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Polyphase Power Systems
14 questions By Tony R. Kuphaldt
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Question 4 of 14
How is polyphase (especially three-phase) electric power generated? Single-phase power is easy to understand, but how do we create three-phase AC voltage?
Reveal answerPolyphase AC voltages are usually created by special alternators, with different “phase” windings spaced around the circumference of the stator.
Notes:An illustration of polyphase alternator construction would be excellent to aid understanding of this concept. Your students should have no trouble finding illustrations of three-phase alternator design from textbooks and from the internet.
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Question 5 of 14
Suppose a set of three neon light bulbs were connected to an alternator with three sets of windings labeled A, B, and C:
The schematic diagram for this alternator/lamp system is as follows:
If the alternator spins fast enough (clockwise, as shown), the AC voltage induced in its windings will be enough to cause the neon lamps to “blink” (neon bulbs have very fast reaction times and thus cannot maintain a glow for very long without current, unlike incandescent lamps which operate on the principle of a glowing-hot metal filament). Most likely this blinking will be too fast to discern with the naked eye.
However, if we were to video record the blinking and play back the recording at a slow speed, we should be able to see the sequence of light flashes. Determine the apparent “direction” of the lamps’ blinking (from right-to-left or from left-to-right), and relate that sequence to the voltage peaks of each alternator coil pair.
Reveal answerThe lamps will blink from left to right (C-B-A-C-B-A).
Follow-up question: suppose lamp B stopped blinking, while lamps A and C continued. Identify at least two possible causes for this failure.
Notes:This question is really a prelude to discussing phase rotation in polyphase systems.
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Question 6 of 14
What is meant by the term phase rotation sequence, in a polyphase electrical system?
Reveal answer“Phase rotation sequence” refers to the successive order in which the various phase voltages reach their positive peaks.
Notes:Ask your students how the phase rotation sequence of a three-phase system is typically denoted. What symbols are used to describe the direction in which the phase voltages “rotate”?
