Physics
Operation of a Transformer
The operation of a transformer involves the transfer of electrical energy between two or more circuits through electromagnetic induction. It consists of two coils, the primary and secondary, which are magnetically coupled but electrically isolated. When an alternating current flows through the primary coil, it creates a changing magnetic field, inducing a voltage in the secondary coil, thus enabling energy transfer.
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12 Key excerpts on "Operation of a Transformer"
eBook - PDF- Michael Brumbach(Author)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
194 C H A P T E R 12 Transformers O B J E C T I V E S After studying this chapter, the student will be able to: ■ Describe the construction and operating characteristics of transformers. ■ Explain the theory of operation of various types of transformers. ■ Describe the various methods used to prevent transformers from overheating. ■ Illustrate various types of transformer connections and discuss the results of these connections. ■ Describe the construction, use, and operating characteristics of special transformers. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. TRANSFORMERS 195 TRANSFORMER PRINCIPLE The Operation of a Transformer depends upon elec-tromagnetic induction. Most transformers consist of two or more coils. The coil receiving the energy is called the transformer primary , and the coil deliver-ing the power to the load is called the transformer secondary . When an alternating voltage is applied to the primary, an alternating current flows. This current sets up an alternating magnetic field, which moves across the secondary coil (Figure 12–1). The moving magnetic lines of force induce a voltage into this coil. In the transformer shown in Figure 12–1, there is no load connected to the secondary. Thus, no cur-rent is flowing in the secondary coil. Because the primary circuit has high inductance and low resis-tance, the current lags the voltage by nearly 90 de-grees. The magnetic field produced by this current establishes a counter electromotive force (cemf) that is nearly 180 degrees out of phase with the applied voltage.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
____________________ WORLD TECHNOLOGIES ____________________ Chapter-11 Transformer Pole-mounted split-phase transformer with center-tapped secondary winding (note use of grounded conductor, right, as one leg of the primary feeder) ____________________ WORLD TECHNOLOGIES ____________________ A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or voltage in the secondary winding. This effect is called mutual induction. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (V s ) is in proportion to the primary voltage (V p ), and is given by the ratio of the number of turns in the secondary (N s ) to the number of turns in the primary (N p ) as follows: By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be stepped up by making N s greater than N p , or stepped down by making N s less than N p . In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception. Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
____________________ WORLD TECHNOLOGIES ____________________ Chapter-1 Transformer Pole-mounted power distribution transformer with center-tapped secondary winding (note use of grounded conductor, right, as one leg of the primary feeder). It transforms the high voltage of the overhead distribution wires to the lower voltage used in house wiring. A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the ____________________ WORLD TECHNOLOGIES ____________________ transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or voltage in the secondary winding. This effect is called mutual induction. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding ( V s ) is in proportion to the primary voltage ( V p ), and is given by the ratio of the number of turns in the secondary ( N s ) to the number of turns in the primary ( N p ) as follows: By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be stepped up by making N s greater than N p , or stepped down by making N s less than N p . In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception. Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide.
eBook - PDF- S.A. Knight(Author)
- 2014(Publication Date)
- Newnes(Publisher)
Transformer principles Aims: At the end of this Unit Section you should be able to: State the essential constructional forms of radio-frequency and low-frequency transformers Explain and use the relationships between input and output currents and voltages and the transformation ratio of the transformer Draw and understand the phasor diagram of an ideal transformer under no-load conditions Derive the input resistance of a loaded transformer Understand the maximum power transfer theorem for resistive loads and sources Explain iron and resistance losses in a transformer and how these can be minimized by the choice of materials used in the construction. A transformer is essentially a device in which the magnetic flux produced by a current flowing in one coil is arranged to link with the turns of an adjacent coil. This is illustrated in Figure 5.1 . An alternating voltage Vi produces a current I x in the coil having turns Λ^ . This sets up an alternating flux Φ which links with the adjacent coil having turns N 2 and induces an e.m.f. E 2 in this coil. Since the e.m.f. in the second coil is due entirely to the common flux produced by the current in the first coil, the circuits are coupled by mutual induction. This is the fun-damental principle of all transformer action. Transformers take a variety of forms. There are those designed to operate at relatively high-frequencies (radio-frequencies) covering the range from about 100 kHz to possibly as high as 100 MHz. Others are designed to cover the lower range of audio-frequencies from, perhaps, some 100 Hz or so to about 20 kHz. a third group are used exclusively at the power frequency range of 45 to 55 Hz. The physical appearance of each group of transformers varies considerably, particularly between those used at radio-frequencies and those used at power frequency. We will consider briefly the general features of construction of these trans-former forms.
