Teaching Electric Machines and Drives: A Re-examination for the New Millennium

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1 Teaching Electric Machines and Drives: A Re-examination for the New Millennium Tore M. Undeland Ned Mohan Norwegian University of Science and Technology University of Minnesota N-7491 Trondheim Norway Minneapolis, MN USA Tore.Undeland@elkraft.ntnu.no mohan@ece.umn.edu Keywords: Education, Education methodology, Education tools, Drives Abstract All over the world, courses in electric machines and electric drives are suffering from lack of student interest, leading to their cancellation and eventual elimination from the curriculum. This is happening just when we need trained students to make use of tremendous opportunities in this field. This article presents a proven strategy that has tripled student enrollment from its low point at the University of Minnesota. This approach, presented at three National Science Foundation-sponsored Faculty Workshops held at the University of Minnesota [1], is now backed up by recently published textbooks [2,]. The development underway of a dspace/dsp-based laboratory [4] has the potential of making the first course on this topic one of the the most sought-after in the EE curriculum, attracting a large number of students from other disciplines such as mechanical, civil and environmental engineering. A second-semester course on machine dynamics, control and modeling using SIMULINK and dspace will be discussed. Opportunities HVAC 16% Lighting 19% IT 14% Motors 51% World s electricity consumption. 1

2 This pie chart shows that motors consume over one-half of the electricity generated in the world. Using these motors more efficiently would obviate the need for more power plants and the energy conserved would be green money saved and less damage to the environment. Electric Machines versus Electric Drives The introductory course in electric machines has not changed in decades no wonder students look upon the subject of electric machines as old-fashioned, staid and boring. But the solution couldn t be simpler in the same course we can shift the focus from electric machines to electric drives (Fig. 1), which allow us to introduce this subject in the context of exciting new applications. Even a mundane application of driving compressors within airconditioners becomes exciting if adjustable-speed-driven compressors by capacity modulation can be shown to consume 0% less energy compared to their conventional counterparts with on/off cycling to maintain temperature [5]. Meeting the Challenge - Developing A New Approach Without the prerequisite of electric machine theory, how can we discuss the subsystems that make up electric drives (Fig. 1) in a single-semester course? Such a course, consistent with the university mission, must stress the fundamentals thus providing continuity to advanced graduate-level courses in this field, while preparing students for industrial assignments. This challenge has several components that are discussed below. Electric Drive Electric Source (utility) Fixed form Power Processing Unit (PPU) Adjustable Form Motor speed / position Load Sensor Controller Power Signal Input command (speed / position) Fig. 1 Block diagram of an electric drive system. 2

3 This course restructuring has several objectives. The undergraduate courses should prepare students for industry as well as for advanced courses and for research and development oriented careers. They should be appealing and exciting so that students are drawn to them. These courses should provide the requisite information about power electronics and electric drives in a way that provides motivation and allows time to take related courses in areas such as digital signal processing applications, programmable logic, and digital control. This way, students will learn what is needed to meet industry needs and create opportunities for future engineers. Deleting Irrelevant Topics. The above challenge required a completely fresh look at what topics are covered and how they are covered in the traditional course in this field. Discussion of the Power Processing Unit. In electric drives (Fig. 2), power electronic converters play an important role, as a power-processing unit (PPU), of efficiently converting utility-supplied power to voltages and currents of appropriate phase, frequency and amplitude, best suited for the operating conditions. Direct Computation of Electromagnetic Torque. With the goal of discussing speed and position control by electric drives (uncontrolled machines form a subset of these), the machine analysis requires a new approach to understanding the physical basis on which ac and dc machines operate, thus allowing a clear understanding of how they ought to be controlled in electric drives for optimum performance in speed and position control applications. Utilizing Space-Vectors for the Analysis of AC Machines. The key development to the analysis of ac machines in the first course has been to make use of space-vector theory in a very simple form as easy as using phasors (in fact easier, by providing a physical meaning to space vector representations). An important benefit of this approach is the continuity between the first introductory course and more advanced, mathematically based second course. An Integrative Approach Includes Control of Electric Drives. To give students a system s perspective, it is useful to discuss or at least present a one-lecture overview (depending on the time available) of how such drives are controlled.

