Electrical Machines And Electrical Apparatus
1 • Construction and Principles of Power Transformer
Transformer is an indispensable component in many energy conversion systems. It makes possible electric generation at the most economical generator voltage, power transfer at the most economical transmission voltage, and power utilization at the most suitable voltage for the particular utilization device. The transformer is also widely ud in low-power, low-current electronic and control circuits for performing such functions as matching the impedances of a source and its load for maximum power transfer, isolating one circuit from another, or isolating direct current while maintaining alternating current continuity between two circuits-
亚答子Esntially, a transformer consists of two or more windings coupled by mutual magnetic flux. If one of the windings, the primary is connected to an alternating voltage source, an alternating flux will be produced who amplitude will depend on the primary voltage, the frequency of the applied voltage, and the number of turns. The mutual flux will link the other winding, the condary and will induce a voltage in it who value will depend on the number of the condary turns as well as the magnitude of the flux and the frequency. By properly proportioning the primary and the condary turns, almost any desired voltage ratio, or ratio of transformation, can be obtained.
The esnce of transformer action requires only the existence of time-varying mutual flux linking two windings. Such action can occur for two windings coupled through air, but coupling between the windings can be made much more effectively using a core of iron or other ferromagnetic material, becau most of the flux is then confined to a definite, high-permeability path linking the windings. Such a transformer is commonly called an iron-core transformer. Most transformers are of this type. The following discussion is concerned almost wholly with iron-core transforme匚In order to reduce the loss caud by eddy current in the core, the magnetic circuit usually consists of a stack of thin laminations. Two common types of construction are shown schematically in Fig. 1.1 • In the core type (Fig. 1.1a) the windings are wound around two legs of a rectangular magnetic core; in the shell type(Figl.lb) the windings are wound around the center leg of a three-legged core. Silicon -steel laminations 0-014 mm in thick are generally ud for transformer ud at frequencies below a few hundred Hz. Silicon steel has the desirable property of low cost, low core loss, and high permeability at high flux densities (1.0 to 1.5T). The cores of small transformer ud in communication circuits at high frequencies
and low energy levels are sometimes made of compresd ferromagnetic alloys known as ferrites-
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Windings
(a) (b)
In each of the configurations, most of the flux is confined to the core and therefore links both windings. The winding also produce additional flux, known as leakage flux, which links one winding without linking the other. Although leakage flux is small fraction of the total flux, it plays an important role in determining the behavior of the transforme 匸In practical transformers, leakage is reduced by subdividing the windings into ctions placed as clo together as possible. In the core-type construction, each winding consists of two ctions, one ction on each of the two legs of the core, the primary and condary windings being concentric coils. In the shell type construction, variations of the concentric-winding arrangement maybe ud, or the windings may consist of a number of thin pancake coils asmbled in a stack with primary and condary coils interleaved.
2. Advantages of Balanced Three-pha Versus Single-pha Systems
In both transformers and rotating machines, a magnetic field is created by the combined action of the currents in the windings. In an iron-core transformer, most of this flux is confined to the core and links all the windings. This resultant mutual flux induces voltages in the windings proportional to their number of their turns and is responsible for the voltage-changing property of a transformer. In rotatin
g machines, the situation is similar, although there is an air gap which parates the rotating and stationary components of the machine. Directly analogous to the manner in which transformer core flux links the various windings on a transformer core, the mutual flux in rotating machines cross the air gap, linking the windings on the rotor and stator. As in a transformer, the mutual flux induces voltage in the winding proportional to the number of turns and time rate of change of the flux.
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A significant difference between transformers and rotating machines is that in rotating machines there is relative motion between the windings on the rotor and stator. This relative motion produces an additional component of the time rate of change of the various winding flux linkages. The resultant
Core
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—Windings
voltage component, known as the speed voltage, is characteristics of the process of electromechanical energy conversion. In a static transformer, however, the time variation of flux linkages is caud simply by the time variation of winding current; no mechanic motion is involved, and no electromechanical energy conversion takes place.
The resultant core flux in a transformer induces a counter Electro-Motive Force(EMF) in the primary which, together with the primary resistance and leakage-reactance voltage drops, must balance the applied voltage. Since the resistance and leakage -reactance voltage drops usually are small, the counter EMF must approximately equal to the applied voltage and the core flux adjust itlf accordingly. Exactly similar phenomena must take place in the armature windings of an AC motor. The resultant air-flux wave must adjust itlf to generate a counter EMF approximately equal to the applied voltage. In both transformers and rotating machines, the Magneto-Motive Force (MMF) of all the currents must accordingly adjust itlf to create the resultant flux required by this voltage balance. In any AC electromagnetic devices in which the resistance and leakage-reactance voltage drops are small, the resultant flux is very nearly determined by the applied voltage and frequency, and the cuiTents must adjust themlves accordingly to produce the MMF required to create this flux.
