Chapter4-Transformer.pptVIP

第一章 Chapter 4 Transformer 二、 变压器的基本结构 （1） 变压器的型号 三、 变压器的铭牌和技术数据 A transformer is a device that couples two AC circuits magnetically rather than through any direct conductive connection and permits a “transformation” of the voltage and current between one circuit and the other. Transformers play a major role in electric power engineering and are a necessary part of the electric power distribution network. The Ideal Transformer The ideal transformer consists of two coils that are coupled to each other by some magnetic medium. There is no electrical connection between the coils. The Ideal Transformer The primary coil is wound so that it has n1 turns, while the secondary has n2 turns. We define the turns ratio (匝数比) N as Voltage and Current Relationship When a time-varying current flows in the primary winding, a corresponding time-varying voltage is generated in the secondary because of the magnetic coupling between the two coils. The relationship between primary and secondary current in an ideal transformer is very simply stated as follows: (3.8.1) N1, the transformer is called a step-up transformer N1, the transformer is called a step-down transformer. Example Problem Problem: We require a transformer to deliver 500 mA at 24 V from a 120-V rms line source. How many turns are required in the secondary? What is the primary current? Known Quantities: Primary and secondary voltages; secondary current. Number of turns in the primary coil. Find: n2 and I1. Schematics, Diagrams, Circuits, and Given Data: V1 = 120 V; V2 = 24 V; I2 = 500 mA; n1 = 3,000 turns. Assumptions: Use rms values for all phasor quantities in the problem. Problem Solution Solution: Analysis: Using equations 3.8.1 we compute the number of turns in the secondary coil as follows: Knowing the number of turns, we can now compute the p rimary current, also from equation 3.8.1: 三、Impedance matching (阻抗匹配) A very common and rather general situat