Chapter 4Electric Machinery section 4-4 Transformer 电气工程及其自动化专业英语课件.ppt

* Chapter 4 Electric Machinery Section 4 Transformer Text New Words and Expressions Exercises End Addition or Omission of Chinese words Section 4 Transformer Types and construction of transformer A transformer is a device that changes AC electric energy at one voltage level into AC electric energy at 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 Transformer One 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 connected 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 Transformer Power transformer are constructed on one of two types of cores. One type of construction consists of a simple rectangular laminated 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. 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 Transformer The primary and secondary windings in a physical transformer are wrapped one on top of the other with the low-voltage winding innermost. 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 f