电动机转速测定系统的设计外文翻译.docx

1、电动机转速测定系统的设计外文翻译外文文献译文设计(论文)题目: 电动机转速测定系统的设计 专业与班级: 测控技术与仪器0601班 学生姓名: 杨 红 伟 指导教师: 周 彬 2010年3月18日Synchronous motorAbstract： In recent decades, with the power electronics, microelectronics technology and the development of modern control theory, medium and small power motors in the industrial and agr

2、icultural production and peoples daily lives are very broad application. Particularly in the township enterprises and household electrical appliances, need a large number of medium and small-power motors. Because of this motor development and wide application, its use, maintenance and repair work ha

3、s become increasingly important，Here are just on the knowledge of synchronous motor1. Principle of operation In order to understand the principle of operation of a synchronous motor, let us examine what happens if we connect the armature winding (laid out in the stator) of a 3-phase synchronous mach

4、ine to a suitable balanced 3-phase source and the field winding to a D.C source of appropriate voltage. The current flowing through the field coils will set up stationary magnetic poles of alternate North and South.(for convenience let us assume as alient pole rotor, as shown in Fig. 50). On the oth

5、er hand, the 3-phase currents flowing in the armature winding produce a rotating magnetic field rotating at synchronous speed. In other words there will be moving North and South poles established in the stator due to the 3-phase currents i.e at any location in the stator there will be a North pole

6、at some instant of time and it will become a South pole after a time period corresponding to half a cycle. (after a time = 12f , where f = frequency of the supply). Let us assume that the stationary South pole in the rotor is aligned with the North pole in the stator moving in clockwise direction at

7、 a particular instant of time, as shown in Fig50. These two poles get attracted and Figure 50: Force of attraction between stator poles and rotor poles - resulting in production of torque in clockwise direction try to maintain this alignment ( as per Lenzs law) and hence the rotor pole tries to foll

8、ow the stator pole as the conditions are suitable for the production of torque in the clockwise direction. However the rotor can not move instantaneously due to its mechanical inertia, and so it needs sometime to move. In the mean time, the stator pole would quickly (a time duration corresponding to

9、 half a cycle) change its polarity and becomes a South pole。So the force of attraction will no longer be present and instead the like poles experience a force of repulsion as shown in Fig. 51. In other words, the conditions are now suitable for the Figure 51: Force of repulsion between stator poles

10、and rotor poles - resulting in production of torque in anticlockwise direction production of torque in the anticlockwise direction. Even this condition will not last longer as the stator pole would again change to North pole after a time of 12f .Thus the rotor will experience an alternating force wh

11、ich tries to move it clockwise and anticlockwise at twice the frequency of the supply, i.e. at intervals corresponding to 12f seconds. As this duration is quite small compared to the mechanical time constant of the rotor, the rotor cannot respond and move in any direction. The rotor continues to be

12、stationary only.On the contrary if the rotor is brought to near synchronous speed by some external means say a small motor (known as pony motor-which could be a D.C or AC induction rotor) mounted on the same shaft as that of the rotor, the rotor poles get locked to the unlike poles in the stator and

13、 the rotor continues to run at the synchronous speed even if the supply to the pony motor is disconnected.Thus the synchronous rotor cannot start rotating on its own or usually we say that the synchronous rotor has no starting torque. So, some special provision has to be made either inside the machi

14、ne or outside of the machine so that the rotor is brought to near about its synchronous speed. At that time, if the armature is supplied with electrical power, the rotor can pull into step and continue to operate at its synchronous speed. Some of the commonly used methods for starting synchronous ro

15、tor are described in the following section.2. Methods of starting synchronous motor Basically there are three methods that are used to start a synchronous motor: To reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in wi

16、th it during one half-cycle of the rotating magnetic fields rotation. This is done by reducing the frequency of the applied electric power. This method is usually followed in the case of inverter-fed synchronous motor operating under variable speed drive applications. To use an external prime mover

17、to accelerate the rotor of synchronous motor near to its synchronous speed and then supply the rotor as well as stator. Of course care should be taken to ensure that the direction of rotation of the rotor as well as that of the rotating magnetic field of the stator are the same. This method is usual

18、ly followed in the laboratory- the synchronous machine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling procedure. Then the power supply to the prime mover is disconnected so that the synchronous machine will continue to operate as a

19、 motor. To use damper windings or amortisseur windings if these are provided in the machine. The damper windings or amortisseur windings are provided in most of the large synchronous motors in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodica

20、lly varying load.Each of these methods of starting a synchronous motor are described below in detail.2.1Motor starting by reducing the supply Frequency If the rotating magnetic field of the stator in a synchronous motor rotates at a low enough speed, there will be no problem for the rotor to acceler

21、ate and to lock in with the stators magnetic field. The speed of the stator magnetic field can then be increased to its rated operating speed by gradually increasing the supply frequency f up to its normal 50or60Hz value.This approach to starting of synchronous motors makes a lot of sense, but there

22、 is a big problem: Where from can we get the variable frequency supply? The usual power supply systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However, variable-frequency voltage source can be obtained from a dedicated generator only in the olden days and such a situat

23、ion was obviously impractical except for very unusual or special drive applications.But the present day solid state power converters offer an easy solution to this. We now have the rectifier-inverter and cycloconverters, which can be used to convert a constant frequency AC supply to a variable frequ

24、ency AC supply. With the development of such modern solid-state variable-frequency drive packages, it is thus possible to continuously control the frequency of the supply connected to the synchronous motor all the way from a fraction of a hertz up to and even above the normal rated frequency. If suc

25、h a variable-frequency drive unit is included in a motor-control circuit to achieve speed control, then starting the synchronous motor is very easy-simply adjust the frequency to a very low value for starting, and then raise it up to the desired operating frequency for normal running.When a synchron

26、ous motor is operated at a speed lower than the rated speed, its internal generated voltage (usually called the counter EMF) EA = Kwill be smaller than normal. As such the terminal voltage applied to the motor must be reduced proportionally with the frequency in order to keep the stator current with

27、in the rated value. Generally, the voltage in any variable-frequency power supply varies roughly linearly with the output frequency.2.2 Motor Starting with an External Motor The second method of starting a synchronous motor is to attach an external starting motor (pony motor) to it and bring the syn

28、chronous machine to near about its rated speed (but not exactly equal to it, as the synchronization process may fail to indicate the point of closure of the main switch connecting the synchronous machine to the supply system) with the pony motor. Then the output of the synchronous machine can be syn

29、chronized or paralleled with its power supply system as a generator, and the pony motor can be detached from the shaft of the machine or the supply to the pony motor can be disconnected. Once the pony motor is turned OFF, the shaft of the machine slows down, the speed of the rotor magnetic field BR

30、falls behind Bnet, momentarily and the synchronous machine continues to operate as a motor. As soon as it begins to operates as a motor the synchronous motor can be loaded in the usual manner just like any motor.This whole procedure is not as cumbersome as it sounds, since many synchronous motors ar

31、e parts of motor-generator sets, and the synchronous machine in the motor-generator set may be started with the other machine serving as the starting motor. More over, the starting motor is required to overcome only the mechanical inertia of the synchronous machine without any mechanical load (load

32、is attached only after the synchronous machine is paralleled to the power supply system). Since only the motors inertia must be overcome, the starting motor can have a much smaller rating than the synchronous motor it is going to start. Generally most of the large synchronous motors have brushless e

33、xcitation systems mounted on their shafts. It is then possible to use these exciters as the starting motors. For many medium-size to large synchronous motors, an external starting motor or starting by using the exciter may be the only possible solution, because the power systems they are tied to may not be able to h