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Page 74

| Figure 5-4 Three-phase connections and voltage relationships. (A) Delta connection. (B) Wye connection. (C) Voltage vectors. |

| generators to supply a three-phase system are shown in Fig. 5-4, A and B. One method (Fig. 5-4A) employs the delta (from the Greek letter D) connection; the other (Fig. 5-4B) the star or wye (Y) connection. The voltage relationships for these connections are represented by straight arrow lines as illustrated in Fig. 5-4C. |

| It will be noted that while the voltages between the terminals A, B, and C are the same in both the delta and wye systems, the voltages across the windings 1,2, and 3 in the two systems are not only different in magnitude, but their directions do not coincide. This is important in connecting transformers and may be the cause of difficulty when such transformers may be connected together. |

| For example, two transformers may be connected to give the same secondary voltage, but these two secondary systems may have a voltage difference, or displacement, between them of 30° (Fig. 5-5). Hence, these two secondary systems cannot be connected together with safety. |

| These angular relationships between voltages of delta and wye systems are of paramount importance and must always be considered when making three-phase (or polyphase) transformer connections. |

| Figure 5-5 Delta and Wye systems-voltage angular displacements. (A) D system. (B) Y system. (C) Comparison of systems. |

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