eBook - PDF- D.F. Warne(Author)
- 2005(Publication Date)
- Newnes(Publisher)
Chapter 6 Transformers Professor D.J. Allan Merlindesign 6.1 Principles of operation In simple form, a transformer consists of two windings connected by a magnetic core. One winding is connected to a power supply and the other to a load. The circuit con- taining the load may operate at a voltage which differs widely from the supply voltage, and the supply voltage is modified through the transformer to match the load voltage. In a practical transformer there may be more than two windings as well as the magnetic core, and there is the need for an insulation system and leads and bushings to allow connection to different circuits. Larger units are housed within a tank for protection and to contain oil for insulation and cooling. With no load current flowing, the transformer can be represented by two windings on a common core, as shown in Fig. 6.1. It has been explained in Chapter 2 (eqns 2.25 and 2.26) that the input and output voltages and currents in a transformer are related by the number of turns in these two windings, which are usually called the primary and secondary windings. These equations are repeated here for convenience. (6.1) (6.2) The magnetic flux density in the core is determined by the voltage per turn: (6.3) Equation 6.3 represents a key relationship between the frequency, the number of turns in a winding and the size of the core. If the winding turns are increased, the core cross- sectional area may be reduced. If the frequency is increased (from 50 to 60 Hz), the size of the core can be reduced. In the no-load case, a small current I 0 flows to supply the magnetomotive force which drives the magnetic flux around the transformer core; this current lags the primary voltage by almost 90°. This I 0 is limited in magnitude by the effective resistance (R c ) and reactance (X c ) of the magnetizing circuit, as shown in Fig. 6.2. The magnet- izing current is typically 2–5 per cent of the full load current and it has a power factor in the range 0.1–0.2.
- Christopher Lavers, Edmund G.R. Kraal(Authors)
- 2014(Publication Date)
- Thomas Reed(Publisher)
Stanley first demonstrated a full A.C. power distribution system using both step-up and step-down transformers in 1886 in Great Barrington, MA, followed by development of the first three-phase transformer in Germany by Russian-born engineer Mikhail Dolivo-Dobrovolsky in 1889, two essential factors of electrical distribution systems used to this day, in land, sea and air applications. Principle of Operation We begin our discussion with the ‘traditional’ view of the transformer, introducing the ‘phasor’ approach before finishing our discussion with modern versions of the device once the basic principle of operation is understood. Since the transformer functions using the principle of mutual induction, it is beneficial to revise briefly some of the earlier theoretical aspects. When an A.C. current passes through a solenoid, an A.C. flux is produced which is in phase with the current, i.e. if the former varies sinusoidally then the flux will also vary sinusoidally. This relationship applies specifically if the solenoid is air-cored; however, in order to use the magnetising ampere-turns to maximum advantage, an iron core is needed. Provided the iron is not worked at too high a flux density, i.e. does not saturate, working conditions are similar to those for air. In basic theory, a condition of flux-linkages exists, and as the current varies 2 • Advanced Electrotechnology in a sinusoidal manner the flux-linkages will vary similarly and an e.m.f. will be induced through self-induction. The e.m.f. magnitude and direction will be in accordance with the laws of electromagnetic induction, which will be used later to develop a formula applicable to transformer operation. Of course the effect of mutual inductance is to cause the primary coil to take more power from the electrical supply in response to an increased load on the secondary. Consider next a second coil, isolated from the first but wound on the same iron core.
eBook - PDFElectronic and Electrical Engineering
Principles and Practice
- Lionel Warnes(Author)
- 2017(Publication Date)
- Red Globe Press(Publisher)
298 Figure 16.1 The parts of a transformer (16.1) 16 Transformers P ROPERLY-DESIGNED transformers are highly efficient (up 99.5% for some multi-MVA transformers) devices that are chiefly used to step down (or step up) alternating voltages and also for matching loads. They range in size from multi-tonne, multi-MVA transformers in electricity distribution systems to 1VA, PCB-mounted transformers for portable instruments. Besides their high efficiencies, they are also notable for their reliability and freedom from the need for maintenance virtues stemming from their lack of moving parts. AC became standard partly because transformers have these attractive properties, relegating DC to special purposes. 16.1 The ideal transformer A transformer comprises two coils wrapped around a magnetic (more correctly ferromagnetic) core which channels nearly all the magnetic flux through them. Figure 16.1 shows a transformer, one coil of which the primary is connected to the supply, while the other coil the secondary is connected to the load. The coils are wound onto the limbs of the core, which are connected by a yoke . Transformers are reversible because primary can become secondary and vice versa: the primary is the coil connected to the primary supply, that is all. Current passing through the primary coil produces magnetic flux in the core; if all this flux passes through the secondary coil then the e.m.f. induced in it can be found from Faraday’s law of electromagnetic induction: The voltages are proportional to the numbers of turns in the coils: the coil with the most turns Chapter 16 299 (16.3) (16.2) (16.4) (16.5) Figure 16.2 The dot convention has the higher voltage . An ideal transformer is 100% efficient and the power put into the primary coil is equal to the power put out by the secondary: P 1 = P 2 . That is Substituting for V 1 / V 2 from Equation 16.1 gives where n is the turns ratio of the transformer.
