Three-phase Y and ¢ conflgurations

Chapter 10.5 Three-phase Y and ¢ conflgurations

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

  • 10.5.THREE-PHASE Y AND ∆ CONFIGURATIONS301• By introducing a phase shift of less than 180o to the magnetic fields in such a motor, a definitedirection of shaft rotation can be established.• Single-phase induction motors often use an auxiliary winding connected in series with a ca-pacitor to create the necessary phase shift.• Polyphase motors don’t need such measures; their direction of rotation is fixed by the phasesequence of the voltage they’re powered by.• Swapping any two “hot” wires on a polyphase AC motor will reverse its phase sequence, thusreversing its shaft rotation.10.5Three-phase Y and ∆configurationsInitially we explored the idea of three-phase power systems by connecting three voltage sourcestogether in what is commonly known as the “Y” (or “star”) configuration. This configuration ofvoltage sources is characterized by a common connection point joining one side of each source.(Figure 310,10.31)+-+--+120 V120 V120 V∠ 0o∠ 120o∠ 240oFigure 10.31: Three-phase “Y” connection has three voltage sources connected to a common point.If we draw a circuit showing each voltage source to be a coil of wire (alternator or transformerwinding) and do some slight rearranging, the “Y” configuration becomes more obvious in Fig-ure 311,10.32.The three conductors leading away from the voltage sources (windings) toward a load are typicallycalled lines, while the windings themselves are typically called phases. In a Y-connected system,there may or may not (Figure 311,10.33) be a neutral wire attached at the junction point in the middle,although it certainly helps alleviate potential problems should one element of a three-phase load failopen, as discussed earlier.When we measure voltage and current in three-phase systems, we need to be specific as towhere we’re measuring. Line voltage refers to the amount of voltage measured between any two lineconductors in a balanced three-phase system. With the above circuit, the line voltage is roughly208 volts. Phase voltage refers to the voltage measured across any one component (source windingor load impedance) in a balanced three-phase source or load. For the circuit shown above, thephase voltage is 120 volts. The terms line current and phase current follow the same logic: the