Keywords drives; electric machines education; power electronics education; PWM inverters
In universities all over the world, courses in electric machines and drives are suffering from lack of student interest1 and, in some cases, are being cancelled from the curriculum. However, nowadays, industry is demanding more and more trained engineers in this fi eld. From this point of view, universities have to modernise and improve the electrical machines and drives curriculum, meeting these industry demands, since power electronics and electric machines/ drives courses have in many cases not changed in several decades.2 With the aim of attracting more students, a modernisation of both course and the supporting laboratory is thus essential. In this way, laboratory experiments must be updated to fulfi l these requirements, though continuing the conventional way of providing practical experience to electrical and electronic engineering students, through the use of extensive laboratorybased systems. Engineering students must become acquainted with the equipment and techniques used in professional environments. When selecting the practical resources to be used in an engineering course, a trade-off must be found between their industrial signifi cance and the educational requirements.3
One essential element of power engineering education must be a renewed emphasis on the laboratory.4-6. Any new laboratory should address the areas of electrical machines, power electronics and their control, maintaining the conventional way of providing practical experience to electrical and electronic engineering students. In this context, it is necessary to point out that more than 75% of all generated power is processed by power electronics. The extensive usage of switching converter circuits in electronic products and systems makes the fundamental understanding of power electronics a necessity for students and electronic engineers.7 However, these laboratories are costly to build and diffi cult to maintain, being usually designed for their use along the years. Indeed, some authors claim that virtual instruction environments overcome this problem adding safety and security,8,9 and the use of computers do much to alleviate the students' frustration and give them an early sense of satisfaction and accomplishment10. Additionally, during the past decade, Internet technologies have become the basis of computer-assisted education. Web-based systems take advantage of giving access to many resources using a standard, universal, and well-known user interface11. In this context, computer games have also been integrated in a basic automatic control course at the undergraduate university level.12
In our opinion, it would be desirable if engineering education could combine both aspects: computer simulations and laboratory experiments. From this point of view, the main contribution of this paper is focused on describing our satisfactory experience when both simulations and laboratory experiments are proposed as practical sessions in different subjects. In our case, electric drives are considered in the present work as topic of interest, since they appear as a preferred objective in different electrical and electronics undergraduate subjects.
Industrial Engineering Degree
According to the current undergraduate curriculum of the Industrial Engineering degree13, the Electrical Engineering and Electronic Technology Departments of the Technical University of Cartagena (Spain), teach the subjects shown in Table 1, where ECTS represents the European Credit Transfer System equivalent14. This undergraduate curriculum was elaborated from a generalist education point of view, with emphasis on industrial engineering fundamentals and practices.
In this curriculum there is only one compulsory subject directly related to the fi eld of electronics - Power Electronics - , in which the goals are limited and have to be focused on basic concepts. The optional subjects give us the opportunity to extend the basic concepts, but those subjects are normally assigned to different departments. For this reason, a good coordination between these departments would be desirable in order to optimise the contents, avoiding overlap and offering a more effi cient education. In our case, Extension of Electric Machines and Power Electronics optional subjects - both given during the 5th-year course - have been coordinated during the past few years. They involve different areas including electric, electronics, semiconductors, and control, and, from our experience, students do not easily assimilate these interdisciplinary concepts. Moreover, they normally do not relate them, tending to study them as separate - even isolated - disciplines. With the aim of mitigating this problem, different laboratory tests of adjustablespeed drives have been proposed from these optional subjects to relate and strengthen power electronics, electric machine control and inverter performance concepts.
On the other hand, the average ratio between the ECTS credits of subjects related to electronics and the total undergraduate ECTS credits for the Industrial Engineering curriculum approved and given in different Spanish universities is around 40% - nowadays, in Spain, this curriculum includes the equivalent Electrical, Mechanical, Power and Electronics Engineering degree. However, in our case, the percentage of credits (20%) is signifi cantly lower than the average value, which justifi es our interest and necessity to minimise this defi ciency by means of the different proposed laboratory experiments.15
Resource description
In this section, a brief description of the hardware and software is provided. In this case, all the hardware system equipment is common in the industrial environment, and thus, less costly than their educational counterparts. Specifi cally, these items are the following:
* Variable speed a.c. drive.16 Unidrive UNI1403 - 1.5kW - from Control Technniques �. In our case, two a.c. drives have been used: one works in motor mode, and the other one in regenerative braking mode.
Induction electric machine.17 This induction electric machine is a three-phase motor, 4 pole, with squirrel-cage rotor from Lucas-N�lle� laboratory technology.
Electrical measurements.18 Two-channel oscilloscopes and digital voltmeters and ammeters have been used to measure and store the real voltage and current evolution.
In reference to the software packages, an evaluation version of Orcad-PSpice Computer � has been used for the simulation stage.19 In this case, the circuits have been simplifi ed according to our educational and curriculum objectives: for example, power converters are simulated as d.c.-controlled-transformers. In the same way, this freeware distribution allows students to simulate any equivalent circuit as well as share their experiences and contributions.
Laboratory tests: results
From these equipments and software packages, different laboratory experiments have been proposed. Laboratory tests of industrial electric a.c. drives have been implemented inside the Extension of Electric Machines subject, being their equivalent circuits simulated inside the Power Electronics subject. The main aim of these simulations is to show clearly how converters work, avoiding complex control systems according to our curriculum objectives. From this point of view, the proposed real and simulated laboratory experiments include: One-leg switch-mode inverter (PWM), averaged one-leg switch-mode inverter (simplifi cation), threephase inverter (PWM), averaged three-phase inverter, averaged three-phase inverter with 3rd harmonic injection, and regenerative braking process - including energy balance.
No comments:
Post a Comment