page_76

< previous page | page_76 | next page >

Page 76

| Figure 5-7 Three, single-phase transformers, additive polarity, connected D-D. |

| approximately zero between the two transformers closing the delta, a voltage of double that on one phase may be experienced, and closing the delta under these conditions would be equivalent to a short circuit. This condition is indicated in Fig. 5-8. Before the two transformers are connected to close the delta, therefore, a reading between them should be taken by a voltmeter (or appropriate device if on the high voltage side) or a fuse of relatively low capacity (but great enough to withstand exciting current) installed temporarily. |

| When three transformers are operated in a close 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 creating unnecessary heating of the transformers. The same condition may arise if the voltage ratio of the three transformers is not the same. |

| The delta-delta connection is generally used to supply three-phase power loads. Comparatively small single-phase lighting requirements at approximately half the power supply voltage may be supplied from one of the transformers in the delta by establishing a neutral or ground connection at the midpoint of the secondary of one transformer, as shown in Fig. 5-7. |

| Figure 5-8 Correct (A) and incorrect (B) methods of making a delta connection. |

< previous page | page_76 | next page >