Three-phase Electrical Power
Abstract
Utility companies generate three-phase electric power, which consists of three sinusoidal voltages with phase angles of 0, 120, and 240 degrees. The AC generators described in most introductory textbooks are single phase generators, so physics students are not likely to learn much about three-phase power. I have developed a simple way to display the waveforms of the three-phase power supplied to my lab and to demonstrate some interesting features of three-phase power. The waveform displays require three small transformers, a Vernier LabPro (R) data collection device, and a computer; but the demonstrations require only the transformers and some miniature light bulbs. I have also developed a way to demonstrate how six-phase AC can be derived from three-phase AC.
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Six-Phase: Additional Information
Figure A shows a plot I made of the six voltages created by the circuit in Figure B . Because one Vernier LabPro can support at most four voltage probes, I used two Vernier LabPro data collection devices (connected to one PC) and a total of six Vernier voltage probes to create Figure B, but I discovered a problem. The two LabPros are not synchronized; in other words, they do not necessarily take measurements at the same time. (I thank John Gastineau of Vernier Software & Technology for helping me understand this.) Sometimes the waveforms look good, and sometimes they don’t. (Figure B shows one of the good-looking waveforms.) With a single LabPro connected to phases A, B, C, and F as shown in Figure C , I can display up to four of the six phases, as shown in Figure D . Figure E and Figure F show similar plots of four phases, but they were made with LabPro CH4 connected to phase E or phase D instead of phase F. I plan to observe all six phases with a LabJack U3-HV (see labjack.com), but I have not yet done that.
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Links to additional resources regarding Three-phase Electrical Power: The Three Phase Power Resource Site - Three Phase Power Solutions