan be eliminated. Furthermore, if an error is found during theexecution of the experiment (i.e., misreading of an instrument),Mathcad automatically processes the new information.

A typical Mathcad file suitable for the respective laboratory ex-periment is included in the Appendix. Since there are many featuresto enhance the file, the program is just an example of what can bedone. The direct advantages of the proposed approach are discussedin Section VI.

In the laboratory environment, the students having studied thetheoretical analysis and prepared a Mathcad file (Appendix) useall the necessary instruments and methods in order to obtain com-plete information for determining the equivalent circuit of the mo-tor.

The measurements are inserted in the file immediately after theinstruments are read.

VI. DISCUSSION OFRESULTS ACHIEVED

The goal of this laboratory experiment is to analyze a three-phaseinduction motor through experimental work. Due to the extremelyhigh involvement of equations, this task is difficult to achieve in ausual laboratory session (about 3-h duration).

Through the use of the proposed method, understanding of thematerial has been enhanced for the following reasons:

First, essential but repetitive calculations can be dealt with quickly.This leaves more time to concentrate on developing the student’sskills of analysis and correlation of the experimental work. It alsoencourages the discussion of the results while the experiment is beingconducted.

Secondly, the frustration due to the usual student’s fear to workeffectively in a laboratory environment can be virtually eliminated.Furthermore, motivation is enhanced when the students are awareof the quality of their work. This is also referred to as positivefeedback.

The old proverb about hearing and forgetting but doing andremembering is more than just an old proverb when it comes tolearning. The proposed approach further stimulates participation.Another important achievement is giving the students an overallview of the area and facilities which are available in the learningprocess.

The students fully interpret and comprehend not only the respectivematerial but also learn a complete method of organizing, conducting,and documenting work which by virtue of the software underconsideration, is of a higher standard. That is, the students areprovided with a range of experiences that aid learning, experiencesnot necessarily available in any other way.

Analysis of the equivalent circuit of a three-phase induction motorhas been simplified and students’ time and effort has been optimized.It should be noted that the necessary background for the dynamicanalysis of the motor, namely speed control, has been placed inperspective. That further means that the proposed procedure is alsovalid and very useful for a postgraduate course.

IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 1, FEBRUARY 1997 113

VII. CONCLUSION

The advantages of using software tools, other than simulationpackages, during the execution of a laboratory experiment in anelectrical engineering undergraduate program have been discussedin this correspondence. An important accessory to complement un-derstanding of the theory through experimental verification has beenpresented. This further permits the student to closely monitor andappreciate the experimental world. The example of the study of aninduction motor represents, of course, only one of many applicationspossible for Mathcad.

NOMENCLATURE

MathcadNotation

Variable

s SlipV nl No-load voltageInl No-load currentq1 Number of phasesPanl Wattmeter 1 - No-loadPbnl Wattmeter 2 - No loadV br Blocked-rotor voltageIbr Blocked-rotor currentPabr Wattmeter 1 - Blocked rotorPbbr Wattmeter 2 - Blocked rotorXe Equivalent leakage reactanceRe Equivalent resistancens Synchronous speednr Rotating speedV 1 Line-to-neutral voltagePg Gap powerPo Output powerTd Developed torqueTstart Starting torqueIstart Starting currentsmax Maximum slipTmax Maximum torquePgmax Maximum gap powerPmax Maximum powerX� Magnetising reactancer1 Stator winding resistancer02 Stator-referred rotor winding resistance

x1 Stator leakage reactancex02 Stator-referred rotor leakage reactance

APPENDIX

A three-phase squirrel-cage induction motor with the followingdata is used for the experiment:

440 V, 4.6 A, 50 Hz, 2.2 kW, 1420 r/min.For the dc dynamometer test the following motor is used:220 V, 12 A, 2.24 kW, 1200 r/min. The motor is a three-phase

induction motor thereforeq1 := 3.

No-Load Test

The following entries are the direct measurements of the exper-iment.