eBook - PDFPractical Transformer Handbook
for Electronics, Radio and Communications Engineers
- Irving Gottlieb(Author)
- 1998(Publication Date)
- Newnes(Publisher)
3 Operational features of transformers It would be nice if the Operation of a Transformer was just a matter of applying voltage to the primary and meeting the load's requirements from the secondary. One tends to think of the transformer as a passive device with little capability of any gross malperformance. However, experience teaches that the passivity can only too easily translate into active malperformance. Somewhat surprisingly, this pertains to a *good' transformer, not necessarily one with shorted or open windings (although these defects often enough develop from the malper-formance). In the first place, our perfectly 'good' transformer is, in practice, always a considerable departure from the role model ~ the ideal transformer. The practical transformer will be endowed with such blemishes as winding resistance, core losses, leakage inductance, exciting current, non-linearity, and an ineffective thermal situation. The core can have residual magnetism and some core materials will suffer irreversible damage from an excessive temperature rise. As a consequence of these real-life defects, transformers can display undesirable characteristics in a circuit or system. These include audible noise, heat development, transient generation, harmonic production, oscillation and ringing, frequency attenuation, self-resonance, waveform displacements from d.c. components and current in-rush. Also, it would not be far-fetched to add expense, bulk and weight. Lest excessive cynicism sets in, it should be mentioned that many desirable modes of operation can also be realized by paying heed to the transformer's practical behaviour. In this manner, shortcomings can often be exploited profitably as useful responses. As will be seen, much depends on how the transformer is operated. 50 Practical Transformer Handbook Operation of transformers at other than their intended frequencies Hobbyists and experimenters often consider 50 and 60 Hz transformers to be interchangeable.
Available until 25 Jan |Learn more- Turan Gonen(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
The secondaries of both voltage and CTs are normally grounded. A transformer consists of a primary winding and a secondary winding linked by a mutual mag-netic field. Transformers may have an air core, an iron core, or a variable core, depending on their operating frequency and application. Transformers are also quite different in size and shape depending on the application. In power system applications, the single- or three-phase transformers with ratings up to 500 kVA are defined as distribution transformers , whereas those transformers with ratings over 500 kVA at voltage levels of 69 kV and above are defined as power transformers .* Figure 4.1 shows a cutaway view of a single-phase, overhead pole-mounted distribution transformer. Notice that it has two high-voltage bushings since it is built to operate under line-to-line voltage rather than line-to-neutral voltage. Figure 4.2 shows a three-phase, 345/161 kV autotransformer used as a power transformer. Its power ratings are 214/285/357 MVA for its OA/FA/FOA operations. Note that the OA/FA/FOA means oil-immersed, self-cooled/forced-air-cooled/forced-oil-cooled . † A transformer is basically made up of two or more windings coupled by a mutual magnetic field. Ferromagnetic cores are employed to develop tight magnetic coupling and high flux densities. When such a coupling exists, the transformer is called an iron-core transformer . Most distribution and power transformers are immersed in a tank of oil for better insulation ‡ and cooling purposes. The leads of the windings are brought to the outside of the tank through insulating bushings which are attached to the tank, as shown in Figure 4.1. Such transformers are used in high-power applications. § When there is no ferromagnetic material but only air present, such a transformer is called an air-core transformer . ¶ These transformers have poor magnetic coupling and are usually used in lower-power applications such as in electronic circuits.
