IEEE TRANSACTIONS ON EDUCATION, VOL. 49, NO. 3, AUGUST 2006 383

Three-Phase Machines and Drives—Equipment for aLaboratory-Based Course

S. A. Shirsavar, Member, IEEE, Benjamin A. Potter, Member, IEEE, and Isabel M. L. Ridge

Abstract—The hazards associated with high-voltage three-phaseinverters and high-powered large electrical machines have re-sulted in most of the engineering courses covering three-phasemachines and drives theoretically. This paper describes a setof purpose-built, low-voltage, and low-cost teaching equipmentthat allows the hands-on instruction of three-phase inverters androtating machines. The motivation for moving towards a systemrunning at low voltages is that the students can safely experimentfreely with the motors and inverter. The students can also accessall of the current and voltage waveforms, which until now couldonly be studied in textbooks or observed as part of laboratorydemonstrations. Both the motor and the inverter designs are forteaching purposes and require minimal effort and cost.

Index Terms—Brushless dc motor, course, induction motor, in-verter, teaching.

I. INTRODUCTION

UTILIZING a mixed variety of pedagogical approachesto enhance student learning is widely accepted among

educators [1], [2]. By considering adult learning styles, re-searchers can show that using a variety of stimuli promotes“deep learning” [3] and ensures that each adult learner is pre-sented with the material so that it appeals to individual learningpatterns [4].

Laboratory and practical classes play a major role in the edu-cation of scientists and engineers [5]. In the field of engineeringeducation, latest research suggests that the pedagogical learningobjectives of practical sessions generally go much further thansimply supplying an additional stimuli, therefore promoting ad-ditional skills and deep learning [6].

The extra skills learned in practical sessions are an integralpart of engineering as a discipline and, from the engineeringeducator’s point of view, the goals for a practical session aremuch wider than just the promotion of deep learning. Fry et al.[6] suggest that these additional aims may include the following:

• gaining practical skills and experience of practical piecesof equipment;

• making links between theory and practice;• gathering, manipulating, and interpreting data;• forming and testing hypotheses;• developing problem-solving techniques;• motivating and exciting students.

Manuscript received February 20, 2004; revised April 25, 2006.S. A. Shirsavar and B. A. Potter are with the Department of Electronic Engi-

neering, School of Systems Engineering, University of Reading, RG6 6AY U.K.I. M. L. Ridge is with the School of Construction Management and Engi-

neering, University of Reading, RG6 6AY U.K.Digital Object Identifier 10.1109/TE.2006.879266

The results of two recent surveys of the electronic engineeringstudents at the University of Reading, Reading, U.K. [7], [8] cor-roborate many of the above suggestions. Both of these surveysverified that the engineering students overwhelmingly preferan experimental approach to teaching. The vast majority (over80%) of the students in these surveys felt that experiments en-hanced their grasp of the core subject and their understanding ofthe theoretical material and made the course more interesting.

However, the study of three-phase rotating machines and theirassociated power electronics as part of an undergraduate elec-tronic engineering course seems to lack any significant prac-tical work. This scarcity is indeed understandable because, tra-ditionally, hands-on experiments are not easy to introduce intosuch courses. The two main reasons for this difficulty are thehigh voltages present in three-phase inverters and the mechan-ical hazards associated with motors. This problem is particu-larly acute in departments geared toward electronic engineering,where the laboratories in general are not equipped for high volt-ages and rotating machines. Thus, unless explicitly geared to-ward electrical engineering, many electronic engineering stu-dents are only able to follow a mainly theoretical approach.

To solve this problem, the introduction of alternative teachingmethods enhance student learning. Some of these methods usecomputers and simulation packages [9], [10], while most of thework that advocates the use of purpose-built hardware teachesonly dc motors [11], [12]. Indeed, a complete absence of ex-periment-based teaching material seems to exist for three-phaseinverters driving a three-phase induction motor. This lack of ma-terial can be directly attributed to the commercial unavailabilityof low-voltage, three-phase induction machines and their asso-ciated inverters.

A solution to the above problem is presented in the excel-lent work of Undeland and Mohan [13]. In their paper, the au-thors overcome the unavailability of low-voltage, three-phaseinduction machines by designing and constructing their ownthree-phase induction motor running off a 42-V dc bus. Al-though a 42-V dc bus voltage is much lower than in typicalsystems, the resulting inverter voltages will still be too highto allow for hands-on student experimentation, as currently themaximum voltage permitted for use in undergraduate experi-mentation in many of the United Kingdom’s universities standsat 30-V dc.

The above discussion shows the existence of a real practicalproblem, which has so far prohibited the vast majority of un-dergraduate students from experimenting with three-phase ma-chines and inverters.