The two wattmeters are read as follows:Panl := 426; P bnl :=

�606.The sum of the two readings yields:Psnl := Panl + Pbnl.The difference of the two readings yields:Pdnl := Panl�Pbnl.The net input power during the no-load test isjPsnlj = 180.

The no-load line-to-line voltage isV nl := 420.The no-load input line current isInl := 2:45.The stator winding resistance isr1 := 11:4.The respective angle is as follows:

�nl := a tanp3 �

Pdnl

jPsnlj�360

2�:

The power factor at no-load is also found from

�nl := a cosjPsnlj

q1 � V nl � Inl�360

2�:

The reactive component of the magnetizing current isI� := Inl �sin (�nl)

Xnl :=V nl

(p3 � Inl)

� sin (�nl)

X� :=V nl

I�:

Blocked-Rotor Test

The two-wattmeters are read as follows:Pabr := 0; P bbr :=

�354.The sum of the two readings yields:Psbr := Pabr + Pbbr.The difference of the two readings yields:Pdbr := Pabr�Pbbr.The net input power during the blocked-rotor test isjPsbrj = 354.The blocked-rotor input line current isIbr := 4:25.The blocked-rotor line-to-line voltage isV br := 90.

�br := a tanp3Pdbr

Psbr�360

(2�):

The equivalent leakage reactance is

Xe :=V br

p3 � Ibr

� sin (�br):

The effective value of the equivalent resistance is

Re :=jPsbrj

3 � Ibr:

The equivalent per phase impedance is

Ze :=V br

p3 � Ibr

and finally the equivalent leakage reactance is

Xe := (Ze2 �Re2):

Full-Load Test

The synchronous speed isns := 1500:

The speed for the full-load test isnr := 1433:

The slip is s := ns�nr

ns; s = 0:045:

That is, !s := 2 � � � ns60!s = 157:08

The input line-to-line voltage is:V ll := 420

I0

2 := 4; V l :=V llp3

r0

2 := 0:5

The gap power isPg := q1 � I 022 � r 2

s:

The output power isPo := Pg � (1 � s).The developed torque isTd :=

Pg

!s.

The starting torque isTstart := q1 � I 022 � r 2

!sand the starting

current is

Istart :=V l

pRe2 +Xe2

:

114 IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 1, FEBRUARY 1997

The maximum slip is

smax :=r02p

r12 +Xe2:

The maximum torque i:

Tmax :=1

!s�

q1 � vl2

2 � r1 +pr12 +Xe2

:

The maximum gap power then isPgmax := Tmax:!sand themaximum power then is:Pmax := Pgmax:(1� smax):

REFERENCES

[1] A. Bork, Learning with Computers. Bedford, MA: Digital, 1981.[2] J. W. Willis, D. L. Johnson, and P. N. Dixon,Computers, Teaching &

Learning. Dilithium Press, 1983.[3] J. Nievergelt, A. Ventura, and H. Hinterberger,Interactive Computer

Programs for Education. Reading, MA: Addison-Wesley, 1986.[4] A. Keller, When Machines Teach, Designing Computer Courseware.

New York: Harper & Row, 1987.[5] R. Lewis and E. D. Tagg,Trends in Computer Assisted Education.

Oxford, U.K.: Blackwell, 1987.[6] H. F. O’Neil, Computer-Based Instruction—A State-of-the-Art Assess-

ment. New York: Academic Press, 1981.[7] M. Singh, “Role of circuit and logic simulators in EE curriculum,”IEEE

Trans. Educ., vol. 32, pp. 411–413, Aug. 1989.[8] V. Rao and R. Hoelzeman, “A SPICE interactive graphics preprocessor,”

IEEE Trans. Educ., vol. E-29, pp. 150–153, Aug. 1986.[9] T. Giuma and P. Walker, “PSpice circuit generation through the method

of simulated annealing,”IEEE Trans. Educ., vol. 35, pp. 159–163, May1992.

