变压器
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變壓器，是一種利用電磁原理把能量從一個電路轉移到另一個電路，而不需活動零件的電力設備。變壓器包含兩組或以上的線圈或一組tapped winding及一個磁芯以集合Magnetic flux。在一組線圈上的交流電會在磁芯上產生出time-varying magnetic flux，並從而在另一組線圈上產生感應電壓。
它的主要用途是轉變電壓，改變阻抗，及分隔電路。在電力運輸之中，變壓器擔當重要的角色。
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[编辑] 基本原理
一個簡單的單相變壓器由兩塊導電體組成。當其中一塊導電體有一些不定量的電流 (如交流電或脈沖式的直流電) 通過，便會產生變動的磁場。根據電磁的互感原理，這變動的磁場會使第二塊導電體產生電勢差。假如第二塊導電體是一條閉合電路的一部份，那麼該閉合電路便會產生電流。電力於是得以傳送。
在通用的變壓器中，有關的導電體是由 (多數為銅質的) 電線組成的線圈，因為線圈所產生的磁場要比一條筆直的電線大得多。
變壓器無法完成以下工作：
- 把直流電轉換為交流電，或把交流電轉換為直流電
- 變更直流電的電壓或電流
- 變更交流電的頻率
- 把單相電流轉為多相電流
[编辑] 定律
以下兩條電學定律與變壓器有關：
由於變壓器遵守這兩條定律，它不會是放大器。如果處在變壓器兩方的電壓有所不同，那麼流經變壓器兩方的電流也會不同，而兩者的差距則成反比。如果變壓器一方的電流比另一方小，那電流較小的一方會有較大的電壓；反之亦然。然而，變壓器兩方所消耗的功率 (即一方的電壓和電流兩值相乘) 應是相等的。
[编辑] 電壓和電流比例
假設現在有一變壓器以 25:2 的電流轉變比率運行，而雙方的功率皆為 50 瓦。根據歐姆定律，可以確定以下兩點：
至於變壓器兩方之間的電流或電壓比例，則取決於兩方電路線圈的圈數。圈數較多的一方電壓較高但電流較小，反之亦然。
如果撇除泄漏等因素，變壓器兩方的電壓比例相等於兩方的線圈圈數比例，亦即電壓與圈數成正比。以算式表示如下：
在算式中：
- Vp是輸入方的電壓(Primary Voltage)；
- Vs是輸出方的電壓(Secondary Voltage)；
- Np是輸入方的線圈圈數(Numbers of turns in the Primary Winding)；
- Ns則是輸出方的電流圈數(Numbers of turns in the Secondary Winding)。
變壓器的這個性質使它成為轉換電壓的重要設備。
另外，撇除泄漏的因素，變壓器某一方 (線圈) 的電壓可以從以下算式求得：
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- E = 4.44 * F * n * a * b
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在算式中：
- E是流經該線圈的電壓的方根均值；
- F是電流的頻率 (單位為赫茲)；
- n是線圈的圈數；
- a是線圈內空間 (鐵芯) 的切面面積；
- b是通過線圈內空間 (鐵芯) 的磁力。該值是以每單位面積計算，而面積的單位與變數 a 相同。
- 常數值 4.44 是為了使算式結果對應於計算出來的單位而設。
[编辑] 日常生活中的變壓器
變壓器在日常生活中以很多形態存在。原理相同、構造相近的變壓器可以嵌在小型麥克風之中以連接不同的電路，也可以處理數以百萬千瓦計的電功率並聯繫多個為整個國家供電的大型輸電網。
[编辑] 能量流失
一個理想的變壓器沒有能量流失，所以擁有 100% 效率。在現實之中，大功率的變壓器的效率可達 98%；但小型的變壓器流失會較嚴重，而它們的效率可能低於 85%。
變壓器的能量流失可以來自這些現象： (在以下敘述中，線圈內的導電體一律稱為「鐵芯」)
- 線圈的電阻：電流通過導電體時產生熱能，造成能量損失。和其他種類的流失不同，這種流失並不是來自變壓器的鐵芯。
- 渦流：磁力使鐵芯產生環迴電流，導致能量化成熱並流失至外界。把鐵芯切成不相通的薄片可以減少這種流失。
- 磁力流失：所有未被輸出方線圈接收的磁力線均會造成能量流失。
- 滯後：鐵芯的滯後作用使每次磁場改變時造成能量流失。這種流失的大小取決於鐵芯的原料。
- 力流失：交替的磁場使導線、鐵芯與附近的金屬之間的電磁力產生變化，結果形成振動和能量流失。
- 磁致伸縮：交替的磁場使鐵芯出現伸縮。如果鐵芯的原料容易受伸縮影響，分子之間的摩擦會導致能量流失。
- 冷卻設備：大型的變壓器一般配備冷卻用的電風扇、油泵或注水的散熱器。這些設備所使用的能量一般亦算作變壓器的能量流失。
變壓器運作時的噪音一般來自磁力流失或磁致伸縮所造成的振動。
[编辑] 设计
[编辑] 發明
以下人仕與變壓器的發明有關係：
- 法拉第在1831年8月29日發明了一個「電感環」。這是第一個變壓器，但法拉第只是用它來示範電磁感應原理，並沒有考慮過它可以有現實的用途。
- Lucien Gaulard and John Dixon Gibbs, who first exhibited a device called a 'secondary generator' in London in 1881 and then sold the idea to American company Westinghouse. This may have been the first practical power transformer, but was not the first transformer of any kind. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Their early devices used a linear iron core, which was later abandoned in favour of a more efficient circular core.
