电气专业外文翻译 负载运行的变压器及直流电机导论 电气类.doc

The Transformer on load﹠Introduction to DC Machines The Transformer on load It has been shown that a primary input voltage can be transformed to any desired open-circuit secondary voltage by a suitable choice of turn’s ratio. is available for circulating a load current impedance. For the moment, a lagging power factor will be considered. The secondary current and the resulting ampere-turns will change the flux, tending to demagnetize the core, reduce and with it . Because the primary leakage impedance drop is so low, a small alteration to will cause an appreciable increase of primary current from to a new value of equal to . The extra primary current and ampere-turns nearly cancel the whole of the secondary ampere-turns. This being so, the mutual flux suffers only a slight modification and requires practically the same net ampere-turns as on no load. The total primary ampere-turns are increased by an amount necessary to neutralize the same amount of secondary ampere-turns. In the vector equation,; alternatively, . At full load, the current is only about 5% of the full-load current and so is nearly equal to. Because in mind that , the input kVA which is approximately is also approximately equal to the output kVA, . The physical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary, is shown unchanged because the total back e.m.f., （）is still equal and opposite to . However, there has been a redistribution of flux and the mutual component has fallen due to the increase of with . Although the change is small, the secondary demand could not be met without a mutual flux and e.m.f. alteration to permit primary current to change. The net flux linking the secondary winding has been further reduced by the establishment of secondary leakage flux due to , and this opposes . Although and are indicated separately, they combine to one resultant in the core which will be downwards at the instant