[10] D. W. Hart, “Circuit simulation as an aid in teaching the principles ofpower electronics,”IEEE Trans. Educ., vol. 36, pp. 10–16, Feb. 1993.

[11] L. V. Hmurcik, “SPICE applications to an undergraduate electronicsprogram,” IEEE Trans. Educ., vol. 33, pp. 183–189, May 1990.

[12] S. Pricozy, “Novel applications of SPICE in engineering education,”IEEE Trans. Educ., vol. 32, pp. 35–38, Feb. 1989.

[13] N. D. Rao, “Typical applications of microcomputer spreadsheets toelectrical engineering problems,”IEEE Trans. Educ., vol. E-27, pp.237–242, Nov. 1984.

[14] L. P. Huelsman, “Electrical engineering applications of microcomputerspreadsheet analysis programs,”IEEE Trans. Educ., vol. E-27, pp.86–92, May 1984.

[15] T.-F. Chan, “Analysis of electrical machines using Symphony,”IEEETrans. Educ., vol. 35, pp. 76–82, Feb. 1992.

[16] Mathcad User’s Guide, Windows Versions, MathSoft Inc., 201 Broad-way, Cambridge, MA 02139, USA.

[17] V. Del Toro,Electric Machines and Power Systems. Englewood Cliffs,NJ: Prentice-Hall, 1985.

[18] J. F. Lindsay and M. H. Rashid,Electro-mechanics and ElectricalMachinery. Englewood Cliffs, NJ: Prentice Hall, 1986.

A Comparison of Experimental Results to Thosein “Evaluation of Bipolar Junction TransistorTransconductance in Practical Applications”

Ernest M. Kim and Thomas F. Schubert, Jr.

Abstract—An attempt to recreate the experimental results given inthe recent paper by C. D. Ferris was made using a modern transistorcurve tracer. The experimental results were not reproduced. The newexperimental results seem to validate the common usage of� � 1 indescribing bipolar junction transistors.

Index Terms—Electronics transistor parameters.

In the above paper,1 the author advocated the use of a more exactexpression for determining the mutual transconductancegm of bipolarjunction transistors (BJT’s):

gm =jIC j

�VT:

This expression includes the often ignored term�, an empiricalscaling constant. The author’s justification for such inclusion is a setof experimental data that suggest that� � 1:2 is more appropriatethan the usual unity value assumption.

We also feel that inclusion of� in all diode and BJT characterizingequations is appropriate, but the seemingly large value predicted byFerris attracted our attention. An experiment to verify the Ferrisresults was developed.

Ten BJT’s were tested in each of seven distinct types: fournpnandthreepnp. While the available inventory limited selection, an attemptwas made to use as many different manufacturers and production lotsas possible. With a SONY/Tektronix model 370 programmable curvetracer, the base–emitter voltage was sampled at five values of thebase current (20, 40, 60, 80, and 100�A) and read to the nearestmillivolt using the digital cursor display. Other significant settingson the curve tracer were as follows:

Max peak voltage 16 VMax peak power 0.08 WVariable voltage 100%

A least squares, best fit straight line was placed through the agraph of the natural logarithm of the base current versus base–emittervoltage data points and the slope divided by the voltage equivalenttemperatureVT , to determine�: Our choice forVT was the valuesuggested by most electronics textbooks for uncontrolled, room-temperature operation

VT = 26 mV ) T � 28:5�C:

This value takes into account a small warming due to electricalpower dissipation.

The results of our measurements are displayed in Table I. Ourconfidence in the data is high due to a linear regression coefficient of

Manuscript received November 2, 1993; revised June 24, 1996.The authors are with the Department of Electrical Engineering, University

of San Diego, San Diego, CA 92110–2492 USA..Publisher Item Identifier S 0018-9359(97)01665-8.1C. D. Ferris,IEEE Trans. Educ., vol. 36, pp. 293–295, Aug. 1993.

0018–9359/97$10.00 1997 IEEE