tailieunhanh - transformer engineering design and practice 1_phần 6

Sự mất mát tải của máy biến áp bao gồm các khoản lỗ do kháng ohmic của cuộn dây (tổn thất I2R) và một số thiệt hại bổ sung. Những tổn thất này bổ sung thường được gọi là tổn thất lạc, xảy ra do lĩnh vực rò rỉ của cuộn dây và lĩnh vực cao mang dòng dẫn / xe buýt, quán bar | 4 Eddy Currents and Winding Stray Losses The load loss of a transformer consists of losses due to ohmic resistance of windings PR losses and some additional losses. These additional losses are generally known as stray losses which occur due to leakage field of windings and field of high current carrying leads bus-bars. The stray losses in the windings are further classified as eddy loss and circulating current loss. The other stray losses occur in structural steel parts. There is always some amount of leakage field in all types of transformers and in large power transformers limited in size due to transport and space restrictions the stray field strength increases with growing rating much faster than in smaller transformers. The stray flux impinging on conducting parts winding conductors and structural components gives rise to eddy currents in them. The stray losses in windings can be substantially high in large transformers if conductor dimensions and transposition methods are not chosen properly. Today s designer faces challenges like higher loss capitalization and optimum performance requirements. In addition there could be constraints on dimensions and weight of the transformer which is to be designed. If the designer lowers current density to reduce the DC resistance copper loss PR loss the eddy loss in windings increases due to increase in conductor dimensions. Hence the winding conductor is usually subdivided with a proper transposition method to minimize the stray losses in windings. In order to accurately estimate and control the stray losses in windings and structural parts in-depth understanding of the fundamentals of eddy currents starting from basics of electromagnetic fields is desirable. The fundamentals are described in first few sections of this chapter. The eddy loss and circulating current loss in windings are analyzed in subsequent sections. Methods for 127 Copyright 2004 by Marcel Dekker Inc. 128 Chapter 4 evaluation and control of these two .