In order to address this problem, this paper describes theconstruction of low-voltage (12-V), three-phase induction,

0018-9359/$20.00 © 2006 IEEE

384 IEEE TRANSACTIONS ON EDUCATION, VOL. 49, NO. 3, AUGUST 2006

Fig. 1. Power stage of the proposed inverter.

brushless dc motors, and an associated three-phase inverter.The equipment has been designed to be both relatively inex-pensive (circa U.S. $25) and easy to construct. Furthermore,since the inverter runs at only 12 V, the students are able toexperiment freely with the motor and inverter and access thevarious waveforms at different points in the circuitry. Thisability to experiment enhances their understanding of the basicprinciples of three-phase induction machines and drives.

One must note at this point that the purpose of this paper isnot to prove that experimental work enhances student learning inengineering courses, as this concept has already been proven byeducationalists [1]–[6]. The objective of this paper is to informcolleagues of a novel, easy, and low-cost method of introducingexperimental work into a topic that has suffered from lack oflaboratory work in many educational establishments.

All information required to reproduce these components ispresented, and colleagues who are interested in introducing ahands-on, practical element to their power electronic courses areencouraged to make full use of the material presented here. Anycomments or suggestions would be gratefully received. The de-sire is that the information presented here will assist engineeringeducators in improving the instruction of this discipline.

In the following sections, the construction of the inverter andmotor and some experimental results and feedback receivedfrom the students will be described.

II. CONSTRUCTION OF A LOW-VOLTAGE

THREE-PHASE INVERTER

Commercial inverters are not appropriate for undergraduateteaching. Unavoidable safety issues associated with the highvoltages that these units generate are present, making it diffi-cult, if not impossible, to access the waveforms generated bysuch inverters.

In addition, commercially available inverters are far morecomplicated than the simple form of a three-phase H-bridgedepicted in most textbooks. Commercial inverters possess cir-cuitry associated with surge protection, electromagnetic inter-

ference suppression, high-side drivers, current limiting, over-voltage protection, and optical isolation. In addition, most newcommercial inverters use all-in-one power modules containingall the power switches in one package, rendering access to indi-vidual switch signals almost impossible.

A simple H-bridge inverter was designed to show the studentsclearly how the circuitry relates to classroom material or de-scriptions found in electronics textbooks. To allow the studentssome hands-on experience, a further requirement was for theoperating voltage to be lower than the minimum permitted safevoltage. In the case of the United Kingdom, this requirement is50-V ac, equating to a dc bus voltage lower than 30 V.

Since the inverter has been designed as a teaching tool, sev-eral features within the design facilitate ease of functional de-scription, ease of construction, and low cost. One notes thesefeatures will, in general, be at the expense of performance.

Fig. 1 depicts the schematic circuit diagram for the powerstage of the inverter, while Fig. 2 depicts the schematic circuitdiagram for the microprocessor control stage of the inverter.

The use of monolithic, integrated circuits (ICs) incorporatingall six switches has been avoided so that the students can haveeasy access to all signals within the H-bridge inverter when thebasic operation of the three-phase, H-bridge inverter is taught. Inaddition, no isolation is provided in order to simplify the circuitfurther.

P-type, metal–oxide–semiconductor field-effect transistors(MOSFETs) have been used on the high-side of the H-Bridge,i.e., M1, M3, and M5. P-type MOSFETs have the advantagethat they can be switched without the need of expensive andoften delicate high-side drivers, thus reducing circuit com-plexity and cost. A further simplification has been obtained byusing a MOSFET’s parasitic diode for freewheeling purposes[14]. Furthermore, all six power switches are slightly overratedto eliminate the need for heatsinking.

The microcontroller stage of the circuit simply consists ofone bottom of the range 8-b microprocessor with its associated20-MHz clock and voltage regulator. The microcontroller gen-

SHIRSAVAR et al.: THREE-PHASE MACHINES AND DRIVES—EQUIPMENT FOR A LABORATORY BASED COURSE 385

Fig. 2. Microcontroller stage of the proposed inverter.

Fig. 3. Fully constructed inverter.

erates all six pulsewidth-modulated (PWM) outputs, incorpo-rating within them the necessary dead time. The switching fre-quency is set to 3 kHz, while the base speed is fixed at 100 Hz.This specification allows the six PWM signals to be generatedby a lookup table and permits the use of an inexpensive, low-per-formance processor. The microprocessor code, written in the Cprogramming language, is straightforward and has not been in-cluded in this paper. However, if required, the code can be ob-tained by contacting the corresponding author.

The entire circuit is routed on a single-sided board to alloweasy, in-house construction. The complete printed circuit board(PCB) layout, its silkscreen, and the component lists are in-cluded in the Appendix.

