Lessons In Electric Circuits Volume II – AC Book

Chapter 10.3 Phase rotation

Lessons In Electric Circuits Volume II – AC Book
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Lessons In Electric Circuits Volume II – AC Book

  • 10.3.PHASE ROTATION291connection scheme is to draw the windings in the shape of a “Y” like Figure 300,10.16.+--+-+120 V120 V120 V∠ 0o∠ 120o∠ 240oFigure 10.16: Alternator ”Y” configuration.The “Y” configuration is not the only option open to us, but it is probably the easiest tounderstand at first. More to come on this subject later in the chapter.• REVIEW:• A single-phase power system is one where there is only one AC voltage source (one sourcevoltage waveform).• A split-phase power system is one where there are two voltage sources, 180o phase-shifted fromeach other, powering a two series-connected loads. The advantage of this is the ability to havelower conductor currents while maintaining low load voltages for safety reasons.• A polyphase power system uses multiple voltage sources at different phase angles from eachother (many “phases” of voltage waveforms at work). A polyphase power system can delivermore power at less voltage with smaller-gage conductors than single- or split-phase systems.• The phase-shifted voltage sources necessary for a polyphase power system are created in al-ternators with multiple sets of wire windings. These winding sets are spaced around thecircumference of the rotor’s rotation at the desired angle(s).10.3Phase rotationLet’s take the three-phase alternator design laid out earlier (Figure 301,10.17) and watch what happensas the magnet rotates.The phase angle shift of 120o is a function of the actual rotational angle shift of the three pairsof windings (Figure 301,10.18). If the magnet is rotating clockwise, winding 3 will generate its peakinstantaneous voltage exactly 120o (of alternator shaft rotation) after winding 2, which will hitsits peak 120o after winding 1. The magnet passes by each pole pair at different positions in therotational movement of the shaft. Where we decide to place the windings will dictate the amountof phase shift between the windings’ AC voltage waveforms. If we make winding 1 our “reference”voltage source for phase angle (0o), then winding 2 will have a phase angle of -120o (120o lagging,or 240o leading) and winding 3 an angle of -240o (or 120o leading).