Chapter2-transformer 《Electric Machinery 电机学（英汉双语）》课件.ppt

Contents 2.1 Construction and Ratings of Transformers 2.2 Analysis of Single Phase Transformers in No-load Condition 2.3 Operation of Transformers in Loaded condition 2.4 Determining parameters of Transformers by Tests 2.5 Voltage Regulation and Efficiency of Transformers 2.6 Three-phase Transformers 2.7 Parallel Operation of Transformers 2.8 Special Transformers 2.1.1 Construction of transformers a transformer is a device that changes ac electrical power at one voltage level to ac electrical power at another voltage level of the same frequency based upon the laws of electromagnetic induction. It consists of two or more coils of wire wrapped around a common ferromagnetic core. These coils are usually not directly connected. The interaction between the coils is realized through the common magnetic flux in the common core. The transformer winding connected to the power source is called the primary winding (input winding with N1 turns), and the winding connected to the load is called the secondary winding (output winding with N2 turns). If there is a third winding on the transformer, it is called the tertiary winding. Power transformers are constructed using either the core-form construction or the shell-form construction. The former one consists of a simple rectangular laminated piece of steel with the transformer windings wrapped around two sides of the rectangle. The later construction consists of a three-legged laminated core with the windings wrapped around the center leg. 2.2 Analysis of a Single Phase Transformer in No-load condition 2.2.1 Physical Situation of transformers in no-load condition The primary winding is connected to an ac power source. The secondary winding is open-circuited. If the resistor of the primary winding is r1, the voltage drop will be . The open-circuit voltage of the secondary winding is , which is equal to emf. According to Faraday’s law, the induced emfs in two windings can be expressed as: The only difference between e1 and