双语chapter5.pptVIP

授课教师：任爽 5.1 Three-Phase Induction Motor This is one of the most rugged and most widely used machines in industry. The three-phase induction motor consists of stator and rotor. The stator is composed of?stator core and stator? winding. The stator core is composed of laminations of sillicon sheet steel. The inner surface of the stator core is slotted to accommodate a three-phase winding. In Fig.5.2 (a) the three-phase winding is represented by three coils, the axes of which are 120 electrical degrees apart. Coil aa’ represents all the coils assigned to phase a for one pair of poles. Similarly coil bb’ represents phase b coils, and coil cc’ represents phase c coils. When one end of each phase is tied together, as depicted in Fig.5.2 (b), the three-phase stator winding is said to be Y-connected. Such a winding is called a three-phase (symmetrical) winding because the voltage induced in each of the three phase by a revolving flux density field are out of phase by 120 electrical degrees—a distinguishing characteristic of a balanced three-phase system. The rotor also consists of laminations of slotted ferromagnetic material, but the rotor winding may be either the squirrel-cage type or the wound-rotor type. The squirrel-cage winding consists merely of a number of copper bars embedded in the rotor core slots and connected at both ends by means of copper end rings. In modern induction motors, the most common type of rotor has cast-aluminum conductors and short-circuiting end rings. The wound-rotor winding is of a form similar to that of the stator winding. The winding terminal are brought out to three slip rings. This allows an external three-phase resistor to be connected to the rotor winding for the purpose of providing speed control. In conclusion, the squirrel-cage construction is not only simpler and more economical than the wound-rotor type but more rugged as well. There are no slip rings or carbon brushes to be bothered with. When three-phase volta