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1、感应电动机的基本工作原理可以旋转马蹄形磁铁和铜圆盘的实验来说明。当马蹄铁磁铁旋转时，穿过铜圆盘的移动磁场在盘中感应出涡旋电流。这些涡旋电流的方向能够引起圆盘跟随马蹄形磁铁旋转。根据右手定则将会产生涡旋电流。右手定则指出：伸出右手的大拇指、食指、中指，并让他们互相垂直，这样实质的方向为磁场方向，大拇指指向为导体运动方向，中指的方向为感应电压的方向。注意，导体的相对运动方向和磁场旋转的方向相反。运用左手定责得出铜盘受到得力与磁铁旋转方向一致。左手定则指出：伸出左手的大拇指、食指、中指，并令他们互相垂直，让食指指向磁场方向，中指指向电流的方向，则大拇指的方向即为作用力的方向。尽管铜盘将按照旋转磁场的
2、方向运动，但它决不会与磁场的速度相同。因为如二者速度相同，它们之间将不存在相对运动，从而铜盘中也就没有感应电流产生。旋转磁场和铜盘间的速度之差称为转差率。转差率对于感应式电动机的工作至关重要。在感应电动机中，旋转磁场是由定子中的绕组激发的，感应电流是在转子中流动的，旋转马蹄磁铁和铜圆盘在结构上与当今的感应式电动机有很大区别，但是它们基本原理上是相同的。旋转磁场对感应式电动机是至关重要的。实际电机中，它是通过空间上交替布置绕组和时间相位上相互交替的激励电压实现的。转子是采用层叠的钢片冲压而成的，转子绕组有叠片上冲槽内嵌入的导体条构成。这写导体条在两端通过短路环构成短路。不包括层叠片式铁心中铸入铝
3、的方式构成转子。在较大功率的交流电机中，铸铝转子不实用，采用的是在转子槽中插入钢条的作法。这些钢条在两端通过铜短路环短路，短路环是采用纤接或焊接的方式与铜条相连的。有时采用青铜或其他合金代替铜来制作鼠笼和短路环。到底在多大功率上，以铜转子代替铸铝转子因不同的制造商而不同名的是几千马力以上的电动机转子一般都采用条形工艺。另一个与电动机容量有关的结构特性是所采用的线圈组的类型。在中小型交流电动机中，大多数线圈是散绕的。这些线圈采用的是圆截面导线，这些导线绕置在定子槽中，并将其端部绕置成菱形。只不过杂给定绕组中导线是随即分布的，因此得名“散绕”。对于大型交流电动机尤其是高压电动机，如2300V以上的
4、电动机，则采用模绕线圈。这些线圈采用截面为矩形的导线构成，将它们弯曲成型后缠上绝缘带。在制作定子绕组时，线圈被做成适当的尺寸，以便将整个线圈置于定子槽中。模绕式线圈可用作高压线圈，因为它较容易在将绕组置于定子之前为每个线圈采用额外的绝缘措施。.MotorThe basic principe of operation of an induction machine in illustrated by the revolving horsesshou-magnet and copper-disk experiment pictured in Fig 1-1.When the horseshou m
5、agnet 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 cyrrents will be as
6、displayed to Flemings right-hand rule.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 in pointed in the direction of the field，the thumb is in the direction of mot
7、ion of the conductor， and the second finger is in the direction of the induced voltage.Note that the relative motion of the conductor is opposite to the direction of rotation of the magnetic field.By applying Flemings left-hand rule，the force on the copper disk is determined to be in the direction o
8、f rotation of the magnet.Flemings left-hand rule：Place the thumb and the first and second fingers 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
9、the direction of the force.Whereas the copper disk will rotate in the same direction as the rotating magnetic field，it will never reach the same speed ad 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 di
10、fference in speed between the rotartng 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 conductors in the rotor.The
11、 rotating horseshoe magnet and copper disk are considerably different in structure from todays induction motor，but the basic principles of operation are the same.The rotating magnetic field is essential to the functioning of an induction motor.In practical machines this rotating magnetic field is ac
12、hered by a combination of a space displacement of the winding and a time-phase displacement of the exciting voltage.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
13、 ends by a short-circuiting ring.A bar-end ring structure，without the laminated core，is called a squirrel cage.In small-and medium-horsepower sizes，rotors are made by casting aluminum into the rotou core，in the larger sizes of ac motors，cast-aluminum rotors are not pratical，and copper bars are inser
14、ted into rotor slots.These copper bars are short circuited at both ends by a copper end ring，and the end ring is brazed or soldered onto the bars，sometimes bronze or other alloys are used ti replace copper in making the cage and end ring.The size at which the transition between cast-aluminum and cop
15、per rotors takes place varies among motor manyfacturers，but virtually all rotors in motor sizes of several thousand horsepower and above are built with bar-type rotors.Another construction feature dependent in motor size is the type of coil winding used.In small and medium-size ac motors，most coils
16、are random-wound.These coils are made with round wire，which is wound into the stator slots and assumes a diamond shapr 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-voltagemotors，2300V an
17、d above，form-would coils are used.These coils are construted from rectangular wire，which is bent into shape around forms and them taped.The coil is formed to the proper size so that the complete coil can be inserted into the stator slots at the time the stator is wound.Form wound coils are used for high-voltage windings because it is relatively easy to add extra insulation on the individual coils before inserting them into the stator.
