Synthesis Of Low Voltage Three Phase Power For Use In Low Cost Motor And Systems Experiments At The Sophomore Level

The electrical engineering program at the University of San Diego is currently revising its curricular treatment of electrical power and electrical machines at several levels and for students in all engineering majors. During the development of a three-phase system and synchronous motor laboratory experience for sophomore-level engineering students, budgetary and safety concerns led to the decision to work with three-phase systems at voltage levels less than 25V and power levels less than 5W. A three-phase 5V generator used in a commercially available lowcost “mini dynamo massager” served as the motor for this project. The development of a three-phase low-voltage source became the primary challenge presented by the decision to work at low voltages. Since the study of three-phase systems was a significant portion of the exercise, PWM sources, appropriate if only motor characteristics were of interest, were judged to be inappropriate. The laboratory experience required a low-voltage replica of commercial three-phase power: sinusoids with 120° phase separation. The faculty design team was unable to find such a source commercially at low cost and designed, built, and tested several different sources. In addition to a transformer solution, two electronic circuits were implemented in the initial trials of the laboratory exercises: (a) digital synthesis using synchronized counters, D/A conversion, and wave shaping, and (b) digital synthesis using EPROMS and D/A conversion. Each of these three-phase synthesizers was developed with an estimated parts cost of less than US$10 (assuming appropriate DC power availability). The design and implementation of the sources is described as well as the design and assessment of the three-phase system and synchronous motor laboratory experience in which they were used. Student learning was assessed through questionnaires at the beginning and end of the laboratory period. The questionnaires addressed both student knowledge and student confidence levels. The assessment showed a significant overall increase of both student knowledge and student confidence in the application of that knowledge. On a five-point scale, overall student-reported knowledge increased slightly more than one point and overall confidence increased by 1.33 points. Faculty assessment of knowledge, as measured by scoring short answers to knowledge questions, showed good correlation to the student-reported scores with the students reporting somewhat higher knowledge change than the faculty perceived. The low-voltage electronic three-phase synthesizers allowed for a safe and meaningful threephase system and synchronous motor laboratory experience for students who have minimal knowledge of the subject at low cost. While these three-phase sources were initially developed on a protoboard-based trainer, the team is continuing development of them as stand-alone devices. As such, these three-phase sources could provide a useful resource for many programs.