In a transformer, the condary current is determined by the voltage reduced by the condary winding, the condary leakage impedance, and the electric load. In an induction motor, the condary(rotor) current is determined by the voltage induced in the condary, the condary leakage impedance, and mechanical load on its shaft. Esntially the same phenomena place in the
primary winding of the transformer and in the armature (stator) windings of induction and synchronous motors. In all three, the primary, or armature, current must adjust itlf so that the combined MMF of all currents creates the flux the required by the applied voltage.
In addition to the uful mutual fluxes, in both transformers and rotating machines there are leakage fluxes which link individual windings without linking others. Although the detailed picture of the leakage fluxes in rotating machines is more complicated than that in transformers, their effects are esntially the same. In both, the leakage fluxes induce voltage in AC windings which are accounted for as leakage-reactance voltage drops. In both, the reluctances of the leakage-flux paths are dominated by that of a path through air, and hence the leakage fluxes are
nearly linearly proportional to the current producing them・ The leakage-reactance therefore is often assumed to be constant, independent of the degree of saturation of the main magnetic circuit.
Further examples of the basic similarities between transformer and rotating machines can be cited. Except for friction and windage, the loss in transformer and rotating machines are esntially the same. Tests for determining the loss and equivalent circuit parameters are similar: an open circuit, or no-load, test gives information regarding the excitation requirements and core loss(along with fri
ction and windage loss in rotating machines), while a short-circuit test together with DC resistance measurements gives information regarding leakage reactance and winding resistances. 3. Elementary Knowledge of Rotating Machines
安全案例Electromagnetic energy conversion occurs when changes in the flux linkage result from mechanical motion. In rotating machines, voltage are generated in windings or groups of coils by rotating the windings mechanically through a magnetic field, by mechanically rotating a magnetic field past the winding, or by designing the circuit so that the reluctance varies with rotation of the motor. By any of the methods, the flux linking a specific coil is changed cyclically, and a time-varying is generated.
A t of such coils connected together is typically referred to an armature winding. In general, the term armature winding is ud to refer to a winding or a t of windings on a rotating machine which carry AC currents. In AC machines such as synchronous or induction machines, the armature winding is typically on the stationary portion of the motor refeiTed to as the stator, in which ca the windings may also be referred to as stator windings.夏季星空
In a DC machine, the armature winding is found on the rotating member, referred to as the rotor. The armature winding of a DC machine consists of many coils connected together to form a clod loop. A rotating mechanical contact is ud to supply current to the armature winding as the rotor rotates.
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Synchronous and DC machine typically include a cond winding (or t of ttings) which carry DC current and which are ud to produce the main operating flux in the machine. Such a winding is typically refeiTed to as field winding. The field winding on a DC machine is found on the stator, while that on a synchronous machine is found on the rotor, in which ca current must be supplied to the field winding via a rotating mechanical contact. As we have en, permanent magnetic also produce DC magnetic flux and are ud in the place of field windings in some machines.
In most rotating machines, the stator and rotor are made of electrical steel, and the windings are installed in slots on the structures. The u of such high-permeability material maximizes the coupling between the coils and increa the magnetic energy density associated with the interaction.
It also enables the machine designer to shape and distribute the magnetic fields according to the requirements of each particular machine design. The time varying flux prent in the armature structures of the machines tends to induce cuiTents, known as eddy currents, in the electrical steeL Eddy currents can be a large source of loss in such machine and can significantly reduce machine performance. In order to minimize the effects of eddy currents, the armature structure is typically built from thin laminations of electrical steel with are insulated from each other.
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In some machines, such as reluctance machines and stepper motors, there are no windings on the roton Operation of the machines depends on the nonuniformity of air-gap reluctance associated with variations in rotor position in conjunction with time-varying currents applied to their stator windings. In such machines, both the stator and rotor structures are subjected to time-varying magnetic flux and, as a result, both may require lamination to reduce eddy-current loss.
Rotating electric machines take many forms and are known by many names: DC, synchronous, permanent-magnet, induction, variable reluctance, hysteresis, brushless, and so on. Although the machines appear to be quite dissimilar, the physical principles governing their behavior are quite similar, and it is often helpful to think of them in the same physical picture.