Fig. 3 depicts the fully constructed inverter. The most ex-pensive component in the circuit is the microprocessor at U.S.$3.80, and hence educational establishments will be able tomake their own inverters with minimal cost and effort.

III. CONSTRUCTION OF LOW-VOLTAGE THREE-PHASE MOTORS

The three-phase induction motor and the three-phase brush-less dc motor that will be described in this section share the same

Fig. 4. Low-voltage, three-phase, PCB-mounted induction motor.

simple stator assembly. Therefore, it is only necessary to ex-change the rotors in order to switch between the different motortypes.

A. Miniature Three-Phase Induction Motor

Winding the electromagnets is a major problem when con-structing any kind of motor by hand. In the case of inductionand brushless dc machines, construction is easier because norotor windings are present; only the stator needs to be wound.However, even the smallest motors require thousands of turns ofvery fine wire for each phase to produce enough magnetomotiveforce at a reasonable current.

The easiest means of producing suitable windings was to re-move the solenoids from simple, proprietary relays. Althoughnot the cheapest solution, relay solenoids provided compact,uniformly wound coils with sufficient electromagnetic force atreasonable current. The relays, from which the solenoids usedin the motor shown in Fig. 4 were taken, cost around U.S. $3each. Of course, the winding distribution of the above motor isdifferent from a commercial machine and will not be able to pro-duce any usable torque. However, for understanding the basicprinciples, this arrangement will suffice, especially for coursesgeared towards electronic engineering where the main emphasiswill be on the power electronic inverter.

386 IEEE TRANSACTIONS ON EDUCATION, VOL. 49, NO. 3, AUGUST 2006

Fig. 5. Interchangeable rotors used in the motors.

The rotor was made from a piece of copper pipe with end capsfitted top and bottom and is shown in Fig. 5 (bottom left). Therotor shaft was taken from a miniature dc motor.

Having chosen the coils and rotor, the stator assembly wassimply mounted on a PCB that provided the electrical connec-tions for the coils. The stator circuit was designed so that thestator configuration could be changed between star and deltawith the use of a switch. If desired for further simplicity, theswitch could be removed and the coils permanently connectedin one configuration. The delta configuration is preferable togive the greatest start-up torque and, hence, the lowest operatingvoltage.

B. Miniature Three-Phase Brushless dc Motor

The stator of the brushless dc motor is identical to the in-duction motor assembly described above. However, the rotorfor this type of motor is made of permanent magnets. Manu-facture of magnets, specifically for the rotor, would be prohib-itively expensive and unnecessary. A permanent-magnet rotorcan be made by removing the stator magnets of the miniatureU.S. $1 dc motor and attaching them to the rotor as is shownin Fig. 5. By removing the windings from the rotor shown inFig. 5 (center) and attaching the two semicircular magnets thatwere used to make the stator of the dc motor, the finished rotorshown in Fig. 5 (right) can be constructed. The rotor of the in-duction and brushless dc motors are fully interchangeable in thestator assembly.

IV. RESULTS

Both motors, with both stator assemblies, were found to op-erate smoothly with dc bus voltages as low as 12 V when con-nected to the inverter described earlier in this paper. The overallsystem is well within safe voltage limits and presents no signif-icant electrical risk to the user. In addition, since the kinetic en-ergy in the rotor is very low because of its small size, no signif-icant mechanical risks are present. Students can now have full,safe access to all the voltages and currents, which would nor-mally be impossible with commercial motors and inverters.

As an example, the statement Fig. 6 depicts the stator voltageand the stator currents for a delta-connected induction motor. Ascan be seen from Fig. 6, although the stator voltage is clearly a

Fig. 6. Stator PWM voltage and sinusoidal current.

Fig. 7. Dead time between the gate drive signals.

PWM square wave, the stator current is a sine wave with slightharmonics.

By expanding the time base of the voltage waveform, the stu-dents can for the first time observe the dead time generated bythe inverter under safe low-voltage conditions. A typical wave-form is depicted in Fig. 7.

In addition, since most modern oscilloscopes provide aFourier Transform function, students can now also study theswitching harmonics experimentally. The frequency spectrum,displayed using a digital oscilloscope, is depicted in Fig. 8;harmonic analysis is a prime example of one of the manyimportant topics that so far could only be studied theoretically.

By way of summary, the inverter and motors presented in thispaper allow students to carry out hands-on, practical investiga-tions into, but not limited to, the following topics:

• the generation and use of PWM waveforms;• inverters, gate drive signals, and the use of dead time;• harmonic content of the inverter voltage outputs;• running three-phase machines and start-up characteristics;• differences and similarities between dc brushless motors

and ac induction motors;• currents and voltages in the motors’ stator windings;• differences between star and delta stator configurations;• harmonic content of the stator currents and the low-pass

filtering effect of the coils.

