The basic principle of operation of an induction machine is illustrated by the revolving horseshoe magnet and copper-disk experiment pictured in Fig.4-1. When the horseshoe magnet is rotated, the moving magnetic field passing across the copper disk induces eddy currents in the disk. These eddy currents are in such a direction as to cause the disk to follow the rotation of the horseshoe magnet. With the direction of rotation shown in the figure, the eddy currents will be as displayed in Fig.4-1 according to Flemings right-hand rule 一台感应电机的基本工作原理如图 41 中的旋转的 U 形磁铁和铜圈盘的实验图所示。当 U 形铁旋转时，运动的磁场穿过铜圆盘从而在铜圆盘中产生涡流。这些涡流的方向是这样 的以致于使圆盘顺着 U 形磁铁的转向旋转。在图示的旋转方向下，根据弗莱明右手定则涡 流的方向将如图 41 所示。 Flemings right-hand rule: Place the thumb and the first and second fingers of the right hand so that all three are mutually perpendicular. With the hand in this position, the first finger is pointed in the direction of the field, the thumb is in the direction of motion of the relative motion of the conductor, and the second finger is the direction of the induced voltage. Note that the relative motion of the conductor is opposite to the rotation of the direction of rotation of the magnetic field. 弗莱明右手定则：伸出右手大拇指、食指以及中指并使它们相互垂直。保持右手处于这种 状态，使食指指向磁场的方向，拇指指向导体相对运动的运动方向，那么中指所指的方向 就是感应电压的方向，要注意的是导体的相对运动与磁场的旋转方向的运动相反。 By applying Flemings right-hand rule, the force on the copper disk is determined to be in the direction of rotation of the magnet. 通过应用弗莱明右手定则，我们确定作用在铜圆盘上的力与磁铁的旋转方向一致 Flemings left-hand rule: Place the thumb and the first and second finger of the left hand so that all three are mutually perpendicular to each other. With the first finger in the direction of the field and the second finger in the direction of the current, the thumb indicates the direction of the force. 弗莱明左手定则：伸出左手大拇指、食指以及中指并使它们相互垂直。使食指指向磁场的 方向，中指指向电流的方向，那么拇指所指的方向是力的方向 Whereas the copper disk will rotate in the same direction as the rotating magnetic field, it will never reach the same speed as the rotating magnet, because if it did, there would be no relative motion between the two and therefore no current induced in the copper disk. The difference in speed between the rotating magnetic field and the copper disk is known as slip, which is essential to the operation of an induction motor. In induction motors the rotating magnetic field is set up by windings in the stator, and the induced currents are carried by conductorsin the rotor. The rotating horseshoe magnet and copper disk are considerably different in structure from todays induction motor, but the basic principles of operation are the same. 尽管铜圆盘将顺着旋转磁场的方向而旋转，然而它却永远也达不到旋转磁铁的速度，因为 如果铜圆盘的速度等于旋转磁铁的速度，那么在两者之间就不存在相对运动因而在铜圆盘 中不会感应出电流。旋转磁铁和铜圆盘之间的速度差被称为转差（率） ，转差（率）对于感 应电机的运行是必要的，在感应电机中旋转磁场是通过定子绕组建立的，并且感应电流是 由转子导体承载的。旋转 U 形铁和铜圆盘与目前感应电动机的结构有相当大的不同，但基 本工作原理是一样的。 The rotating magnetic field is essential to the functioning of an induction motor. In practical machines this rotating magnetic field is achieved by a combination of a space displacement of the windings and a time-phase displacement of the exciting voltage. 旋转磁场对于一台感应电动机来说是必要的。在实际的电机中，旋转磁场是通过绕组在空 间上的交替布置和励磁电压在时间相位上的相互交替这两者的结合来实现的。 The rotor is formed from laminated electrical steel punching, and the rotor winding consists of bars contained in slots punched in the laminations. These bars are short-circuited at both ends by a short-circuiting ring. A bar-end ring structure, without the laminated core, is called a squirrel cage, as shown in the Fig.4-2. In small- and medium-horsepower sizes, rotors are made by casting aluminum into the rotor core. In the larger sizes of ac motors, cast-aluminum rotor are not practical, and copper bars are inserted into the slots. Thesecopper bars are short-circuited at both ends by a copper end ring, and the end ring is brazed or soldered onto the bars. Sometimes bronzes or other alloys are used to replace copper in making the cage and end ring. The sizes at which the transition between cast-aluminum and copper rotors takes place varies among rotor manufacturers, but virtually all rotors in motor sizes of several thousand horsepower and above are built with bar-type rotors. 转子是由电工钢冲压叠片形成的，并且转子绕组是由叠片中冲出的槽中的导条组成的。这 些导条被两端的短路环所短路。一个没有叠片铁心的导条端环结构被称为鼠笼，如图 42 所示。在中小型电机中，转子是通过在转子槽中铸铝来制成的。在大型交流电动机中， 铸铝转子是不实用的，我们将铜导条插入转子槽中。这些铜导体被两端的铜环所短路，并 且端环被焊接在导条上。有时我们在制作笼条和端环时用青铜或其他合金来代替铜。铸铝 与铜转子之间的尺寸的变化在不同的电机制造商之间有所不同，但实际上功率等于几千马 力以及功率大于几千马力的所有电动机的转子都是导条类型的转子。 Another construction feature dependent on motor size is the type of coil winding used. In small- and medium-size ac motors, most coils are random-wound. These coils are made with round wire, which is wound into the stator slots and assumes a diamond shape in the end turns; however, the wires are randomly located within a given coil, and hence the name “random-wound“. For large ac motors and particularly for high-voltage motors, 2300V and above, form-wound coils are used. These coils are constructed from rectangular wire, which is bentinto sha