- William Stanley, an engineer for Westinghouse, who built the first practical device in 1885 after George Westinghouse bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886.
- Hungarian engineers Ottó Bláthy, Miksa Déri and Károly Zipernowsky at the Ganz company in Budapest in 1885, who created the efficient "ZBD" model based on the design by Gaulard and Gibbs.
- Nikola Tesla, who is often incorrectly credited with its invention, although his true achievement was to develop and patent (in 1888) a complete polyphase AC system, including a polyphase transformer, for power distribution. He sold his patents to Westinghouse in the same year. In 1891 he invented the Tesla transfomer or Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.
[编辑] 电路符号
标准电路符号
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有铁芯两绕组变压器。 |
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三绕组变压器。 图中的点标示绕组极性相同的端子（同名端）。 |
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降压或升压变压器。 图中所示绕组匝数比例并非实际比例。 |
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带静电屏蔽的变压器， 可以防止绕组之间静电耦合。 |
[编辑] 参考文献
[编辑] 构造
一個變壓器必須包括：
- 兩組或以上的線圈，以傳輸電流；
- 一个铁芯, 它把互感的磁场与线圈耦合在一起.
变压器一般运行在低频，导线围绕铁芯缠绕成绕组。虽然铁芯会造成一部分能量的损失，但这有助于将磁场限定在变压器内部，并提高效率。
电力变压器按照铁芯和绕组的结构分为芯式结构和壳式结构，以及按照磁通的分支数目（三相变压器有3，4或5个分支）分类。它们的性能各不相同。
[编辑] 铁芯
变压器通常采用硅钢材料的铁芯作为主磁路。这样可以使线圈中磁场更加集中，变压器更加紧凑。 电力变压器的铁芯在设计的时候必须保防止达到磁路饱和，有时需要在磁路中设计一些气隙减少饱和。 实际使用的变压器铁芯采用非常薄，电阻较大的硅钢片叠压而成。 这样可以减少每层涡流带来的损耗和产生的热量。
电力变压器和音频电路有相似之处。典型分层铁芯一般为E和I字母的形状，称作“EI变压器”。 这种铁芯的一个问题就是当断电之后铁芯中会保持剩磁。 当再次加电后，剩磁会造成铁芯暂时饱和。 对于一些容量超过数百瓦的变压器会造成的严重后果，如果没有采用限流电路，涌流可造成主熔断器熔断。 更严重的是，对于大型电力变压器，涌流可造成主绕组变形、损害。
[编辑] 实芯铁芯
在如开关电源之类的高频电路中，有时使用具有较高的磁导率和电阻率的铁磁材料粉末铁芯。 在更高的频率下，需要使用绝缘体导磁材料，常见的有各种称作铁素体的陶瓷材料。 在一些调频无线电电路中的一些变压器铁芯采用可调铁芯，来配合耦合电路达到谐振。
[编辑] 空气芯
High-frequency transformers in low-power circuits may have air cores. These eliminate the loss due to hysteresis in the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.
[编辑] 圆环柱心
Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel wound into a coil. This construction ensures that all the grain boundaries are pointing in the optimum direction, making the transformer more efficient by reducing the core's reluctance, and eliminates the air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to prevent the core's magnetic field from generating electromagnetic interference.
Toroidal cores for use at frequencies up to a few tens of kilohertz may also be made of ferrite material to reduce losses. Such transformers are used in switch-mode power supplies.
Toroidal transformers are more efficient (around 95%) than the cheaper laminated EI types. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantage is higher cost.
When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork.
[编辑] 线圈
The winding material depends on the application. Small power and signal transformers are wound with insulated solid copper wire. Larger power transformers may be wound with wire, copper or aluminum rectangular conductors, or strip conductors for very heavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire, to minimize the skin effect losses in the conductors. Very large power tranformers will also have multiple strands in the winding, for the same reason (see skin effect).
Windings on both primary and secondary of a power transformer may have taps to allow adjustment of the voltage ratio; taps may be connected to automatic on-load tapchanger switchgear for voltage regulation of distribution circuits.
[编辑] 绝缘保护
The conductor material must have insulation to ensure the current travels around the core, and not through a turn-to-turn short-circuit.
In power transformers, the voltage difference between parts of the primary and secondary windings can be quite large. Layers of insulation are inserted between layers of windings to prevent arcing.
[编辑] 屏蔽物
Although an ideal transformer is purely magnetic in operation, the close proximity of the primary and secondary windings can create a mutual capacitance between the windings. Where transformers are intended for high electrical isolation between primary and secondary circuits, an electrostatic shield can be placed between windings to minimize this effect.
Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer or to prevent the transformer from affecting the operation of other devices (such as CRTs in close proximity to the transformer). Transformers may also be enclosed for reasons of safety, both to prevent contact with the transfomer during normal operation and to contain possible fires that occur as a result of abnormal operation. The enclosure may also be part of the transformer's cooling system.
[编辑] 冷却剂
Small transformers up to a few thousand watts in size usually are adequately cooled by air circulation. Larger "dry" type transformers may have cooling fans.
High-power or high-voltage transformers are bathed in highly-refined mineral oil that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl, "PCB" was used as it was not a fire hazard in indoor power transformers. Due to the stability of PCB and its environmental accumulation, it is no longer permitted in new equipment. Today, nontoxic, stable silicone-based or fluorinated hydrocarbons may be used, where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil.
The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. To improve cooling of large power transformers, the oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Large and high-voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load.
Experimental power transformers in the 2000 kVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.
[编辑] 接线端
Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must both provide electrical insulation, and contain oil within the transformer tank.
[编辑] 自耦变压器
An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed DC power is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. Autotransformers are used to compensate for voltage drop in a distribution system or for matching two transmission voltages, for example 115,000 V and 138,000 V. For voltage ratios, not exceeding about 3:1, an autotransformer is less costly,lighter, smaller and more efficient than a two-winding transformer of a similar rating.
Variac is a trademark of General Radio (mid-20th century) for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. The term is often used to describe similar variable autotransformers made by other makers. A variable autotransformer is an efficient and quiet method for adjusting the voltage to incandescent lamps. While lightweight and compact semiconductor light dimmers have replaced variacs in many applications such as theatrical lighting, variable autotransformers are still used when an undistorted variable voltage sine wave is required.
[编辑] 多相变压器
For three phase power, three separate transformers can be used, or all three phases can be connected to a single polyphase transformer.
[编辑] 谐振变压器
A resonant transformer is one that operates at the resonant frequency of one or more of its coils. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. If the primary coil is driven by a periodic source of alternating current, such as a square or sawtooth wave, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrostatic breakdown. These devices are therefore used to generate high alternating voltages. The current available from this type of coil can be much larger than that from electrostatic machines such as the Van de Graaff generator and Wimshurst machine.
Examples:-
- Tesla coil
- Oudin coil (or Oudin resonator; named after Paul Marie Oudin, 1851-1923)
- D'Arsonval apparatus
- ignition coil or induction coil used in the ignition system of a petrol engine
Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where a large measure of the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.
A voltage regulating transformer uses a resonant winding and allows part of the core to go into saturation on each cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an ac power supply. However, due to the hysteresis losses accompanying this type of operation, efficiency is low.
[编辑] 目前的变压器
A 电流互感器 is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.
Current transformers are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly.
Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary as in this circumstance a very high voltage would be produced across the secondary.
Current transformers are often constructed with a single primary turn either as an insulated cable passing through a toroidal core, or else as a bar to which circuit conductors are connected.
[编辑] Uses of transformers
- Electric power transmission over long distances. The simplicity, reliability, and economy of conversion of voltages by static transformers was the principle factor in the selection of alternating current power transmission ( see War of Currents).
- High-voltage direct-current HVDC power transmission systems .
- Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking.
- Some transformers are designed so that one winding turns or slides, while the other remains stationary. These can pass power or radio signals from a stationary mounting to a turning mechanism, such as a machine tool head or radar antenna.
- Some moving coil transformers are precisely constructed in order to measure distances or angles. Most often, they have several primaries, and electronic circuits measure the shape of the wave in the different secondaries. See linear variable differential transformer, synchro and resolver.
- Small transformers are often used to isolate and link different parts of radio receivers and audio amplifiers. See electronics and impedance match.
- Balanced-to-unbalanced conversion. A special type of transformer called a balun is used in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads. A balanced line is one in which the two conductors (signal and return) have the same impedance to ground: twisted pair and "balanced twin" are examples. Unbalanced lines include coaxial cables and strip-line traces on printed circuit boards.
[编辑] See also
- Main : Distributed generation, Electronic power supply, Electronics, Inductor, Pickup, Electrical network, Electricity distribution, List of electronics topics
- Circuits: Ballast, Clamp meter, Repeating coil, Inverter (electrical), Ignition system, Electricity generation, Linear variable differential transformer, Neon signage, Regulator, Substation, Switched-mode power supply, Technological applications of superconductivity, Tesla coil, Transducer
- Electromagnetism: Alternating current, Electric power, Electric power transmission, Electromagnetic induction, Equivalent series resistance, High-voltage direct current, Impedance matching, Inductive coupling, Potential difference, Skin effect, Leakage inductance, Superconductivity
- People: Ottó Bláthy, Miksa Déri, John Ambrose Fleming, Otto A. Knopp, William Stanley, Nikola Tesla, Milan Vidmar, George Westinghouse, Károly Zipernowsky
- Other: DI unit, Polychlorinated biphenyl, Stafford, Timeline of invention, War of Currents, World Columbian Exposition, X10
- See how to make a transformer for instructions on how to make a very simple transformer suitable for demonstration of the principles in a school classroom setting.