SHIRSAVAR et al.: THREE-PHASE MACHINES AND DRIVES—EQUIPMENT FOR A LABORATORY BASED COURSE 387

Fig. 8. Switching harmonics on the current waveform.

Fig. 9. PCB layout for the inverter.

V. STUDENT FEEDBACK

As stated in the Introduction of this paper, the objective of thispaper is not to prove that experimental work enhances studentunderstanding. This concept has already been proven and hasbeen referenced in this paper. Nevertheless, the students in theSchool of Systems Engineering at the University of Readingwho used this equipment as part of their course were surveyed.

The vast majority of students in this survey (over 80%) ex-pressed that experimentation with the inverter and motor, in-cluding the ability to carry out hands-on tests, made the sub-ject of electrical machines and power electronics much easierto understand and far more interesting. In addition, the studentswanted to see a similar approach adopted in their other subjects.

VI. CONCLUSION

The hazards, both electrical and mechanical, associated withtypical three-phase inverters and machines have meant thatthe vast majority of related course material is theoretical. Thispaper describes a miniature, low-voltage three-phase inverterand motor that avoids these hazards and allows the studentdirect and full access to all signals and motor elements.

Fig. 10. Silkscreen for the inverter.

TABLE IBILL OF MATERIALS FOR THE INVERTER

The stator design allows for the study of both a three-phase in-duction machine and a three-phase brushless dc motor by simplychanging the rotor. The designs of all parts of the system havebeen carefully considered to produce a system which is simpleto manufacture and has a low overall cost (circa U.S. $25).Typical oscilloscope readings covering voltages, currents, andFourier transforms of both voltages and currents have been in-cluded and show the wealth and range of practical investigationsthat could be made by the student.

The feedback received from the students who have used thepresented teaching material was very positive. The feedbackshowed that enabling students to have hands-on ready access tosuch equipment not only rekindles interest in the subject, butalso significantly improves their understanding and promotesdeep learning.

APPENDIX

The complete printed circuit board layout for the inverter isshown in Fig. 9, and the corresponding silkscreen is shown inFig. 10. The bill of materials for the inverter is shown in Table I.

388 IEEE TRANSACTIONS ON EDUCATION, VOL. 49, NO. 3, AUGUST 2006

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S. A. Shirsavar (M’94) received the B.Eng. (Hons.) degree in electronic engi-neering and the Ph.D. degree from the University of Reading, Reading, U.K., in1992 and 1998, respectively.

After a period of working in the industry designing embedded controllerhardware, switch-mode power supplies, and high-performance three-phase in-verters, he returned to the University of Reading as a Lecturer, where he hasbeen teaching courses at all levels. His main research interests are in power elec-tronics and renewable energy resources, in particular high-efficiency grid-con-nected inverters for use with solar panels.

Benjamin A. Potter (M’04) received the M.Eng. (Hons.) degree in engineeringscience from the University of Oxford, Oxford, U.K., in 2001. He is currentlyworking towards the Ph.D. degree at the University of Reading, U.K., and isactively involved in lecturing and laboratory-based teaching.

His main research interests include electric machines and drives and powerelectronics, in particular the modeling of the high-frequency behavior of induc-tion machines.

Mr. Potter is a member of the Institution of Electrical Engineers (IEE) in theUnited Kingdom.

Isabel M. L. Ridge was awarded the B.Sc. degree in mechanical engineering(with mathematics) and the Ph.D. degree from the University of Reading,Reading, U.K., in 1988 and 1992, respectively. Her Ph.D. research focused onthe Bending-tension fatigue of wire rope.

Currently, she is a Principal Research Fellow in the School of ConstructionManagement and Engineering at the University of Reading, where she managesthe Rope Research Programme. Her main field of interest is in the behavior andcondition assessment of offshore mooring lines for oil platforms, which she hasstudied for over 15 years. She is a member of the Organisation InternationalePour l’Etude de l’Endurance des Câbles (OIPEEC) Management Committeeand President of the OIPEEC Scientific Committee and is currently editor oftheir bulletin.

Dr. Ridge is a member of the following professional societies: a Memberof the Institute of Mechanical Engineers (CEng, MIMechE); Registered withFEANI (Eur. Ing.); a Member of the British Institute of Non-Destructive Testing(MInstNDT); and a Fellow of the Royal Society for the Encouragement of Arts,Manufacturers and Commerce (FRSA). In addition, she has received the fol-lowing awards and prizes in recognition of her work: the Institution of Mechan-ical Engineers project prize (1988), the OIPEEC Award (1993), and the Wor-shipful Company of Turners design prize Silver Medal (1994).