- S. B. Lal Seksena, Kaustuv Dasgupta(Authors)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
The electric energy is measured by the product of voltage and current. Thus, in a transformer V × I of input side should be equal to the V × I of the output side. This implies the current and voltage of output and input sides varies inversely proportional. When a transformer lowers down or steps down the voltage it also steps up the current and vice-versa. We generally call a transformer to be step up transformer if the voltage level is elevated from input side to output side. A step up transformation always steps down the current to maintain the constant power. Likewise a step down transformer always decreases the voltage and increases the current from input to output side. Fig. 9.1 will help us to understand the interrelation between an electric generator and transformer. 9 TRANSFORMER Transformer 433 Let us suppose there are two electromagnets A and B. B is placed inside a slit of A. A has winding terminals a 1 a 2 and that of B is b 1 b 2 . Now let us suppose our intention is to get some output voltage at b 1 b 2 terminals. According to Faraday’s law we must establish a rate of change in flux in B to get the voltage output at b 1 b 2 . A and B are magnetically coupled due to mutual induction effect. If we can set up a d ϕ – dt (rate of change of magnetic flux w.r.t. time) in coil A, the same rate of change of flux will be induced in B. There are two ways by which we can vary the flux associated with coil B with respect to time. a. By varying the position of the coil If we rotate B inside A the position of B with respect to A will vary. If we excite the coil of A by a constant DC source there will be no variation of ϕ in coil A. But due to the rotation there will be a continuous change in position of B with respect to A. Thus, the flux cut by B will be varied due to change in positional angle between the axis of B and constant magnetic lines of force. This is the principle of operation of an electric generator.
eBook - PDF- Jan De Kock, Cobus Strauss(Authors)
- 2004(Publication Date)
- Newnes(Publisher)
5 Transformer theory 5.1 Transformer theory Transformer is an essential device in electrical AC power distribution system, which is used to transform AC voltage magnitudes of any value obtained from a source to any desired value for the purpose of distribution and/or consumption. The development of power transformer dates back to 19th century. The main feature of a transformer is its constant VA rating whether referred to its primary side or the secondary side. With VA being constant ( V refers to the voltage magnitude and A refers to the current magnitude in a transformer winding), it is possible to get a higher V with lower A or a lower V with a higher A , by choosing suitable ratio for the transformer. The major benefit of such a device is its ability to take in the high current produced at relatively low voltage from the electrical generators and transform this power at a higher voltage level with lower current. This ensures that the power generated in the order of several megavolt amperes (MVA) is being transmitted at low current magnitudes in a cable of practical dimensions over very long distances. Today’s transmission and distribution systems are heavily dependent upon this technology and transformers are used extensively throughout the world. The standard transformer A standard transformer typically consists of a pair of windings, primary and secondary linked together by a magnetic core. The windings can be connected in any of the following types, which in turn decides to which category a transformer belongs. There are two basic types of the transformer viz., (a) shell type and (b) core type. These are illustrated in Figure 5.1. In Figure 5.1, the windings are the concentric portions and the white solid portions refer to the metallic part of a transformer (laminations), which either surround the windings (shell type) or where the windings surround the core (core type).
- P. C. Sen(Author)
- 2013(Publication Date)
- Wiley(Publisher)
Transformers are used to step up and step down voltage 40 at various stages of power transmission, as shown in Fig. 2.3. A large power transformer used to step up generator voltage from 24 to 345 kV is shown in Fig. 2.4. A distribution transformer used in a public utility system to step down voltage from 4:6 kV to 120 V is shown in Fig. 2.5. Transformers are widely used in low-power electronic or control circuits to isolate one circuit from another circuit or to match the impedance of a source with its load for maximum power transfer. Transformers are also used to measure voltages and currents; these are known as instrument transformers. FIGURE 2.2 Schematic representation of a two-winding transformer. FIGURE 2.3 Power transmis- sion using transformers. FIGURE 2.1 Transformer core construction. (a) Core-type, (b) Shell-type, (c) L-shaped lamination, (d) E-shaped lamination. Transformers 41 2.1 IDEAL TRANSFORMER Consider a transformer with two windings, a primary winding of N 1 turns and a secondary winding of N 2 turns, as shown schematically in Fig. 2.6. In a schematic diagram it is a common practice to show the two windings in the two legs of the core, although in an actual transformer Courtesy of Westinghouse Electric Corporation FIGURE 2.5 Distribution transformer. Courtesy of Westinghouse Electric Corporation FIGURE 2.4 Power transformer, 24 to 345 kV. 42 chapter 2 Transformers the windings are interleaved. Let us consider an ideal transformer that has the following properties: 1. The winding resistances are negligible. 2. All fluxes are confined to the core and link both windings; that is, no leakage fluxes are present. Core losses are assumed to be negligible. 3. Permeability of the core is infinite (i.e., μ ! 1). Therefore, the exciting current required to establish flux in the core is negligible; that is, the net mmf required to establish a flux in the core is zero.
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