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| 2. The two windings of a transformer may be wound in two ways. If the windings of the primary and secondary are in opposite direction, the applied voltage and the induced voltage will have opposite direction, and the transformer is said to have subtractive polarity. If the windings of the two coils are in the same direction, the applied and induced voltages will have the same direction, and the transformer is said to have additive polarity (Fig. 5-2). |

| 3. In addition to polarity, transformers connected in parallel should have the same primary and secondary voltage ratings and should have nearly the same impedance. |

| 4. Transformers may be connected to supply single-phase loads, with secondary windings connected to supply a two- or three-wire output (Fig. 5-3). They may also be connected in banks to supply two-phase or three-phase outputs. Two-phase supply may have three-, four-, or five-wire output. Three-phase supply may have three-, four-, or six-wire outputs. |

| 5. Three-phase loads may be supplied by two or three transformers connected in delta (D) or wye (Y), or combinations of both, to accommodate the desired input and output voltages (Figs. 5-7 through 5-14). |

| 6. When three transformers are operated in a closed delta bank, the impedance of the three units should be very closely the same. Otherwise, the difference in voltages caused by unequal voltage drops will cause currents to circulate in the delta, causing unnecessary heat in the transformers. Care should be taken in closing the delta, for, if one of the transformers is connected so that its voltage is in improper relation to the others, a dangerous voltage of approximately twice line to line voltage may result. |

| 7. Open delta or open wye banks of two transformers are sometimes used for economy when the third transformer may not be immediately required, or in emergency, when the third unit is unavailable for any reason. |

| 8. For economy and space reasons, and for ease in installing, three-phase transformers are used, which may comprise two or three single-phase units connected internally to supply the desired three-phase output. Hence, when such units are connected in parallel, it is again necessary to establish polarity, but, in addition to relative voltage directions, their relative angular displacement from each other is also necessary (Figs. 5-15, 5-16, 5-17, and 5-18). |

| 9. Transformers may also be connected to change three-phase to two-phase supply, or vice versa, (Figs. 5-20 and 5-21). Also for boosting or bucking supply voltage for higher or lower output voltages (Fig. 5-22). |

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