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2025-01-27 08:12本頁(yè)面
【正文】 verage value of the induced voltage given by Eavg = turns c h a n g e in flu x in a g iv e n tim eg iv e n tim e which is Faraday’s law applied to a finite time interval. It follows that Eavg = N 21/(2 )mf? = 4fNφm which N is the number of turns on the winding. Form ac circuit theory, the effective or rootmeansquare (rms) voltage for a sine wave is times the average voltage。, therefore, they are 180186。 in phase behind the primary voltage VP. It is this ponent that sets up the flux in the core。 and (2) it provides a ponent to account for the hysteresis and eddy current losses in the core. There bined losses are 鄭州大學(xué)電氣工程學(xué)院畢業(yè)論文 6 normally referred to as the core losses. The noload current Iθ is usually few percent of the rated fullload current of the transformer (about 2 to 5%). Since at noload the primary winding acts as a large reactance due to the iron core, the noload current will lag the primary voltage by nearly 90186。 譯自 科技英語(yǔ) 5 原文 TRANSFORMER 1. INTRODUCTION The highvoltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transformer makes it possible for various parts of a power system to operate at different voltage levels. In this paper we discuss power transformer principles and applications. 2. TOWWINDING TRANSFORMERS A transformer in its simplest form consists of two stationary coils coupled by a mutual magic flux. The coils are said to be mutually coupled because they link a mon flux. In power applications, laminated steel core transformers (to which this paper is restricted) are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers. The current flowing in the coil connected to the ac source is called the primary winding or simply the primary. It sets up the flux φ in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing。這證明了它對(duì)我們分析變壓器時(shí)所產(chǎn)生的影響微乎其微。由于分支勵(lì)磁體現(xiàn)在電流里，為了分析我們可以將它忽略。另外，兩側(cè)繞組同樣具有阻抗，這也將產(chǎn)生一個(gè)電阻壓降。一次側(cè)漏磁也一樣。 另外，流過(guò)一次側(cè)繞組的負(fù)載電流只在一次側(cè)繞組中產(chǎn)生磁通，這個(gè)磁通被稱(chēng)為一次側(cè)的漏磁。 當(dāng)一個(gè)電流流過(guò)二次側(cè)繞組，它的磁動(dòng)勢(shì)（ NsIs）將產(chǎn)生一個(gè)磁通，于空載電流 I0產(chǎn)生的磁通 φ0不同，它只停留在二次側(cè)繞組中。因?yàn)樵谶@種狀況下鐵芯的 磁通是恒定的。故一次側(cè)電流 Ip 是電流 Ip’與 I0’的和。它的磁動(dòng)勢(shì) NpIp’只停留在一次側(cè)。其需要的條件是 E 降落很多來(lái)使電流 Ip 增加。 總的來(lái)說(shuō)，變壓器為了保持磁通是常數(shù)，對(duì)磁通變化的響應(yīng)是瞬時(shí)的。感應(yīng)電壓的減小將使外施電壓和感應(yīng)電動(dòng)勢(shì)之間的差值更大，它將使初級(jí)線圈中流過(guò)更大的電流。因此， NsIs 所產(chǎn)生的磁動(dòng)勢(shì)會(huì)使主磁通 φ0減小。這個(gè)磁通的方向在任何一個(gè)時(shí)刻都和主磁通反向。當(dāng)變壓器帶負(fù)荷運(yùn)行時(shí)，將有電流 Is 流過(guò)二次側(cè)，因?yàn)?Es 產(chǎn)生的感應(yīng)電動(dòng)勢(shì)相當(dāng)于一個(gè)電壓源。換句話說(shuō)，極性的標(biāo)注可以表明當(dāng)電流流過(guò)兩側(cè)的線圈時(shí)，線圈中的磁動(dòng)勢(shì)會(huì)增加。如果點(diǎn)號(hào)同在線圈的上端，就意味著它們的極性相同。這個(gè)公式應(yīng)用于變壓器的阻抗匹配。根據(jù) Vs 和 Is，可得 Vp 和 Ip 的比例是 ppVI = /ssaVIa = 2ssaVI 但是 Vs / Is 負(fù)荷阻抗 ZL，因此我們可以這樣表示 Zm (primary) = a2ZL 這個(gè)等式表明二次側(cè)連接的阻抗折算到電源側(cè)，其值為原來(lái)的 a2 倍。 從電源側(cè)來(lái)看變壓器，其阻抗可認(rèn)為等于 Vp / I
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