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1、Chapter 4Electric Machinery Section 4 Transformern Textn New Words and Expressionsn Exercisesn Endn Addition or Omission of Chinese wordsSection 4 TransformerTypes and construction of transformerA transformer is a device that changes AC electric energy at one voltage level into AC electric energy at
2、 another voltage level through the action of a magnetic field. It consists of two or more coils of wire wrapped around a common ferromagnetic core. These coils are (usually) not directly connected. The only connection between the coils is the common magnetic flux present within the core. Section 4 T
3、ransformerOne of the transformer windings is connected to a source of AC electric power, and the second (and perhaps third) transformer winding supplies electric power to loads. The transformer winding connected to the power source is called the primary winding or input winding, and the winding conn
4、ected to the loads is called the secondary winding or output winding. If there is a third winding on the transformer, it is called the tertiary winding. Section 4 TransformerPower transformer are constructed on one of two types of cores. One type of construction consists of a simple rectangular lami
5、nated piece of steel with the transformer windings wrapped around two sides of the rectangle. This type of construction is known as core form. The other type consists of a three- legged laminated core with the windings wrapped around the center leg. This type of construction is known as shell form.
6、In either case, the core is constructed of thin laminations electrically isolated from each other in order to reduce eddy currents to a minimum. Section 4 TransformerThe primary and secondary windings in a physical transformer are wrapped one on top of the other with the low-voltage winding innermos
7、t. Such an arrangement serves two purposes: (i) It simplifies the problem of insulating the high- voltage winding from the core. (ii) It results in much less leakage flux than would be the two windings were separated by a distance on the core. Section 4 TransformerPower transformers are given a vari
8、ety of different names, depending on their use in power systems. A transformer connected to the output of a generator and used to step its voltage up to the to transmission levels is sometimes called a unit transformer. The transformer at the other end of the transmission line, which steps the volta
9、ge down from transmission levels to distribution levels, is called a substation transformer. Finally, the transformer that takes the distribution voltage and steps it down to the final voltage at which theSection 4 Transformerthe power is actually used is called a distribution transformer. All these
10、 devices are essentially the same in their construction, the only difference among them is their intended use.In addition to the various power transformers, two special-purpose transformers are used with electric machinery and power systems. The first of these special transformers is a device specia
11、lly designed to sample a high voltage and produce a low secondary voltage directly proportional to it. Such a transformer is called a potential transformer. Section 4 TransformerA power transformer also produces a secondary voltage directly proportional to its primary voltage; the difference between
12、 a potential transformer and a power transformer is that the potential transformer is designed to handle only a very small current. The second type of special transformer is a device designed to provide a secondary current much smaller than but directly proportional to its primary current. This devi
13、ce is called a current transformer. Section 4 TransformerEquivalent circuit of transformerThe losses that occur in real transformers have to be accounted for in any accurate model of transformer behavior. The major items to be considered in the construction of such a model are ： （1）Copper (Rr) losse
14、s. Copper losses are the resistive heating losses in the primary and secondary windings of the transformer. They are proportional to the square of the current in the windings. Section 4 Transformer（2）Eddy current losses. Eddy current losses are resistive heating losses in the core of the transformer
15、. （3）Hysteresis losses. These losses are associated with the rearrangement of the magnetic domains in the core during each half-cycle. （4）Leakage flux. The fluxes which escape the core and pass through only one of the transformer windings are leakage fluxes. These escaped fluxes produce a self-induc
16、tance in the primary and secondary coils, and the effects of this inductance must be accounted for. Section 4 TransformerIt is possible to construct an equivalent circuit that takes into account all the major imperfections in real transformers. Each major imperfection will be considered in turn, and its effect will be included in the transformer model.The easiest effect to model is the copper losses. Copper losses are resistive losses in the primary and secondary windings of the transformer core
