电气专业英语（论文）.doc

PAGE PAGE 22 院（系、部、中心） 专 业 班 级 学 生 姓 名 学 号 任 课 教 师 Page1 Generators and Motors(发电机和电动机) 1 English text From reference 1 1. Direct-current generators impress on the line a direct or continuous emf, one that is always in the same direction. Commercial dc generators have commutators, which distinguish them from ac generators. The function of a commutator and the elementary ideas of generation of emf and commutation are discussed in Div. 1. Additional information about commutation as applied to dc motors, which in general is true for dc generators, is given below. 2. Excitation of generator fields. To generate an emf, conductors must cut a magnetic field which in commercial machines must be relatively strong. A permanent magnet can be used for producing such a field in a generator of small output, such as a telephone magneto or the magneto of an insulation tester, but in generators for light and power the field is produced by electromagnets, which may be excited by the machine itself or be separately excited from another source.Self-excited machines may be of the series, shunt, or compound type, depending upon the manner of connecting the field winding to the armature. In the series type of machine,the field winding (the winding which produces the magnetic field) is connected in series with the armature winding. In the shunt type, the field winding is connected in parallel,shunt, with the armature winding. Compound machines have two field windings on each pole. One of these windings is connected in series with the armature winding, and the other is connected in parallel or shunt with the armature winding. 3. Armature winding of dc machines may be of the lap or the wave type. The difference in the two types is in the manner of connecting the armature coils to the commutator.A coil is the portion of the armature winding between successive connections to the commutator.In the lap