电动机的结构特点英文翻译.doc

Construction Features of Electrical Machines The energy-conversion process usually involves the presence of two important features in a given electromechanical device. These are the field winding, which produces the flux density, and the armature winding, in which the “working emf is induced. In this section the salient construction features of the principal types of electric machines are described to show the location of these windings, as well as to demonstrate the general composition of such machines. 1.Three-Phase Induction Motor This is one of the most rugged and most widely used machines in industry. Its stator is composed of laminations of high-grade sheet steel. The inner surface 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 winding because the voltage induced in each of the three phases by a revolving flux density field are out of phase by 120 electrical degrees梐 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 latter is of a form similar to that of the stator winding. The winding terminals 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. As a matter of fact, it is the need for speed control which in large measure accounts for the use of the wound-rotor type induction motor. Otherwise the squirrel-cage induction motor would be