4 TABLE 1 TOPICS IN ELECTRIC DRIVES COURSES AND ASSOCIATED LAB SESSIONS No. Lecture Topic Lects. Laboratory Sessions Labs 1 Introduction to Electric Drive Systems 1 Lab Safety, Familiarity with Simulink 1 2 Understanding Mechanical System Requirements Mechanical System Modeling, Introduction to dspace 2 Review of Electric Circuits Basic Understanding of Power Electronics Switch Mode Converters for Drives 2 5 Magnetic Circuits 4 Line-Frequency Transformer 1 6 Basic principles of Electro-mechanical Energy Conversion 0 7 DC Motor Drives 5 DC Motors, DC-Motor Drives 2 8 Feedback Controller Design in Drives Feedback Control of DC Drives 1 9 Introduction to AC Machines & Space Vectors 5 Space Vectors in Simulink & dspace 1 10 Sinusiodal PMAC Drives 4 PMAC Drives 1 11 Induction Machines: Steady State Analysis 5 Induction Machines 1 12 Adjustable Speed Induction Motor Drives Adjustable Speed Induction Motor Drives 1 1 Reluctance Drives 1 Stepper Motor Drive 1 Real-Time, DSP-Based Electric Drives Hardware Laboratory Being developed by NSF and NASA funding in consultation with the consortium of nine universities and four government labs, this laboratory is intended to serve as the model for adoption at other universities. Therefore, safety, transportability, ease of usage and low cost are very important factors. This laboratory is intended for use in the first course, but its flexible nature will allow it to be used in advanced courses as well as in graduate level research. The block diagram of this software-reconfigurable lab setup will is shown in Fig. 2. The dcbus is opted to be at 42 V for safety reasons, mindful of large automotive applications as the auto industry switches from 12/14-V system to 6/42-V system in the coming years. It consists of a dc machine as an active load, controllable in all four quadrants. The common dc bus for both the converters results in only the system losses to be supplied by a small dc power supply. DC Power Supply M optical encoder ( tachometer ) switch signals DSP Board switch signals Fig.2 Block diagram 4 of the DSP controlled electric drives laboratory

5 This laboratory has three components: special motors are being built under a contract to operate from the converter fed by the 42-V dc bus. The converters are being developed on a single board and the controller using pulse-width-modulation for speed and position control is based on a rapid prototyping tool from DSPACE [6]. In this system, students design the controller in SIMULINK by bringing various blocks together, and then a program (completely transparent to the users) converts the code and downloads it in an embedded micro-controller for real-time control of electric drives. It will offer an unparallel experience and will be a great means of attracting students and exciting their interest, where they will compare results of computer simulations with those of hardware experiments, providing them with an education that will be invaluable. A photo of the prototype setup is shown in Fig.. [7] Induction motor PMDC active load 4 quadrants controlled PMAC motors (BLDC) PMDC motor Fig. Motor Lab Kit 42 V motors with matching mechanical and electrical characteristics, special designed for the laboratory 5

6 Second-Semester Course In the second-semester course, steady state models of electric machines are seamlessly extended to the study of advanced topics such as dynamic analysis, control and modeling of electric drives using Simulink. Simulink-based design examples allow meaningful designoriented problems to be assigned as homework. Students find that confirmation of analytical discussion by simulation results is extremely satisfying. q i sq ω d N 2 s i rq N 2 s at t r i s r 2 i sd i r i rd 2 i sq θ da 2 i rq θ m 2 i rd θ da ω d N 2 s i sd i sd ω m ω m d A rotor a stator Figure Stator Figure and rotor - Stator representation and rotor by equivalent mmf representation dq winding currents. by The dq winding voltages are equivalent defined as positive dq winding at the dotted currents. terminals. Note that the relative positions of the stator and the rotor current space vectors are not actual, rather only for definition purposes. In analysis and control of ac machines, this course provides a physical picture as much as possible for visualization purposes. Most research literature and textbooks in this field treat dq- transformation of a-b-c phase quantities on a purely mathematical basis, without relating this transformation to a set of windings that could be visualized. The approach adopted in this course is different but leads to the same results - a set of representative dq windings are visualized along an orthogonal set of axes (Fig. ) and then their currents and voltages are related to the a-b-c phase quantities. This discussion follows seamlessly from the treatment of space vectors and the equivalent winding representation in steady state in the previous course. 6

7 References: 1. NSF-Sponsored Faculty Workshops on Teaching of Electric Drives and Power Electronics, held at the University of Minnesota, 1994, 1997 and N. Mohan, Electric Drives: An Integrative Approach, textbook published by MNPERE, Minneapolis, N. Mohan, Advanced Electric Drives: Analysis, Control and Modeling using SIMULINK, textbook published November 2001 by MNPERE, Minneapolis, MN NSF/NASA-Sponsored Project, DSP-based, Software-Reconfigurable Laboratory to Nationally Revitalize Electric Drives and Power Electronics Curricula, University of Minnesota, June 1, 2000 May 1, Instructor s CD to accompany [2]. 6. dspace GmbH, Technologiepark 25, 100 Paderborn, Germany. 7. R. C. Panaitescu, N. Mohan et al An Instructional Laboratory for the Revival of Electric Machines and Drives Courses IEEE Power Electronics Specialists Conference June 2002, Cairns, Australia 7