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| two-phase load. The same observations concerning the secondary three-, four-, five-wire arrangements made above also apply in this case. |

| Boost or Buck Connection |

| Standard transformers are sometimes used to boost the voltage of a long or overloaded line. The high-voltage winding (Fig. 5-22) is connected across the line and the low-voltage winding is in series with the line to add its voltage to the line voltage. The low-voltage winding is subjected both to the voltage of the high-voltage circuit and to voltage stresses which may occur on that circuit. Since the secondary windings must carry the load current of the high-voltage circuit, the size of the transformer required is determined by multiplying the expected maximum load current to be carried (for example, 100 amperes) by the voltage of the secondary (for example, 240 volts), and the result expressed in volt-amperes (24,000 or 24 kVA; the nearest standard size would be 25 kVA). |

| Figure 5-22 Standard single-phase transformer, additive polarity, used as a booster. |

| By reversing the connections to the secondary winding, the action can be reversed from boosting to bucking. Other considerations, such as insulation stress and transformer size, remain the same. This connection constitutes an autotransformer. |

| Review. |

| 1. Unlike a direct current, there are no fixed positive and negative poles in alternating current; hence, transformers cannot have fixed positive and negative terminals, but the relative direction in which primary and secondary windings are wound around the core determines the relative direction of the voltage across the windings. Since it is essential if two or more transformers are to be connected together, to know the relative direction of the voltage transformed, their polarity, or direction of the voltages, needs to be established (Fig. 5-1). |

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